SYSTEM AND METHOD FOR DELIVERY OF CARDIAC PACING IN A MEDICAL DEVICE IN RESPONSE TO ISCHEMIA

TECHNICAL FIELD

The invention relates generally to medical devices, and, in particular, to .medical devices capable of controlling delivery of cardiac pacing in response to ischemia.

BACKGROUND

Recently, cardiac pacing protocols employed by implantable cardiac pacemakers and implantable cardioverter defibrillators (ICDs) are aimed at promoting intrinsic conduction and intrinsic depolarization of the ventricles as often as possible while reducing or minimizing ventricular pacing. Such a pacing protocol generally includes pacing in an atrial mode, such as AAJ, ADl, AAlR or ADJR mode, while monitoring intrinsic AV conduction. As long as AV conduction, is intact, the atria! pacing mode is maintained. If AV conduction is absent, ventricular pacing is delivered, for example in a dual chamber pacing mode such as DDI or DDIR, Io ensure ventricular depolarization during AV block.

Many patients implanted with cardiac pacemakers or ICDs have underlying coronary artery disease and are susceptible to myocardial ischemia. When the pacing røode is programmed in a rate responsive mode (e.g. ADIR ₅ ^' DDDR, or DDIR), a higher pacing rate is provided in response to a sensed increase in metabolic need. The higher pacing rate increases the metabolic demand placed on the myocardium, which could lead to ischemia, Patients subjected to minimum ventricular pacing may have long AV conduction times thai result in inefficient AV coupling. Inefficient AV coupling can adversely affect cardiac output and subsequently reduce myocardial perfusion leading to myocardial ischemia. Other causes of myocardial ischemia may be disease related, It is desirable to provide cardiac pacing In a .manner that does not worsen an ischemic state and may act to alleviate myocardial ischemia.

BRIEF DESCRIPTION OF TTIE DRAWINGS

Aspects and features of She present invention will be appreciated as the same becomes better understood by reference to the following detailed description of the

embodhnents of the invention when considered m connection with {he accompanying drawings, wherein:

FiO. 1 deplete an implantable, cardiac stimulation device embodied as an ICD 14, in which one embodiment of the present invention may be implemented;

FIG. 2 is a functional block diagram showing some of the components typically included in an implantable cardiac stimulation device such as the ICD shown in F IG. 1 ;

FIG. 3 illustrates the general physical configuration of one external device that may be used to manually indicate an ischemic episode;

FϊG, 4 Is a block functional diagram of the patient activator shown in FIG.3;

FiG. 5 is a flow chart providing an overview of a method for monitoring for myocardial ischemia aid providing a« ischemia response; and

FIG. 6 is a flow chart summarizing a method for responding to myocardial ischemia defection according to one embodiment of the invention.

DETAILED DESCRIPTION

In the following description, references are made Io illustrative embodiments for carrying out the invention, It is understood that other embodiments may be utilized without departing from Ae scope of Ihe invention.

FϊG. 1. depicts an implantable, cardiac stimulation device embodied as an ICD 14, in which one embodiment of the present invention may be implemented. ICD 14 is provided for sensing intrinsic heart activity and delivering cardiac stimulation pulses as appropriate to one or more heart chambers. ϊCD 14 is adapted for delivering minimum ventricular pacing according to a programmed mode of operation.

ϊCD 14 is shown in communication, with a patienf s heart \ O by way of three