[0001] This application claims the benefit of U.S. Provisional Patent Application Serial No. 63/374,175, filed 31 August 2022, the entire content of which is incorporated herein by reference.

TECHNICAL FIELD

[0002] This disclosure is related to an implantable medical systems, such as an implantable medical device.

BACKGROUND

[0003] Implantable medical devices are often placed in a subcutaneous pocket and coupled to one or more transvenous medical electrical leads carrying pacing and sensing electrodes positioned in the heart. Intracardiac pacemakers have recently been introduced that are implantable within a ventricular chamber of a patient’s heart for delivering ventricular pacing pulses without the use of electrical leads. Such pacemakers or other implantable medical devices may also be able to detect the occurrence of arrhythmias, such as fibrillation, tachycardia and bradycardia, in the patient’ s heart. An implantable cardiac defibrillator may deliver electrical shocks to the patient’s heart in response to detection of a tachycardia or fibrillation to restore a normal heartbeat in the patient. In some cases, a single implantable medical device functions as both an implantable pacemaker and implantable cardiac defibrillator.

[0004] Implantable medical devices may include electrodes and/or other elements for physiological sensing and/or therapy delivery. The electrodes and/or other elements may be implanted at target locations selected to detect a physiological condition of the patient and/or deliver one or more therapies. For example, the electrodes and/or other elements may be delivered to a target location within an atrium or ventricle to sense intrinsic cardiac signals and deliver pacing or antitachyarrhythmia shock therapy from a medical device coupled to a lead. SUMMARY

[0005] This disclosure describes an implantable medical device (IMD) configured to position within a heart of a patient. The IMD is configured to receive imparted forces from a tissue wall of a beating heart when a fixation element of the IMD is implanted in the tissue wall. The imparted forces may cause motion of the IMD. The IMD includes a motion sensor configured to sense the motion of the IMD and includes processing circuitry configured to compare the motion sensed with a representative cardiac activity. The processing circuitry is configured to assess the engagement of the fixation element and the tissue wall based on the comparison. The medical system may assist a clinician in assessing whether the IMD is being and/or has been properly deployed in the heart. For example, the medical system may provide feedback to the clinician during implantation of the IMD when fluoroscopy, ultrasound, and/or other imaging modalities provide limited visibility.

[0006] In an example, a medical system comprises: an implantable medical device configured to engage tissue within a heart of a patient, wherein the implantable medical device includes: a housing, a fixation element configured to progressively embed within the tissue to couple the implantable medical device to the tissue, wherein the fixation element is configured to transmit a force from the tissue to the housing as the fixation element progressively engages, and a motion sensor configured to provide an sensor signal indicative of a motion of the housing; and processing circuitry configured to: compare the sensor signal and a representative cardiac activity when the fixation element engages the tissue, and evaluate the engagement of the implantable medical device to the tissue based on the comparison. The representative cardiac activity may be based on, for example, a sensed cardiac activity of the patient, an expected cardiac activity based on historical cardiac activity of the patient or other patients, or another indicator indicative of a representative cardiac activity.

[0007] In an example, a method comprises: engaging, using a fixation element, an implantable medical device to tissue within a heart of a patient, wherein engaging the fixation element includes progressively engaging the fixation element to the tissue, and wherein the fixation element is configured to transmit a force from the tissue to a housing of the implantable medical device as the fixation element progressively engages the tissue; comparing, using processing circuitry, a sensor signal and a representative cardiac activity as the fixation element progressively engages the tissue, wherein the sensor signal is provided by a motion sensor, and wherein the sensor signal is indicative of a motion of the housing; and assessing, using processing circuitry, the engagement of the implantable medical device to the tissue based on the comparison.

[0008] The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

[0009] FIG. 1 is a conceptual diagram illustrating an example medical system including an implantable medical device.

[0010] FIG. 2 is a schematic illustration of an implantable medical device and a tissue wall.

[0011] FIG. 3 is a schematic illustration of the implantable medical device of FIG. 2 with a fixation element contacting with the tissue wall.

[0012] FIG. 4 is a schematic illustration of the implantable medical device of FIG. 2 and FIG. 3 with the fixation element engaged with the tissue wall.

[0013] FIG. 5 is a graphical example of a representative cardiac activity.

[0014] FIG. 6 is a graphical example of motion signals provided by a motion sensor of an implantable medical device when the implantable medical device is displaced from a tissue wall.

[0015] FIG. 7 is a graphical example of motion signals provided by a motion sensor of an implantable medical device illustrating perturbations caused by motion of a tissue wall. [0016] FIG. 8 is a graphical example of motion signals provided by a motion sensor of an implantable medical device illustrating rotations of a device housing relative to a tissue wall.

[0017] FIG. 9 is an example illustration of a relative orientation between an implantable medical device and a tissue wall.

[0018] FIG. 10 is a functional block diagram illustrating an example configuration of an example medical system.

[0019] FIG. 11 illustrates an example technique for assessing an engagement of an implantable medical device and a tissue wall. DETAILED DESCRIPTION

[0020] This disclosure describes a medical system including an implantable medical device (IMD) configured to position within a heart of a patient, such as within an atrium, ventricle, coronary sinus, or other portions of the heart. The IMD includes a fixation element configured to engage tissues of the heart to substantially affix the IMD to a target site within the heart. The fixation element may be, for example, a helix, one or more times, or another structure and/or component configured to engage the tissues. The medical system is configured to assist a clinician in assessing whether the IMD is being and/or has been properly deployed in the heart. For example, the medical system may provide feedback to the clinician during implantation of the IMD when fluoroscopy, ultrasound, and/or other imaging modalities provide limited visibility.

[0021] The IMD is configured such that forces imparted to the fixation element from the engaged tissues of the heart (e.g., due to a heartbeat) cause a corresponding motion of the IMD along one or more geometric axes of the IMD. The IMD includes a motion sensor (e.g., an accelerometer) configured to sense the IMD motion along the one or more geometric axis. The medical system is configured to compare the IMD motion to a representative cardiac activity of the heart, in order to evaluate the coupling between the structure mechanism and the engaged tissues. In examples, the medical system is configured to compare the IMD motion and the representative cardiac activity during implantation of the IMD within the heart. The medical system may be configured to provide an output during the implantation process, such that a clinician may assess the coupling between the IMD and the heart tissues as the implantation process progresses. [0022] IMD motion may refer to motion indicative of a movement of the IMD along one or more principal axes in a geometric space. In examples, the IMD motion is indicative of a movement of the housing of the IMD. In examples, the motion indicative of the IMD and/or housing of the IMD may be the motion of a proof mass and/or other component sensed by a motion sensor mechanically supported by the IMD (e.g., the IMD housing). In some examples, the motion indicative of a movement of the IMD may be caused by motion of the IMD. For example, the motion indicative of a movement of the IMD may be a motion of a proof mass of an accelerometer configured such that movement of the IMD causes movement of the proof mass. [0023] The medical system may be configured such that an action by a clinician causes the fixation element of the IMD to progressively embed within the heart tissue during an implantation procedure. For example, the fixation element may be helix configured to progressively embed within the heart tissue when the helix contacts the heart tissue and clinician causes an impartation of a torque (e.g., using a delivery member) to the IMD. The fixation element may be one or more tines configured to progressively embed when one or more of the tines contacts the heart tissue and the clinician causes a proximal retraction of a delivery catheter and/or exertion of an axial force to the IMD. The medical system is configured to sense motion of the IMD and correlate the motion to a representative cardiac activity during the implantation, such that the clinician may assess when a contact between the fixation element and the heart tissue has occurred, assess a progression of the coupling between the fixation element and the heart tissue as the clinician causes the fixation element to progressively embed, assess a quality of the coupling between the fixation element and the heart tissue, assess an orientation of the IMD relative to a surface of the heart tissue, and/or assess other aspects of the IMD implantation.

[0024] The fixation element may be mechanically supported by a housing of the IMD, such that forces imparted to the fixation element by the heart tissue (e.g., caused by beating of the heart) cause motion of the IMD housing. The IMD housing may define one or more axes, such as a principal axis extending through a distal portion and a proximal portion of the IMD, a first radial axis perpendicular to the principal axis, and a second radial axis perpendicular to the first radial axis and the perpendicular axis. The motion sensor may be configured to sense motion (e.g., sense motion by sensing an acceleration) of the IMD along one or more of the principal axis, the first radial axis, and/or the second radial axis. Processing circuitry may receive one or more signals indicative of the IMD motion along one or more of the principal axis, the first radial axis, and/or the second radial axis, and assess a deviation between the IMD motion and a motion which might be expected based on a representative cardiac activity.

[0025] The motion sensor may be configured to sense, for example, an acceleration indicative of an acceleration of the IMD housing caused by forces imparted to the IMD housing (e.g., via the fixation element) as the heart beats and imparts forces to the fixation element. In examples, the motion sensor is configured to sense acceleration of a proof mass. The processing circuitry is configured to compare the IMD motion sensed to the representative cardiac activity and provide an output indicative of the coupling between the fixation element and the heart tissue. The representative cardiac activity may be based on, for example, a sensed cardiac activity of the patient, an expected cardiac activity based on historical cardiac activity of the patient or other patients, or another indicator indicative of a representative cardiac activity. In examples, the medical system is configured to sense a frequency of an IMD motion (e.g., a frequency based on perturbations of a measured acceleration) along the principal axis, the first radial axis, and/or the second radial axis to determine a frequency-based sensor parameter, and compare the frequency-based sensor parameter to one or more frequencies of the representative cardiac activity. In examples, the medical system is configured to sense an amplitude of an IMD motion (e.g., an amplitude based on perturbations of a measured acceleration) along the principal axis, the first radial axis, and/or the second radial axis to determine an amplitude-based sensor parameter, and compare the amplitude-based sensor parameter to one or more amplitudes of the representative cardiac activity. In examples, medical system may be configured to determine a force on the IMD (e.g., based on the acceleration sensed by the motion sensor) and compare the determined force to a force that might be expected on the IMD by the representative cardiac activity. The processing circuitry may be configured to compare the determined force to the representative cardiac activity and provide an output indicative of the coupling between the fixation element and the heart tissue.

[0026] The medical system may include an output device configured to provide indications to a clinician during an implantation of the IMD. For example, the medical system may compare the IMD motion sensed along one or more of the principal axis, the first radial axis, and/or the second radial axis to the representative cardiac activity and provide an indication that contact between the fixation element of the IMD and the heart tissue is present. Such an indication may allow the clinician to assess whether, for example, the IMD is still substantially free-floating in the heart and/or experiencing intermittent contact with the heart tissue prior to the clinician taking action to progressively embed the fixation element within the tissue. The medical system may compare the IMD motion to the representative cardiac activity and provide an indication of a pressure (e.g., a force) exerted by the fixation element on the heart tissue to, for example, allow the clinician to assess whether an existing pressure is too low or too high for implantation of the IMD in the heart tissue. In examples, when an action by the clinician (e.g., a rotation of the IMD, and/or withdrawal of a delivery catheter) causes the fixation element to progressively embed in the heart tissue, the medical system may substantially monitor the IMD motion to provide an indication of the progression of the fixation element into the heart tissue as the clinician takes the action. For example, when the fixation element is a helix, the medical system may assess the IMD motion to determine rotations of the IMD housing around the principal axis as the medical system compares the IMD motion to the representative cardiac activity, in order to determine a number of rotations of the IMD caused by the clinician as the helix progressively embeds within the tissue. In examples, the medical system is configured to assess and/or indicate the likely success of an implantation based on, for example, the number of rotations, the comparison of the IMD motion and the representative cardiac activity, and/or other parameters and/or indications.

[0027] In examples, the medical system is configured to assess an alignment of the IMD relative to a tissue wall (e.g., a septum) of the heart during and/or following the implantation. For example, the processing circuitry may compare the IMD motions sensed among the principal axis, the first radial axis, and/or the second radial axis to indicate a substantial perpendicularity of the IMD to the tissue wall. The processing circuitry may evaluate the perpendicularity based on the IMD motions sensed along the axes either individually, in comparison with one another, and/or in comparison with the representative cardiac activity. In some examples, the processing circuitry is configured to detect and/or assess a pivoting of the principal axis relative to the tissue wall (e.g., a “wobbling” of the IMD) as the heart beats and imparts forces to the IMD, such that the clinician may assess and/or alter the orientation. The detection and/or assessment of the pivoting of the principal axis may also provide an indication that the IMD has prematurely moved distal to a delivery cup or other structure configured to provide stability to the IMD during implantation.

[0028] The IMD is configured to transit through vasculature of the patient to position the IMD in the vicinity of a target area, such as an area within a chamber of the heart. For example, the IMD may be configured to allow a clinician to navigate the IMD through a vein of the heart (e.g., an innominate vein, an interior vena cava (IVC), a superior vena cava (SVC), or another venous pathway) to a target location within a right ventricle (RV), right atrium (RA), or another area of the heart. In examples, the IMD is configured to position with a lumen of a delivery catheter configured to transit the IMD through vasculature. For example, the delivery catheter may include a delivery receptacle (e.g., delivery receptacle 111 (FIGS. 2-4)) at a distal end of the catheter configured to substantially hold the IMD as the delivery catheter transits through vasculature.

[0029] FIG. 1 is a conceptual diagram illustrating a portion of an example medical system 100 configured to deliver therapy (e.g., pacing) to a heart 102 of a patient.

Medical system 100 includes IMD 104 including device housing 106 and fixation element 108 extending from device housing 106. IMD 104 (e.g., device housing 106) may define a proximal portion 105 of IMD 104 (“IMD proximal portion 105”) and a distal portion 107 of IMD 104 (“IMD distal portion 107”) substantially opposite IMD proximal portion 105. In examples, medical system 100 includes a delivery member 110 configured to position IMD 104 within the vicinity of a target site 112 within heart 102.

[0030] In examples, as illustrated in FIG. 1, target site 112 is a region in a ventricular wall of the right ventricle (RV) of heart 102. Target site 112 may include a tissue wall 113 with surface tissue 115 at least partially defining an anatomical volume (e.g., the RA and/or RV) of heart 102. In other examples, delivery member 110 and/or IMD 104 may be configured to position in the vicinity of a target site at another portion of heart 102. For example, delivery member 110 and/or IMD 104 implantable medical lead may be configured to position in the vicinity of a target site in the right atrium (RA) of heart 102, the left atrium (not shown), the left ventricle (not shown), or within or around the coronary sinus 114. Delivery member 110 and IMD 104 may be configured to extend through vasculature of a patient (e.g., an interior vena cava (IVC)) to position IMD 104 within heart 102. In examples, delivery member 110 may include a cup section (not shown) defining a lumen configured to engage IMD 104.

[0031] Fixation element 108 is configured to secure IMD 104 to tissues of heart 102. In examples, device housing 106 mechanically supports fixation element 108. Fixation element 108 is configured to penetrate cardiac tissue (e.g., tissue wall 113 and/or surface tissue 115) at or near a target site, such as target site 112. For example, fixation element 108 may be configured to penetrate cardiac tissue of a septal wall in a RV, RA, LV, and/or LA of heart 102, or penetrate cardiac tissue in another area of heart 102. Fixation element 108 may be configured to substantially maintain IMD 104 at or in the vicinity of the target site when fixation element 108 penetrates tissues at or in the vicinity of the target site. [0032] Fixation element 108 may be configured to allow a clinician to cause fixation element 108 to engage the tissue within heart 102, such that the clinician may affix IMD 104 once delivered to target site 112. For example, fixation element 108 may be configured to progressively embed within tissue wall 113 when fixation element 108 contacts surface tissue 115 and an action by a clinician causes fixation element 108 to engage tissues of tissue wall 113. For example, fixation element 108 may be configured to define an embedded depth between a distal end 116 of fixation element 108 (“fixation element distal end 116”) and surface tissue 115 as the clinician causes fixation element 108 to engage tissue wall 113. Fixation element 108 may be configured to increase the embedded depth as a clinician manipulates one or more components of medical system 100, such that fixation element 108 progressively embeds within tissue wall 113 during an implantation of IMD 104 in heart 102. For example, fixation element 108 may be configured to progressively embed to increase the embedded depth as the clinician causes a rotation of IMD 104 about device axis LD, causes exertion of an axial force (e.g., substantially parallel to device axis LD) on IMD 104, causes movement of one or more components of medical system 100 relative to IMD 104 (e.g., proximal retraction of a catheter 132), and/or manipulates medical system 100 in another manner. Fixation element 108 may be, for example, a helix, one or more times, or another structure and/or component configured to progressively embed within tissue wall 113. Device axis LD may be an axis defined by a portion of IMD 104, such as device housing 106 and/or another portion of IMD 104. In examples, device axis LD extends through IMD proximal portion 105 and/or IMD distal portion 107.

[0033] Medical system 100 is configured to sense movement of IMD 104 and compare the movement of IMD 104 to a representative cardiac activity of heart 102 prior to, during, and/or following an implantation procedure. IMD 104 is configured such that forces imparted to fixation element 108 from the engaged tissues of heart 102 (e.g., due to a heartbeat) cause a corresponding motion of IMD 104 along one or more geometric axes of the IMD, such as a principal axis substantially parallel to a device axis LD defined by IMD 104 (e.g., device housing 106), one or more axes substantially perpendicular to the principal axis, and/or another geometric axis. IMD 104 includes a motion sensor 118 configured to sense the motion of IMD 104 and/or a motion indicative of the motion of IMD 104 along one or more geometric axis. Motion sensor 118 may be, for example, an accelerometer, gyroscope, magnetometer, inertial measurement unit (IMU), and/or another motion sensor. In some examples, motion sensor 118 is an accelerometer.

[0034] Medical system 100 is configured to compare the IMD motion to the representative cardiac activity of the heart, in order to evaluate the coupling between fixation element 108 and the engaged tissues within tissue wall 113. For example, medical system 100 may include processing circuitry 120 configured to receive one or more signals from motion sensor 118 (e.g., via communication link 122) indicative of the motion of IMD 104. Processing circuitry 120 may compare the motion of IMD 104 with the representative cardiac activity of heart 102 using the one or more signals. Processing circuitry 120 may be configured to provide an output to an output device 124 (e.g., via communication link 126) based on the comparison, such that a clinician may assess the coupling between IMD 104 and the tissues within tissue wall 113 as the implantation process progresses.

[0035] Motion sensor 118 may be configured to sense, for example, an acceleration of at least some portion of motion sensor 118 (e.g., a proof mass) and/or an acceleration of device housing 106 caused by forces imparted to device housing 106 via fixation element 108 as heart 102 beats and imparts forces to fixation element 108. Motion sensor 118 may sense the acceleration (e.g., of a proof mass) using electrical, piezoelectric, piezoresistive, capacitive, quantum tunneling, convective, motors, and/or other components. Processing circuitry 120 is configured to compare the IMD motion sensed to the representative cardiac activity of heart 102 and provide an output indicative of the coupling between fixation element 108 and tissue wall 113 based on the comparison. The representative cardiac activity may be based on, for example, a sensed cardiac activity of heart 102, an expected cardiac activity based on historical cardiac activity of the patient or other patients, or another indicator indicative of a representative cardiac activity.

[0036] In some examples, medical system 100 includes a medical device 125 configured to generate one or more signals indicative of the electrical and/or mechanical activity of heart 102. The electrical and/or mechanical activity may be, for example, an electrical signal indicative of a muscular movement of the heart of the patient. For example, medical device 125 may be configured to sense ECG and/or EGM signals generated by heart 102. Medical device 125 may be configured to provide the signals indicative of the electrical and/or mechanical activity of heart 102 to processing circuitry 120 (e.g., via communication link 127). Processing circuitry 120 may be configured to determine the representative cardiac activity using the signals provided by medical device 125. In some examples, instead of or in addition to medical device 125, processing circuitry 120 is configured to receive the representative cardiac activity from an external device 129 (e.g., via communication link 131). External device 129 may include a database or other data structure configured to provide the representative cardiac activity based on, for example, a historical cardiac activity of heart 102 and/or a historical cardiac activity of one or more subject hearts in other patients.

[0037] Medical system 100 may be configured to sense motion of IMD 102 (e.g., using motion sensor 118) and correlate the motion to the representative cardiac activity (e.g., using processing circuitry 120) such that a clinician may assess the contact between fixation element 108 and tissue wall 113. For example, medical system 100 may compare the motion of IMD 104 to the representative cardiac activity to provide an indication of a pressure (e.g., a force) exerted by fixation element 108 on tissue wall 113 (e.g., a surface tissue 115 of tissue wall 113). The indication of the pressure may allow the clinician to assess whether an existing pressure is too low or too high to, for example, commence causing fixation element 108 to progressively embed within tissue wall 113 (e.g., by causing a rotation of the IMD 104, imparting an axial force on IMD 104, proximally withdrawing delivery catheter 132, and/or taking another action).

[0038] Medical system 100 may substantially monitor the motion of IMD 102 (e.g., using processing circuitry 120) as the clinician causes fixation element 108 to progressively embed within tissue wall 113 to, for example, provide an indication of the resulting mechanical coupling between fixation element 108 and tissue wall 113. In some examples, fixation element 108 is configured (e.g., as a helix) such that rotation of IMD 104 about IMD axis LD causes fixation element 108 to engage and/or disengage tissues within target site 112. For example, fixation element 108 may be configured such that rotation of IMD 104 in a first rotational direction W1 about IMD axis ED causes fixation element 108 to engage (or alternately, disengage from) tissues within target site 112. Fixation element 108 may be configured such that rotation of IMD 104 about IMD axis LD in a second rotational direction W2 substantially opposite first rotational direction W 1 causes fixation element 108 to disengage from (or alternately, engage) tissues within target site 112.

[0039] Delivery member 110 may be configured to couple with IMD 104 (e.g., IMD proximal portion 105), such that a torque on delivery member 110 causes a torque on IMD 104 to cause the rotation of IMD 104 about device axis LD. In examples, delivery member 110 includes a distal portion 128 (“delivery member distal portion 128”) configured to be intracorporeal to a patient when IMD 104 resides within heart 102 and a proximal portion 130 (“delivery member proximal portion 130”) configured to be extracorporeal to the patient when IMD 104 resides within heart 102. Delivery member 110 may be configured such that, when a clinician causes a torque to be exerted on delivery member proximal portion 130, delivery member distal portion 128 transfers some portion of the torque to IMD 104 (e.g., IMD proximal portion 105) to cause a rotation of IMD 104 about device axis LD. Processing circuitry 120 may be configured to detect the rotation of IMD 104 about device axis LD as processing circuitry 120 compares the motion of IMD 104 to the representative cardiac activity, such that processing circuitry 120 may substantially track a number of rotations of IMD 104 which occur as fixation element 108 progressively embeds within tissue wall 113. Processing circuitry 120 may provide an output (e.g., via output device 124), such that the clinician may be apprised of the number of turns applied to IMD 104 as fixation element 108 progressively embeds. In examples, processing circuitry 120 may be configured to assess that fixation element is likely not progressively embedding as IMD 104 rotates about device axis LD based on, for example, the comparison of the motion of IMD 104 to the representative cardiac activity.

[0040] In some examples, fixation element 108 includes one or more tines (not shown) configured to progressively embed within tissue wall 113 when the one or more of the tines contact tissue wall 113 and the clinician causes a proximal retraction of delivery catheter 132 and/or an exertion of an axial force to IMD 104 (e.g., using delivery member 110). Delivery catheter 132 may define a lumen 134 (delivery catheter lumen 134”) an opening 136 (“delivery catheter opening 136”) which opens to delivery catheter lumen 134. Delivery catheter lumen 134 and delivery catheter opening 136 may be configured such that IMD 104 and/or at least some portion of delivery member 110 may pass therethrough. In examples, delivery catheter 132 includes a distal portion 138 (“delivery catheter distal portion 138”) configured to be intracorporeal to a patient when IMD 104 resides within heart 102 and a proximal portion 140 (“delivery catheter proximal portion 140”) configured to be extracorporeal to the patient when IMD 104 resides within heart 102. Delivery catheter 132 may be configured such that movement of delivery catheter proximal portion 140 (e.g., caused by a clinician) causes movement of delivery catheter distal portion 138 and/or delivery catheter opening 136 relative to IMD 104. The one or more tines which may be included in fixation element 108 may be configured to progressively engage tissue wall 113 to increase an embedded depth of fixation element 108 when delivery catheter opening 136 moves (e.g., moves proximally) relative to IMD 104 (e.g., when the one or more tines are caused to extend distally beyond delivery catheter opening 136). Processing circuitry 120 may be configured to compare the motion of IMD 104 to the representative cardiac activity as the one or more tines progressively embed and provide an output to output device 124, such that the clinician may be apprised of the engagement of the one or more tines with tissue wall 113 as fixation element 108 progressively embeds.

[0041] Medical system 100 may be configured to assess an alignment of IMD 104 relative to tissue wall 113 during and/or following an implantation of IMD 104 within heart 102. For example, processing circuitry 120 may be configured to compare IMD motions sensed along the principal axis, the first radial axis, and/or the second radial axis of motion sensor 118 as tissue wall 113 imparts forces to IMD 102. Processing circuitry 120 may assess the orientation of IMD 104 relative to tissue wall 113 using the IMD motions sensed along the principal axis, the first radial axis, and/or the second radial axis. For example, processing circuitry 120 may be configured to indicate a substantial perpendicularity of IMD 104 to tissue wall 113 and/or other relative orientations of IMD 104 and tissue wall 113. In some examples, processing circuitry 120 is configured to detect and/or assess a pivoting of the principal axis relative to tissue wall 113 (e.g., detect and/or asses a “wobbling” of IMD 104 ) as heart 102 beats and imparts forces to IMD 104. Medical system 100 may provide an indication (e.g., using output device 124) indicative of the pivoting of the principal axis, such that the clinician may assess and/or alter the relative orientation of IMD 104 and tissue wall 113. The detection and/or assessment of the pivoting of the principal axis may also provide an indication that IMD 104 has prematurely moved distal to a delivery cup or other structure of delivery catheter 132 configured to provide stability to IMD 104 (e.g., configured to prevent wobbling) during implantation.

[0042] IMD 104 may include therapy delivery circuitry 142 configured to deliver therapy (e.g., pacing) to and/or sense signals from heart 102. In examples, device housing 106 is configured to mechanically support therapy delivery circuitry 142. Therapy delivery circuitry 142 may be configured to deliver therapy to and/or sense signals from heart 102 using one or more electrodes configured to electrically communicate with tissues of heart 102. In examples, therapy delivery circuitry 142 is configured to deliver therapy to and/or sense signals from heart 102 using a first electrode 144 mechanically supported by fixation element 108. In some examples, therapy delivery circuitry 142 is configured to deliver therapy to and/or sense signals from heart 102 using a second electrode 146 mechanically supported by device housing 106 or another portion of IMD 104 (e.g., a leadlet extending from device housing 106). In some examples, device housing 106 defines a return electrode 148 electrically connected to therapy delivery circuitry 142. Therapy delivery circuitry 142 may be configured to deliver therapy to and/or sense signals from heart 102 using one or more of first electrode 144, second electrode 146, return electrode 148, and/or another electrode of IMD 104. In examples, processing circuitry 120 is configured to communicate with therapy delivery circuitry 142 via, for example, communication link 122. Processing circuitry 120 and/or therapy delivery circuitry 142 may be located within and/or mechanically supported by device housing 106, medical device 125, external device 129, output device 124, and/or within another device or group of devices not illustrated in FIG. 1.

[0043] Hence, medical system 100 is configured to sense motion of IMD 104 and correlate the motion to a representative cardiac activity prior to, during, and/or following an implantation, such that the clinician may assess when contact between fixation element 108 and tissue wall 113 has occurred, assess a progression of the coupling between fixation element 108 and tissue wall 113 as the clinician causes fixation element 108 to progressively embed within tissue wall 113, assess a quality of the coupling between fixation element 108 and tissue wall 113, assess a relative orientation of IMD 104 and tissue wall 113, and/or assess other aspects of the IMD implantation.

[0044] FIG. 2 schematically illustrates an IMD 104 including fixation element 108 and motion sensor 118, with fixation element 108 displaced from surface tissue 115 of tissue wall 113. FIG. 3 illustrates IMD 104 contacting tissue wall 113 such that fixation element 108 defines an embedded depth DI between fixation element distal end 116 and surface tissue 115. FIG. 4 illustrates IMD 104 embedded in tissue wall 113 such that fixation element 108 defines an embedded depth D2 between fixation element distal end 116 and surface tissue 115, with embedded depth D2 greater than embedded depth DI. FIGS. 2-4 illustrate IMD 104 positioned within a delivery receptacle 111 defined by and/or mechanically supported by delivery member 110. Delivery receptacle 111 may be configured to substantially limit movement of device housing 106 in linear directions other than those directions substantially parallel to device axis LD.

[0045] Fixation element 108 is configured to progressively embed (e.g., transition from defining the embedded depth DI to defining the embedded depth D2) when fixation element 108 contacts surface tissue 115 and medical system 100 causes movement of fixation element 108 relative to surface tissue 115. For example, fixation element 108 may be configured to progressively embed with tissue wall 113 when fixation element 108 contacts surface tissue 115 and delivery member 110 imparts a torque on IMD 104 causing device housing 106 to rotate around device axis LD relative to surface tissue 115. Fixation element 108 may be configured to progressively embed with tissue wall 113 when fixation element 108 contacts surface tissue 115 and delivery member 110 imparts an axial force on IMD 104 causing device housing 106 to translate proximally (e.g., in the proximal direction P) toward surface tissue 115. In examples, medical system 100 is configured such that a clinician may cause the movement of fixation element 108 relative to surface tissue 115, such that the clinician may control the engagement of fixation element 108 with tissue wall 113. For examples, medical system 100 may be configured such that a clinician may cause relative movement between fixation element 108 and surface tissue 115 such that fixation element 108 transitions from the relative orientation depicted in FIG. 2, where fixation element 108 is distally displaced from surface tissue 115, to the relative orientation depicted in FIG. 3, where fixation element 108 is in contact with tissue wall 113 and defining the embedded depth DI. Medical system 100 may be configured such that a clinician may cause relative movement between fixation element 108 and surface tissue 115 such that fixation element 108 transitions from the relative orientation depicted in FIG. 3 to the relative orientation depicted in FIG. 4, where fixation element 108 is in contact with tissue wall 113 and defining the embedded depth D2. [0046] When fixation element 108 configured to progressively embed within tissue wall 113, this may mean fixation element 108 is configured to increase an embedded depth within tissue wall 113 as a result of relative movement between fixation element 108 and surface tissue 115. In examples, the embedded depth is a distance from fixation element distal end 116 and surface tissue 115. Fixation element 108 may be configured to progressively embed within tissue wall 113 to such that a clinician may control (e.g., increase or decrease) the embedded depth by controlling the relative movement between fixation element 108 and surface tissue 115. For example, fixation element 108 may be configured (e.g., as a helix) such that the clinician may control the embedded depth by controlling a rotation of device housing 106 about device axis LD. Fixation element 108 may be configured such that the clinician may control the embedded depth by controlling an axial force imparted to device housing 106.

[0047] Fixation element 108 may be supported (e.g., mechanically supported) by device housing 106, such that when forces are imparted to fixation element 108, fixation element 108 transmits a force to device housing 106. For example, as fixation element 108 progressively embeds and/or contacts tissue wall 113 of heart 102 (FIG. 1), beating of heart 102 may cause movement of tissue wall 113 and impartation of a force Fl and/or a force F2 on fixation element 108 (e.g., due to accelerations and contractions of tissue wall 113). Fixation element 108 may transmit at least some portion of force Fl and/or force F2 to device housing 106, causing movement of device housing 106. The resulting movement of device housing 106 caused by the engagement between fixation element 108 and tissue wall 113 may be correlated with the motions of tissue wall 113 which occur during the heartbeat. Hence, the engagement of fixation element 108 with tissue wall 113 (e.g., as the clinician causes fixation element 108 to progressively embed) may be assessed by comparing the motion of device housing 106 with a representative cardiac activity indicative of the motion of tissue wall 113 during a heartbeat.

[0048] Medical system 100 is configured to sense the movement of device housing 106 (e.g., using motion sensor 118) and compare the movement to the representative cardiac activity (e.g., using processing circuitry 120) in order to assist the clinician in assessing the engagement between fixation element 108 and tissue wall 113 prior to, during, and/or following an implantation of fixation element 108 within tissue wall 113. For example, motion sensor 118 may be configured to sense motion of device housing 106 along a principal axis P, a first radial axis Rl, and/or a second radial axis R2. The motion of device housing 106 along one or more of principal axis P, first radial axis Rl, and second radial axis R2 may be indicative of the engagement of fixation element 108 and tissue wall 113, and/or be indicative of a substantial lack of engagement between fixation element 108 and tissue wall 113.

[0049] Motion sensor 118 may be configured to provide one or more signals indicative of a motion device housing along (e.g., substantially parallel to) principal axis P, first radial axis Rl, and second radial axis R2. Principal axis P, first radial axis Rl, and second radial axis R2 are configured relative to each other such that vector sum of a first vector parallel to principal axis P, a second vector parallel to first radial axis Rl, and a third vector parallel to second radial axis R2 describe a vector having a direction in a three- dimensional space. In examples, first radial axis Rl is substantially perpendicular to principal axis P. In examples, second radial axis R2 is substantially perpendicular to principal axis P and first radial axis R2. In some examples, principal axis P is substantially parallel to device axis LD. Motion sensor 118 may provide the one or more signals indicative of the movement of device housing 106 to processing circuitry 120. For example, motion sensor 118 may provide one or more of a principal signal indicative of movement of device housing 106 along principal axis P, a first radial signal indicative of movement of device housing 106 along first radial axis Rl, and/or a second radial signal indicative of movement of device housing 106 along second radial axis R2.

[0050] Processing circuitry 120 may be configured to compare one or more of the principal signal, the first radial signal, and/or the second radial signal to the representative cardiac activity to assess the engagement of fixation element 108 to tissue wall 113. Processing circuitry 120 may be configured to process the principal signal, the first radial signal, and/or the second radial signal in order to substantially isolate portions of the respective signals which might be indicative of the cardiac activity of heart 102 (e.g., the acceleration and contraction of tissue wall 113 during heartbeats of heart 102). For example, processing circuitry 120 may be configured to evaluate one or more frequencies of the principal signal, the first radial signal, and/or the second radial signal to determine one or more frequency-based sensor parameters, and compare the principal signal, the first radial signal, and/or the second radial signal to the representative cardiac activity using the frequency-based sensor parameters. The frequency-based sensor parameter may be, for example, a portion of the principal signal, the first radial signal, and/or the second radial signal exhibiting frequencies that might be expected as a result of motion caused by a tissue wall during the representative cardiac activity, such as portions exhibiting frequencies in a frequency window between and including a lower frequency threshold and a higher frequency threshold. The lower frequency threshold may be, for example, greater than or equal to about 1 Hertz (Hz), 5 Hz, 10 Hz, or another frequency. The higher frequency threshold may be, for example, less than or equal to about 20 Hz, 40 Hz, 100 Hz, 250 Hz, or another frequency.

[0051] In examples, instead of or in addition to evaluating one or more frequencies, processing circuitry 120 may be configured evaluate one or more to amplitudes to assess the engagement of fixation element 108 to tissue wall 113. For example, processing circuitry 120 may be configured to evaluate one or more amplitudes of the principal signal, the first radial signal, and/or the second radial signal to determine one or more amplitude-based sensor parameters, and compare the principal signal, the first radial signal, and/or the second radial signal to the representative cardiac activity using the amplitude-based sensor parameters. The amplitude-based sensor parameter may be, for example, a portion of the principal signal, the first radial signal, and/or the second radial signal exhibiting an amplitude, a change in amplitude, a peak-to-peak amplitude, a peak amplitude and/or other fluctuation of principal signal, the first radial signal, and/or the second radial signal that might be expected as a result of motion caused by a tissue wall during the representative cardiac activity.

[0052] Processing circuitry 120 may be configured to compare the principal signal, the first radial signal, and/or the second radial signal to the representative cardiac activity, and/or compare one of the principal signal, the first radial signal, and/or the second radial signal to another of the principal signal, the first radial signal, and/or the second radial signal, using any signal matching method intended to enable comparison of a first signal with a second signal and/or a template of an expected signal (e.g., a template representative of at least a portion of the representative cardiac activity). In examples, processing circuitry 120 is configured to compare the principal signal, the first radial signal, and/or the second radial signal to the representative cardiac activity using a matching methodology encompassing techniques such as a Fourier transform (FT) comparison (e.g., an FFT comparison), cross-correlation, correlation-coefficient, variation standard deviation, template matching, and/or one or more other waveform similarity analyses.

[0053] Medical system 100 may be configured to provide an indication (e.g., via output device 124) of whether fixation element 108 appears to be displaced from surface tissue 115 or appears to be in contact with surface tissue 115. Such an indication may allow the clinician to assess whether, for example, IMD 104 is substantially free-floating in the heart and/or experiencing intermittent contact with the heart tissue prior to the clinician taking action to progressively embed fixation element 108 within tissue wall 113. Medical system 100 may be further configured to assess and/or provide an indication that fixation element 108 is only partially fixated. For example, processing circuitry 120 may be configured to compare one or more of the principal signal, the first radial signal, and/or the second radial signal to the representative cardiac activity (e.g., using a frequency-based sensor parameter and/or an amplitude-based sensor parameter). Processing circuitry 120 may assess that fixation element 108 is only partially fixated based on, for example, a correlation, similarity, and/or other comparison of the one or more of the principal signal, the first radial signal, and/or the second radial signal to the representative cardiac activity. For example, processing circuitry 120 may be configured to determine a matching coefficient and/or other similarity parameter using the one or more of the principal signal, the first radial signal, and/or the second radial signal and the representative cardiac activity. Processing circuitry 120 may be configured to compare the matching coefficient and/or other similarity parameter to a threshold to assess whether fixation element 108 is only partially fixated to tissue wall 113.

[0054] For example, processing circuitry 120 may compare one or more frequencies of the principal signal, the first radial signal, and/or the second radial signal to the representative cardiac activity and assess that fixation element 108 is likely not in contact with tissue wall 113 (e.g., as depicted in FIG. 2). Processing circuitry 120 may assess the lack of contact based on a reduced amount and/or substantial lack of frequencies in the principal signal, the first radial signal, and/or the second radial signal that might be expected as a result of motion caused by the representative cardiac activity. Processing circuitry 120 may communicate with output device 124 to cause output device 124 to provide an indication representative of the assessed lack of contact, such that the clinician may be apprised of the relative orientation of fixation element 108 and tissue wall 113. The clinician may subsequently cause IMD 104 and fixation element 108 to translate distally toward tissue wall 113, such that contact between fixation element 108 and surface tissue 115 is indicated prior to attempting to cause fixation element 108 to progressively embed within tissue wall 113. Processing circuitry 120 may subsequently compare one or more frequencies of the principal signal, the first radial signal, and/or the second radial signal to the representative cardiac activity and assess that fixation element 108 likely is in contact with tissue wall 113 (e.g., as depicted in FIG. 3 and FIG. 4) based on, for example, a presence of frequencies in the principal signal, the first radial signal, and/or the second radial signal that might be expected as a result of motion caused by the representative cardiac activity.

[0055] As an example, FIG. 5 illustrates an example representative cardiac activity 151 indicative of the electrical and/or mechanical activity of a heart, such as heart 102. In examples, representative cardiac activity 151 is based on one or more signals produced by heart 102 and sensed by medical device 125, and/or one or more signals produced by one or more other patients and received from external device 129 (FIG. 1). The one or more signals may be indicative of, for examples, ECG and/or EGM signals expected from and/or generated by heart 102, an acceleration produced and/or expected during cardiac cycles of a heart of the patient (e.g., heart 102) and/or other patients, a movement produced and/or expected during cardiac cycles of the heart of the patient and/or other patients, and/or other signals produced by the heart of the patient and/or other patients. Processing circuitry 120 may be configured to substantially correlate the representative cardiac activity 151 to an actual and/or expected motion of tissue wall 113 (e.g., during a heartbeat of heart 102). Hence, processing circuitry 120 may compare the representative cardiac activity 151 with the motion of IMD 104 sensed by motion sensor 118 in order to assess an engagement of fixation element 108 and tissue wall 113 (e.g., to assess a likelihood that tissue wall 113 is transferring force Fl and/or force F2 (FIG. 2) to IMD 104).

[0056] FIGS. 6-8 illustrate example motion signals 152 produced by motion sensor 118. FIG. 6 is indicative of motion signals 152 when fixation element 108 displaced from surface tissue 115 and/or tissue wall 113 (e.g., as depicted in FIG. 3). FIG. 7 and FIG. 8 are indicative of motion signals 152 when fixation element 108 is engaged with tissue wall 113. Motion signals 152 may include one or more of a principal signal 154, a first radial signal 156, and/or a second radial signal 158. Principal signal 154 may be indicative of a movement of device housing 106 along (e.g., substantially parallel to) principal axis P. First radial signal 156 may be indicative of a movement of device housing 106 along first radial axis Rl. Second radial signal 158 may be indicative of a movement of device housing 106 along second radial axis Rl. In examples, principal signal 154, first radial signal 156, and/or second radial signal 158 are indicative of a movement of point Pl defined by device housing 106.

[0057] Processing circuitry 120 may compare one or more frequencies of principal signal 154, first radial signal 156, and/or second radial signal 158 to representative cardiac activity 151 to assess whether it is likely that fixation element 108 is in contact with tissue wall 113. For example, when motion sensor 118 provides the motion signals 152 of FIG.

6, processing circuitry 120 may define a window 160 (FIG. 6) over which principal signal 154 exhibits one or more frequencies. Processing circuitry 120 may compare the portion of principal signal 154 within window 160 to one or more windows defined for representative cardiac activity 151, such as representative window 162. Processing circuitry 120 may determine a degree of waveform similarity between the portion of principal signal 154 within window 160 and the portion of representative cardiac activity 151 within representative window 162. Processing circuitry 120 may assess contact or lack of contact between fixation element 108 and tissue wall 113 based on the degree of waveform similarity. For example, processing circuitry 120 may assess, based on a lack of similarity between the waveforms in window 160 and representative window 162, that fixation element 108 is not in contact with tissue wall 113 (as depicted in FIG. 3). Processing circuitry 120 may communicate with output device 124 to cause output device 124 to provide an indication representative of the assessed lack of contact, such that the clinician may be apprised of the relative orientation of fixation element 108 and tissue wall 113.

[0058] In contrast, when motion sensor 118 provides the motion signals 152 of FIG. 7, processing circuitry 120 may define a window 164 (FIG. 7) over which principal signal 154 exhibits one or more frequencies. Processing circuitry 120 may compare the portion of principal signal 154 within window 164 to the portion of representative cardiac activity 151 within representative window 162 and determine a degree of waveform similarity between the window 164 and representative window 162. Processing circuitry 120 may be configured to assess, based on the degree of similarity between waveforms in window 164 and representative window 162, that fixation element 108 is likely in contact with tissue wall 113 (as depicted in FIG. 4 and FIG. 5). Processing circuitry 120 may communicate with output device 124 to cause output device 124 to provide an indication representative of the contact, such that the clinician may be apprised of the relative orientation of fixation element 108 and tissue wall 113.

[0059] In examples, such as when representative cardiac activity 151 is indicative of a signal sensed by medical device 125, window 160, 164 and representative window 162 may be substantially contemporaneous signals (e.g., may be substantially synchronized in time). In some examples, such as when representative cardiac activity 151 is provided by external device 129, window 160, 164 may be substantially unsynchronized with representative window 162. In some examples, processing circuitry 120 is configured to detect representative window 162 using a template matching algorithm configured to search an interval (e.g., a continuous interval) of representative cardiac activity 151 for one or more representative windows which substantially match the signal represented by window 160, 164. Further, although window 162 and window 164 are illustrated as windows encompassing a portion of principal signal 154 in FIGS. 6-7, processing circuitry 120 may be configured to define one or more windows encompassing portions of first radial signal R1 and/or second radial signal R2 and assess a likelihood of contact between fixation element 108 and tissue wall 113 by comparing the one or more windows with representative window 164 is substantially the same way as that described for window 162.

[0060] In some examples, medical system 100 (e.g., processing circuitry 120) is configured to assess that fixation element 108 is likely in contact with and/or progressively embedding in tissue wall 113 by comparing a pattern of fluctuations present in one of principal signal 154, first radial signal 156, or second radial signal 158 to a pattern of fluctuations present in another of principal signal 154, first radial signal 156, or second radial signal 158. Processing circuitry 120 may be configured to assess that fixation element 108 is likely embedded within tissue wall 113 based on the similarities of the patterns of fluctuations detected in principal signal 154, first radial signal 156, and/or second radial signal 158. Processing circuitry 120 may communicate with output device 124 to cause output device 124 to provide an indication to the clinician that fixation element 108 is likely embedded in tissue wall 113, based on patterns of fluctuations detected in principal signal 154, first radial signal 156, and/or second radial signal 158. [0061] For example, with reference to FIG. 7, processing circuitry 120 may detect a pattern of fluctuations FP in principal signal 154, where the pattern FP includes fluctuations detected at time Tl, T2, T3, T4, T5, T6, T7, and T8. A fluctuation may be defined by, for example, a rate of change of principal signal 154, such as a rate of change causing a shifting up of principal signal 154 followed by a shifting down of principal signal 154. Similarly, processing circuitry 120 may detect a pattern of fluctuations Fl in first radial signal 156, with the first pattern Fl including fluctuations in first radial signal 156 detected at time Tl, T2, T3, T4, T5, T6, T7, and T8. Likewise, processing circuitry 120 may detect a pattern of fluctuations F2 in second radial signal 158, with the second pattern F2 including fluctuations in second radial signal 158 detected at time Tl, T2, T3, T4, T5, T6, T7, and T8. Processing circuitry 120 may compare the pattern FP, the pattern Fl, and/or the pattern F2 to assess that fixation element 108 is likely embedded within tissue wall 113 based on, for example, similar time distributions of the perturbations within the pattern FP, the pattern Fl, and/or the pattern F2. Processing circuitry 120 may be configured to assess that fixation element 108 is likely embedded within tissue wall 113 based on the similarities of the patterns of fluctuations detected in principal signal 154, first radial signal 156, and/or second radial signal 158. Processing circuitry 120 may communicate with output device 124 to cause output device 124 to provide an indication representative of the assessment that fixation element 108 is embedded in tissue wall 113. [0062] In examples, medical system 100 is configured to assess that IMD 104 is experiencing movement relative to tissue wall 113 and provide an indication of the relative movement (e.g., via output device 124). For example, when a clinician uses delivery member 110 to cause relative motion between IMD 104 and tissue wall 113 (e.g., to cause IMD 104 to rotate relative to tissue wall 113), medical system 100 (e.g., processing circuitry 120) may assess principal signal 154, first radial signal 156, and/or second radial signal 158 to recognize the motion. Processing circuitry 120 may substantially correlate the sensed relative motion with an assessment of whether the fixation element 108 is or is not likely to be embedded in tissue wall 113, such that a clinician may be apprised of whether fixation element 108 is embedded in tissue wall 113 as the clinician causes the motion of IMD 104. [0063] Processing circuitry 120 may be configured to use one or more of principal signal 154, first radial signal 156, and/or second radial signal 158 to assess the movement of fixation element 108 and/or device housing 106 relative to tissue wall 113 and/or surface tissue 115. For example, processing circuitry 120 may be configured to assess principal signal 154, first radial signal 156, and/or second radial signal 158 to indicate a rotation of fixation element 108 and/or device housing 106 about device axis LD (e.g., caused by a clinician during an implantation), or another relative movement of fixation element 108 and/or device housing 106 relative to surface tissue 115 and/or tissue wall 113. Processing circuitry 120 may be configured to process principal signal 154, first radial signal 156, and/or second radial signal 158 in order to substantially isolate portions of the respective signals which might be indicative of the relative movement.

[0064] For example, processing circuitry 120 may be configured to evaluate one or more lower frequencies of principal signal 154, first radial signal 156, and/or second radial signal 158 to determine one or more lower frequency-based sensor parameters, and assess the lower frequency-based sensor parameters to provide an indication of the relative movement of fixation element 108 and/or device housing 106 relative to surface tissue 115 and/or tissue wall 113. The lower frequency-based sensor parameter may be, for example, a portion of principal signal 154, first radial signal 156, and/or second radial signal 158 exhibiting frequencies outside of those might be expected as a result of motion caused by a tissue wall during the representative cardiac activity, such as portions exhibiting frequencies below a lower frequency limit. The lower frequency limit may be, for example, less than or equal to about 1 Hertz (Hz), 5 Hz, 10 Hz, 50Hz, or another frequency.

[0065] Processing circuitry 120 may assess principal signal 154, first radial signal 156, and/or second radial signal 158 to provide an indication that fixation element 108 and/or device housing 106 has or is rotating about device axis LD relative to surface tissue 115 and/or tissue wall 113. Processing circuitry 120 may assess the rotation based on a perturbation (e.g., a cycling) of principal signal 154, first radial signal 156, and/or second radial signal 158. For example, processing circuitry 120 may be configured to detect a perturbation of principal signal 154, first radial signal 156, and/or second radial signal 158 based on an expected influence of a gravity vector (e.g., a non-zero gravity vector) acting on motion sensor 118. Processing circuitry 120 may be configured to assess that fixation element 108 and/or device housing 106 has or is rotating about device axis LD based on the expected influence of the gravity vector. In examples, processing circuitry 120 is configured to detect a perturbation in an amplitude of principal signal 154, first radial signal 156, and/or second radial signal 158 and compare the amplitude perturbation to that which might be expected when motion sensor 118 experiences a rotation while under the influence of the gravity vector. Processing circuitry 120 may communicate with output device 124 to cause output device 124 to provide an indication representative of the assessed rotation of fixation element 108 and/or device housing 106, such that the clinician may be apprised of the movement of fixation element 108 and/or device housing 106 relative to surface tissue 115 and/or tissue wall 113.

[0066] As an example, FIG. 6 illustrates motion signals 152 exhibiting perturbations due to rotation of IMD 104 about device axis LD (e.g., due to a torque transferred to IMD 104 via delivery member 110). As discussed, FIG. 6 illustrates motion signals 152 when IMD 104 is displaced from tissue wall 113 (e.g., as depicted in FIG. 2). Processing circuitry 120 is configured to detect a perturbation of principal signal 154, first radial signal 156, and/or second radial signal 158, such as perturbation Pl of first radial signal 156 and/or perturbation P2 of second radial signal 158. Processing circuitry 120 may be configured to recognize the perturbation based on a cycling of principal signal 154, first radial signal 156, and/or second radial signal 158 across a substantial mean, such as a cycling of first radial signal 156 across mean MR1 and/or a cycling of second radial signal 158 across mean MR2. In examples, processing circuitry 120 is configured to determine mean MR1 and/or mean MR2 based on first radial signal 156 and/or second radial signal 158. Processing circuitry 120 may be configured to assess that motion sensor 118 (and device housing 106) are likely to be rotating about device axis LD based on detection of perturbation Pl and/or perturbation P2. Although FIG. 6 illustrates a substantial absence of perturbations in principal signal 154 due to the constraint of IMD 104 by delivery receptacle 111, processing circuitry may be configured to assess rotation of motion sensor 118 (and device housing 106) around an axis defined by IMD 104 by detecting perturbations of principal signal 154 in substantially the same manner as that described for first radial signal 156 and/or second radial signal 158. Processing circuitry 120 may be configured to assess that fixation element 108 is likely not progressively embedding within tissue wall 113 based on a substantial lack of IMD movement corresponding to a representative cardiac activity (e.g., a lack of corresponding IMD movement and/or IMD movement below a threshold).

[0067] In similar manner, processing circuitry 120 may be configured to assess rotation of motion sensor 118 (and device housing 106) around an axis defined by IMD 104 when fixation element 108 is embedded within tissue wall 113. For example, FIG. 7 represents motion signals 152 exhibiting perturbations due to rotation of IMD 104 about device axis LD when IMD 104 is engaged with tissue wall 113 (e.g., as depicted in FIG. 3 and FIG. 4). Processing circuitry 120 may be configured to detect a perturbation of principal signal 154, first radial signal 156, and/or second radial signal 158 in manner substantially similar to that described for the detection of perturbation Pl and/or perturbation P2. For example, processing circuitry 120 may be configured to recognize a perturbation P3 based on a cycling of first radial signal 156 across a mean MR3. Processing circuitry 120 may be configured to recognize a perturbation P4 based on a cycling of second radial signal 158 across a mean MR4. Processing circuitry 120 may be configured to assess that motion sensor 118 (and device housing 106) are likely to be rotating about device axis LD based on detection of perturbation P3 and/or perturbation P4.

[0068] Processing circuitry 120 may be configured to assess that fixation element 108 is progressively embedding within tissue wall 113 by assessing a substantially contemporaneous occurrence of contact between fixation element 108 with tissue wall 113 and a movement (e.g., a rotation) of fixation element 108 and/or device housing 106 relative to tissue wall 113 and/or surface tissue 115. For example, as discussed, processing circuitry 120 may utilize principal signal 154, first radial signal 156, and/or second radial signal 158 to assess that fixation element 108 is likely embedded in tissue wall 113. Processing circuitry 120 may utilize principal signal 154, first radial signal 156, and/or second radial signal 158 to assess that fixation element 108 and/or device housing 106 are likely experiencing movement relative to tissue wall 113 and/or surface tissue 115. Processing circuitry 120 may be configured to assess that the apparent embedding of fixation element 108 and the apparent movement of fixation element 108 and/or device housing 106 are substantially contemporaneous events (e.g., occurring substantially simultaneously). Processing circuitry 120 may be configured to indicate that fixation element 108 is progressively embedding within tissue wall 113 based on the substantially contemporaneous occurrences detected. Processing circuitry 120 may provide an indication to output device 124 indicating the detection of the substantially contemporaneous occurrences, such that the clinician be apprised that a motion of IMD 104 (e.g., caused by the clinician) is likely causing fixation element 108 to progressively embed with tissue wall 113.

[0069] In some examples, processing circuitry 120 may be configured to evaluate (e.g., count) a number of rotations of IMD 104 about device axis LD when fixation element 108 is assessed to be embedded within tissue wall 113. Processing circuitry 120 may evaluate the rotations based on perturbations exhibited by one or more of principal signal 154, first radial signal 156, and/or second radial signal 158. In example, processing circuitry 120 is configured to evaluate the number of rotations based on a number of completed cycles of one or more perturbations exhibited by principal signal 154, first radial signal 156, and/or second radial signal 158. For example, processing circuitry 120 may be configured to assess that IMD 104 has completed a complete rotation around device axis LD when a motion signal (e.g., one of principal signal 154, first radial signal 156, and/or second radial signal 158) indicating motion over a motion axis (e.g., one of principal axis P, first radial axis Rl, and/or second radial axis Rl) exhibits a waveform indicating that the axis over has swept over an angle of 360 degrees relative to the direction of a reference vector (e.g., a gravity vector). In examples, the processing circuitry 120 is configured to assess that IMD 104 has completed a complete rotation around device axis LD based on recognizing the waveform has substantially completed a waveform cycle over a waveform periodicity.

[0070] For example, FIG. 8 illustrates motion signals 152 which might be expected as IMD 104 rotates substantially about device axis LD. Processing circuitry 120 may be configured to recognize a first rotation of IMD 104 about device axis LD based on recognizing that first radial signal 156 has completed a first waveform cycle WC1 over a waveform periodicity WP1. Processing circuitry 120 may be configured to recognize subsequent rotations of IMD 104 about device axis LD using first radial signal 156. For example, processing circuitry 120 may be configured to recognize a second rotation of IMD 104 about device axis LD based on recognizing completion of a second waveform cycle WC2 over a waveform periodicity WP2, recognize a third rotation of IMD 104 about device axis LD based on recognizing completion of a third waveform cycle WC3 over a waveform periodicity WP3, and so on. In examples, processing circuitry 120 may be configured to recognize a waveform cycle based on a behavior (e.g., an amplitude) of first radial signal 156 relative to the mean MR1. For example, processing circuitry 120 may be configured to recognize a waveform cycle of first radial signal 156 based on a substantial cycling of an amplitude of first radial signal 156 around the mean MR1 and substantially between a first threshold UR1 and a second threshold LR1. Processing circuitry 120 may be configured to provide an indication to output device 124 indicating a number of completed turns of IMD 104 about device axis LD. In examples, processing circuitry 120 is configured to assess a number of completed turns of IMD 104 about device axis LD when fixation element 108 is engaged with tissue wall 113. Processing circuitry 120 may be configured to communicate with output device 124 to cause output device 124 to provide an indication reflective of the number of completed as fixation element 108 is engaged with tissue wall 113, such that the clinician may be apprised of a number of engaged turns experienced by IMD 104.

[0071] Medical system 100 may be configured to assess a pressure (e.g., a force) exerted by fixation element 108 on surface tissue 115 to, for example, allow a clinician to assess whether an existing pressure is too low or too high before causing IMD 104 to implant within tissue wall 113. Medical system 100 (e.g., processing circuitry 120) may be configured to compare the motion of IMD 104 to representative cardiac activity 151 to assess the pressure. For example, and regarding FIG. 7, processing circuitry 120 may compare the portion of principal signal 154 within window 164 to the portion of representative cardiac activity 151 within representative window 162 to determine a pressure between fixation element 108 and surface tissue 115. Processing circuitry 120 may determine the degree of waveform similarity between window 164 and representative window 162, and assess the pressure based on the waveform similarity. Processing circuitry 120 may communicate with output device 124 to cause output device 124 to provide an indication representative of the pressure, such that, for example, the clinician may be apprised of the pressure between fixation element 108 and surface tissue 115 prior to causing IMD 104 to implant within tissue wall 113.

[0072] Medical system 100 may be configured to assess an alignment of IMD 104 relative to tissue wall 113 during and/or following an implantation. In examples, processing circuitry 120 is configured to compare principal signal 154, first radial signal 156, and/or second radial signal 158 to assess the alignment of IMD 104. For example, processing circuitry 120 may be configured to assess the alignment of IMD 104 by assessing a pivoting of one or more principal axis P, first radial axis Rl, and/or second radial axis R2 relative to surface tissue 115 (e.g., assess “wobbling” of IMD 104) as heart 102 beats and imparts forces to IMD 104. Processing circuitry 120 may cause output device 124 to provide an indication representative of the alignment, such that a clinician may assess and/or alter alignment between IMD 104 and surface tissue 115. The detection and/or assessment of the pivoting of principal axis P, first radial axis Rl, and/or second radial axis R2 may also provide an indication that IMD 104 has prematurely moved distal to delivery receptacle 111 or other structure configured to provide stability to IMD 104 during implantation.

[0073] For example, FIG. 9 schematically illustrates IMD 104 with fixation element 108 implanted within tissue wall 113. In the example of FIG. 9, motion sensor 118 is supported such that principal axis P is substantially parallel to device axis LD, although this not required. Processing circuitry may be configured to compare principal signal 154, first radial signal 156, and/or second radial signal 158 to assess a pivoting of one or more principal axis P, first radial axis Rl, and/or second radial axis R2 relative to surface tissue 115. For example, processing circuitry 120 may use first radial signal 156 to detect motion of IMD 104 substantially along first axis Rl. Processing circuitry 120 may determine that the beating of heart 102 and the impartation of forces Fl and F2 to IMD 104 is causing a relatively large degree of motion of IMD 104 in the direction of first radial axis Rl (e.g., a cycling, periodic motion). For example, motion along first radial axis Rl may indicate that device axis LD is pivoting between a position indicated by LD’ and a position indicated by LD” as tissue wall 113 imparts the forces Fl and F2 as heart 102 beats. Likewise, processing circuitry 120 may use second radial signal 158 to assess that the impartation of forces Fl and F2 to IMD 104 is causing a relatively large degree of motion of IMD 104 in the direction of second radial axis R2. Processing circuitry 120 may cause output device 124 to provide an indication representative of the pivoting of principal axis P as heart 102 imparts the forces Fl and F2 to IMD 104, such that, for example, a clinician may assess and/or alter the alignment between IMD 104 and surface tissue 115.

[0074] FIG. 10 is a functional block diagram illustrating an example configuration of medical system 100 of FIGS. 1-4 and 9 in accordance with one or more examples and/or techniques described herein. Medical system 100 includes electrodes 177 including first electrode 144, second electrode 146, return electrode 148, and/or other electrodes 178 within medical system 100. Medical system 100 may include antenna 180, therapy delivery circuitry 142, operating circuitry 166, sensing circuitry 184, processing circuitry 120, communication circuitry 186, memory 188, switching circuitry 190, sensors 192 including motion sensor 118, and/or power supply 133. In some examples, memory 188 includes computer-readable instructions that, when executed by operating circuitry 166, cause operating circuitry 166 to perform various functions attributed to operating circuitry 166 herein. Memory 188 may include any volatile, non-volatile, magnetic, optical, or electrical media, such as a random access memory (RAM), ferroelectric RAM (FRAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other digital media.

[0075] Operating circuitry 166 (e.g., processing circuitry 120, therapy delivery circuitry 142, sensing circuitry 184, and/or switching circuitry 190) may include fixed function circuitry and/or programmable operating circuitry. Operating circuitry 166 may include any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or equivalent discrete or analog logic circuitry. In some examples, operating circuitry 166 may include multiple components, such as any combination of one or more microprocessors, one or more controllers, one or more DSPs, one or more ASICs, or one or more FPGAs, as well as other discrete or integrated logic circuitry. The functions attributed to operating circuitry 166 herein may be embodied as software, firmware, hardware or any combination thereof. Processing circuitry 120, therapy delivery circuitry 142, sensing circuitry 184, and/or switching circuitry 190 may have attributes of and/or perform any functions of operating circuitry 166.

[0076] Sensing circuitry 184 and communication circuitry 186 may be selectively coupled to electrodes 177 via switching circuitry 190, as controlled by operating circuitry 166. Sensing circuitry 184 may monitor signals from electrodes 177 in order to monitor electrical activity of heart 102 (e.g., to produce an ECG/EGM), deliver pacing signals to heart 102, and/or perform other functions related to one or more physiological signals of a patient. Sensing circuitry 184 also may monitor signals from sensors 192, which includes motion sensor 118 and may include impedance sensors, accelerometers, a gyroscope, a PPG sensor, a tissue oxygen sensor, a blood pressure monitor, respiration rate sensors, respiration effort sensors, respiration pattern sensor, a temperature sensor, or other such sensors configured to monitor one or more physiological signals of a patient. In some examples, sensing circuitry 184 may include one or more filters and amplifiers for filtering and amplifying signals received from one or more of electrodes 177 and/or sensors 192. [0077] Communication circuitry 186 may include any suitable hardware, firmware, software or any combination thereof for communicating with another device, such as a patient input/put device and/or a clinician input/output device. Communication circuitry 186 may be configured to communicate using communication links 122, 126, 127, 131 and/or others. Under the control of operating circuitry 166, communication circuitry 186 may receive downlink telemetry from, as well as send uplink telemetry to, another device (e.g., with the aid of antenna 180). In addition, operating circuitry 166 may communicate with a networked computing device and a computer network, such as the Medtronic CareLink® Network developed by Medtronic, pic, of Dublin, Ireland.

[0078] A clinician or other user may retrieve data from operating circuitry 166 using a clinician input/output device, a patient input/output device, and/or another local or networked computing device configured to communicate with operating circuitry 166 via communication circuitry 186. The clinician may also program parameters of operating circuitry 166 using a clinician input/output device, a patient input/output device, and/or another local or networked computing device.

[0079] Power supply 133 is configured to deliver operating power to the components of medical system 100. Power supply 133 may include one or more batteries and a power generation circuit to produce the operating power. In some examples, the one or more batteries may include a battery that is rechargeable to allow extended operation. In some examples, recharging is accomplished through proximal inductive interaction between an external charger and an inductive charging coil within medical system 100 (e.g., within device housing 106. Power supply 133 may include any one or more of a plurality of different battery types, such as nickel cadmium batteries and lithium ion batteries. A non- rechargeable battery may be selected to last for several years, while a rechargeable battery may be inductively charged from an external device, e.g., on a daily or weekly basis. In some examples, power supply 133 may include both a rechargeable battery and a non- rechargeable battery. [0080] In examples, processing circuitry 120 and/or therapy delivery circuitry 142 is located within device housing 106 of IMD 104. In other examples, processing circuitry 120 and/or therapy delivery circuitry 142 is located within another device or group of devices external to IMD 104 (e.g., within a device or group of devices not illustrated in FIG. 1). As such, techniques and capabilities attributed herein to processing circuitry 120 and/or therapy delivery circuitry 142 may be attributed to any combination of IMD 104 and other devices that are not illustrated in FIG. 1. Hence, medical system 100 (FIG. 1) may represent a system wherein portions are configured to be implanted within a patient and/or configured to be extracorporeal to a patient, and may include any fixed or mobile computer system (e.g., a controller, a microcontroller, a personal computer, minicomputer, tablet computer, etc.), and may be generally described as including substantially all or some portion of processing circuitry 120 and/or therapy delivery circuitry 142.

[0081] A technique for implanting an IMD 104 within a heart 102 is illustrated in FIG. 11. Although the technique is described mainly with reference to IMD 104 of medical system 100, the technique may be applied to other medical devices in other examples.

[0082] The technique includes engaging, by a fixation element 108, an IMD 104 to a tissue wall 113 of a heart 102 (1002). IMD 104 may move relative to tissue wall 113 to cause fixation element 108 to engage tissue wall. In examples, a device housing 106 and fixation element 108 rotate around a device axis LD of IMD 104 relative to tissue wall 113 to cause fixation element 108 to engage tissue wall 113. In examples, delivery member 110 imparts a torque to IMD 104 to cause device housing 106 and fixation element 108 rotate around a device axis LD of IMD 104 relative to tissue wall 113.

[0083] In examples, fixation element 108 progressively engages tissue wall 113 when IMD 104 moves relative to tissue wall 113. Fixation element 108 may increase an embedded depth defined between a fixation element distal end 116 and surface tissue 115 when fixation element 108 progressively embeds. In examples, fixation element 108 receives a force (e.g., force Fl and/or force F2) imparted from tissue wall 113 during the beating of heat 1102. Fixation element 108 may transmit the force from tissue wall 113 to device housing 106.

[0084] The technique includes comparing, using processing circuitry 120, a sensor signal such as a principal signal 154, a first radial signal 156, and/or a second radial signal 158 to a representative cardiac activity 151 (1004). A motion sensor 118 may sense a motion of device housing 106 over a principal axis P, a first radial axis Rl, and/or a second radial axis R2 and provide principal signal 154, a first radial signal 156, and/or a second radial signal 158 to processing circuitry 120. In examples, processing circuitry 120 evaluates one or more frequencies of principal signal 154, first radial signal 156, and/or second radial signal 158 to determine one or more frequency-based sensor parameters. Processing circuitry 120 may compare principal signal 154, first radial signal 156, and/or second radial signal 158 to representative cardiac activity 151 using the frequency-based sensor parameters. In examples, the frequency-based sensor parameter is a portion of principal signal 154, first radial signal 156, and/or second radial signal 158 exhibiting frequencies expected as a result of motion caused by tissue wall 113 during representative cardiac activity 151, such as portions exhibiting frequencies in a frequency window between and including a lower frequency threshold and a higher frequency threshold. The lower frequency threshold may be, for example, greater than or equal to about 1 Hertz (Hz), 5 Hz, 10 Hz, or another frequency. The higher frequency threshold may be, for example, less than or equal to about 20 Hz, 40 Hz, 100 Hz, 250 Hz, or another frequency. In examples, instead of or in addition to evaluating one or more frequency-based parameters, processing circuitry 120 evaluates one or more amplitudes of principal signal 154, first radial signal 156, and/or second radial signal 158 to determine one or more amplitude-based sensor parameters. Processing circuitry 120 may compare principal signal 154, first radial signal 156, and/or second radial signal 158 to representative cardiac activity 151 using the amplitude-based sensor parameters. In examples, the amplitudebased sensor parameter may be, for example, a portion of the principal signal, the first radial signal, and/or the second radial signal exhibiting an amplitude, a change in amplitude, a peak-to-peak amplitude, a peak amplitude and/or other fluctuation of principal signal, the first radial signal, and/or the second radial signal that might be expected as a result of motion caused by a tissue wall during the representative cardiac activity.

[0085] Processing circuitry 120 compare principal signal 154, first radial signal 156, and/or second radial signal 158 to representative cardiac activity 151 using a signal matching method intended to enable comparison of a first signal with a second signal and/or a template of an expected signal (e.g., a template representative of at least a portion of representative cardiac activity 151). Processing circuitry 120 may compare principal signal 154, first radial signal 156, and/or second radial signal 158 to representative cardiac activity 151 using a matching methodology encompassing techniques such as a Fourier transform (FT) comparison (e.g., an FFT comparison), cross-correlation, correlationcoefficient, variation standard deviation, template matching, and/or one or more other waveform similarity analyses.

[0086] In examples, processing circuitry 120 determines that fixation element 108 is likely not in contact with surface tissue 115 and/or tissue wall 113 based on the comparison of principal signal 154, first radial signal 156, and/or second radial signal 158 to representative cardiac activity 151. In some examples, processing circuitry 120 determines that fixation element 108 likely is in contact with surface tissue 115 and/or tissue wall 113 based on the comparison of principal signal 154, first radial signal 156, and/or second radial signal 158 to representative cardiac activity 151. In some examples, processing circuitry 120 assesses a motion of IMD 104 relative to tissue wall 113 and/or surface tissue 115 using principal signal 154, first radial signal 156, and/or second radial signal 158. For example, processing circuitry 120 may assess that IMD 104 has or is rotating around device axis LD using principal signal 154, first radial signal 156, and/or second radial signal 158. Processing circuitry 120 may communicate with output device 124 to cause output device 124 to provide an output indicative of a likely contact or noncontact of fixation element 108 with surface tissue 115 and/or tissue wall 113 and/or an output indicative of the motion of IMD 104 relative to surface tissue 115 and/or tissue wall 113.

[0087] The technique includes assessing, using processing circuitry 120, an engagement of fixation element 108 with tissue wall 113 (1006). Processing circuitry 120 may assess the engagement of fixation element 108 with tissue wall 113 based on the comparison of principal signal 154, a first radial signal 156, and/or a second radial signal 158 to representative cardiac activity 151. In examples, processing circuitry 120 assesses that fixation element 108 is engaged with tissue wall 113 based on comparison of one or more frequencies of principal signal 154, first radial signal 156, and/or second radial signal 158 (e.g., frequencies within window 160 or window 164) to one or more frequencies of representative cardiac activity 151(e.g., frequencies within window 162). Processing circuitry 120 may assess that fixation element 108 is engaged with tissue wall 113 based on comparison of perturbations of one of principal signal 154, first radial signal 156, or second radial signal 158 with perturbations in another of principal signal 154, first radial signal 156, or second radial signal 158. Processing circuitry 120 may communicate with output device 124 to cause output device 124 to provide an output indicative of likely engagement of fixation element 108 with surface tissue 115 and/or tissue wall 113.

[0088] Processing circuitry 120 may assess that fixation element 108 is progressively embedding within tissue wall 113 by assessing a substantially contemporaneous occurrence of contact between fixation element 108 with tissue wall 113 and a movement (e.g., a rotation) of fixation element 108 and/or device housing 106 relative to tissue wall 113 and/or surface tissue 115. Processing circuitry 120 may be configured to indicate that fixation element 108 is progressively embedding within tissue wall 113 based on the substantially contemporaneous occurrences detected. Processing circuitry 120 may communicate with output device 124 to cause output device to provide an indication representative of the detection of the substantially contemporaneous occurrences, such that, for example, a clinician be apprised that a motion of IMD 104 is likely causing fixation element 108 to progressively embed with tissue wall 113. In examples, processing circuitry 120 may evaluate (e.g., count) a number of rotations of IMD 104 about device axis LD when fixation element 108 is assessed to be embedded within tissue wall 113. Processing circuitry 120 may communicate with output device 124 to cause output device 124 to provide an indication reflective of the number of completed as fixation element 108 is engaged with tissue wall 113, such that, for example, a clinician may be apprised of a number of engaged turns experienced by IMD 104.

[0089] In examples, processing circuitry 120 assesses a pressure (e.g., a force) exerted by fixation element 108 on surface tissue 115 to, for example, allow a clinician to assess whether an existing pressure is too low or too high. Processing circuitry 120 may compare the motion of IMD 104 to representative cardiac activity 151 to assess the pressure. Processing circuitry may assess an alignment of IMD 104 relative to tissue wall 113 by assessing a pivoting of one or more principal axis P, first radial axis Rl, and/or second radial axis R2 relative to surface tissue 115 as heart 102 beats and imparts forces to IMD 104. Processing circuitry 120 may communicate with output device 124 to cause output device 124 to provide an indication representative of the pressure and/or the alignment, such that, for example, the clinician may be apprised of the pressure between fixation element 108 and surface tissue 115 and/or the alignment of IMD 104 relative to tissue wall 113.

[0090] Various examples of the disclosure have been described. Any combination of the described systems, operations, or functions is contemplated. These and other examples are within the scope of the following claims.

[0091] Example 1. A medical system, comprising: an implantable medical device configured to engage tissue within a heart of a patient, wherein the implantable medical device includes: a housing, a fixation element configured to progressively embed within the tissue to couple the implantable medical device to the tissue, wherein the fixation element is configured to ransmit a force from the tissue to the housing as the fixation element progressively engages, and a motion sensor configured provide a sensor signal indicative of a motion of the housing; and processing circuitry configured to: compare the sensor signal and a representative cardiac activity when the fixation element engages the tissue, and assess the engagement of the implantable medical device to the tissue based on the comparison.

[0092] Example 2. The medical system of Example 1, wherein the processing circuitry is configured to provide an output indicative of the assessed engagement of the implantable medical device to the tissue as the fixation element progressively engages the tissue, and further comprising an output device configured to display the output.

[0093] Example 3. The medical system of Example 1 or Example 2, wherein the fixation element is configured to transmit the force to the housing when cardiac activity of the heart causes the heart to impart the force to the fixation element.

[0094] Example 4. The medical system of any of Examples 1-3, wherein the representative cardiac activity is based on a signal indicative of at least one of a muscular movement of the heart of the patient or an expected muscular movement, wherein the expected muscular movement is based on a muscular movement exhibited by one or more subject hearts other than the heart of the patient.

[0095] Example 5. The medical system of any of Examples 1-4, further comprising a delivery member configured to cause the fixation element to progressively embed within the tissue when the delivery member is coupled to the implantable medical device.

[0096] Example 6. The medical system of Example 5, wherein the delivery member includes a distal portion configured to position within the heart and a proximal portion configured to position extracorporeal to the patient, wherein the proximal portion is configured to cause movement of the distal portion when the proximal portion moves relative to the patient, and wherein the distal portion is configured to cause the fixation element to progressively embed within the tissue when the distal portion is coupled to the implantable medical device and the proximal portion causes the movement of the proximal portion relative to the patient.

[0097] Example 7. The medical system of any of Examples 1-6, wherein the motion sensor is configured to sense an acceleration of the housing, wherein the acceleration is indicative of the motion of the housing.

[0098] Example 8. The medical system of any of Examples 1-7, wherein the fixation element is configured to define an embedded depth to engage the implantable medical device to the tissue, wherein the embedded depth is a distance from a surface of the tissue to a distal end of the fixation element, and wherein the fixation element is configured to increase the embedded depth as the fixation element progressively engages the tissue.

[0099] Example 9. The medical system of Example 8, wherein the processing circuitry is configured to assess whether the fixation element is defining the embedded depth based the sensor signal and the representative cardiac activity.

[0100] Example 10. The medical system of any of Examples 1-9, wherein the fixation element comprises a helix configured to progressively embed within the tissue when the housing rotates about a device axis defined by the housing.

[0101] Example 11. The medical system of Example 10, wherein the processing circuitry is configured to detect the rotation of the housing about the device axis using the sensor signal.

[0102] Example 12. The medical system of Example 10 or Example 11, wherein the processing circuitry is configured to determine a number of rotations of the housing about the device axis when the helix defines the embedded depth of claim 6 using the detected rotation of the housing and the evaluation of the engagement of the implantable medical device to the tissue.

[0103] Example 13. The medical system of any of Examples 1-9, wherein the fixation element comprises one or more tines supported by the IMD housing, wherein the one or more tines are configured to progressively embed within the tissue. [0104] Example 14. The medical system of any of Examples 1-10, wherein the processing circuitry is configured to compare the sensor signal and the representative cardiac activity by at least one of comparing one or more frequencies of the sensor signal to one or more frequencies of the representative cardiac activity or comparing one or more amplitudes of the sensor signal to one or more amplitudes of the representative cardiac activity.

[0105] Example 15. The medical system of any of Examples 1-14, wherein the housing defines a principal axis of a coordinate system, a first radial axis of the coordinate system, and a second radial axis of the coordinate system, wherein the motion sensor is configured to sense the motion of the housing along one or more of the principal axis, the first radial axis, or the second radial axis, and wherein the sensor signal includes one or more of a principal signal indicative of a motion of the housing along the principal axis, a first radial signal indicative of a motion of the housing along the first radial axis, or a second radial signal indicative of a motion of the housing along the second radial axis.

[0106] Example 16. The medical system of Example 15, wherein the housing includes a distal portion and a proximal portion, wherein the distal portion supports the fixation element, and wherein the principal axis intersects the distal portion and the proximal portion.

[0107] Example 17. The medical system of Example 16 or Example 17, wherein the principal axis is substantially parallel to the device axis of claim 10.

[0108] Example 18. The medical system of any of Examples 15-17, wherein the first radial axis is substantially perpendicular to the principal axis, and wherein the second radial axis substantially perpendicular to the principal axis and substantially perpendicular to the first radial axis.

[0109] Example 19. The medical system of any of Examples 15-18, wherein the processing circuitry is configured to determine an orientation of the principal axis relative to a surface of the heart using two or more of the principal signal, the first radial signal, or the second radial signal.

[0110] Example 20. The medical system of any of Examples 15-19, wherein the processing is configured to determine a pivot of the device axis of claim 10 relative to a heart surface of the heart using one or more of the principal signal, the first radial signal, or the second radial signal. [0111] Example 21. The medical system of any of Examples 1-22, further comprising an output device, wherein the processing circuitry is configured to cause the output device to provide an output indicative of the assessment of the engagement of the implantable medical device to the tissue.

[0112] Example 22. The medical system of any of Examples 1-22, wherein the processing circuitry is configured to determine a pressure between the fixation element and a surface of the tissue based on the comparison.

[0113] Example 23. The medical system of any of Examples 1-22, wherein processing circuitry is configured to compare a frequency-based sensor parameter and a frequencybased activity parameter, wherein the frequency-based sensor parameter is based on one or more frequencies of the sensor signal, and wherein the frequency-based activity signal is based on one or more frequencies of the representative cardiac activity.

[0114] Example 24. A method, comprising: engaging, using a fixation element, an implantable medical device to tissue within a heart of a patient, wherein engaging the fixation element includes progressively embedding the fixation element to the tissue, and wherein the fixation element is configured to transmit a force from the tissue to a housing of the implantable medical device as the fixation element progressively embeds within the tissue; comparing, using processing circuitry, a sensor signal and a representative cardiac activity as the fixation element progressively engages the tissue, wherein the sensor signal is provided by a motion sensor, and wherein the sensor signal is indicative of a motion of the housing; and assessing, using processing circuitry, the engagement of the implantable medical device to the tissue based on the comparison.

[0115] Example 25. The method of Example 24, further comprising providing, using the processing circuitry, an output to an output device, wherein the output is indicative of the evaluated engagement of the implantable medical device to the tissue as the fixation element progressively embeds within the tissue.

[0116] Example 26. The method of Example 24 or Example 25, further comprising transmitting, using the fixation element, the force to the housing when cardiac activity of the heart causes the heart to impart the force to the fixation element.

[0117] Example 27. The method of any of Examples 24-26, further comprising basing the representative cardiac activity on a signal indicative of at least one of a muscular movement of the heart of the patient or an expected muscular movement, wherein the expected muscular movement is based on a muscular movement exhibited by one or more a subject hearts other than the heart of the patient.

[0118] Example 28. The method of any of Examples 24-27, further comprising causing, using a delivery member coupled to the implantable medical device, the fixation element to progressively embed within the tissue.

[0119] Example 29. The method of Example 28, wherein causing the fixation element to progressively embed within the tissue comprises coupling a distal portion of the delivery member to the implantable medical device and moving the distal portion by moving a proximal portion of the delivery device relative to the patient, wherein the distal portion is configured to position within the heart and the proximal portion configured to position extracorporeal to the patient.

[0120] Example 30. The method of any of Examples 24-29, further comprising increasing an embedded depth as the fixation element progressively embeds within the tissue, wherein the embedded depth is a distance from a surface of the tissue to a distal end of the fixation element.

[0121] Example 31. The method of Example 24-30, further comprising assessing, using the processing circuitry, whether the fixation element is defining the embedded depth of claim 29 based the sensor signal and the representative cardiac activity.

[0122] Example 32. The method of any of Examples 24-31, wherein progressively embedding the fixation element within the tissue includes rotating the housing about a device axis defined by the implantable medical device to cause a helix to progressively embed within the tissue.

[0123] Example 33. The method of Example 32, further comprising detecting, using the processing circuitry, a rotation of the housing about the device axis using the sensor signal.

[0124] Example 34. The method of Example 32 or Example 33, further comprising determining, using the processing circuitry, a number of rotations of the housing about the device axis when the helix defines the embedded depth of claim 30 using the detected rotation of the housing and the evaluation of the engagement of the implantable medical device to the tissue.

[0125] Example 35. The method of any of Examples 24-34, further comprising sensing, using the motion sensor, a motion of the housing along one or more of a principal axis of a coordinate system, a first radial axis of the coordinate system, or a second radial axis of the coordinate system, and wherein the sensor signal includes one or more of a principal signal indicative of a motion of the housing along the principal axis, a first radial signal indicative of a motion of the housing along the first radial axis, or a second radial signal indicative of a motion of the housing along the second radial axis.

[0126] Example 36. The method of Example 35, further comprising determining, using the processing circuitry, an orientation of the principal axis relative to a surface of the heart using two or more of the principal signal, the first radial signal, or the second radial signal.

[0127] Example 37. The method of Example 35 or Example 36, further comprising determining, using the processing circuitry, a pivot of the principle axis relative to a heart surface of the heart using two or more of the principal signal, the first radial signal, or the second radial signal.

[0128] Example 38. The method of any of Examples 24-37, wherein progressively engaging the fixation element to the tissue includes engaging one or more tines to the tissue.

[0129] Example 39. The method of any of Examples 24-38, wherein comparing the sensor signal and the representative cardiac activity includes comparing, using the processing circuitry, one or more frequencies of the sensor signal with one or more frequencies of the representative cardiac activity.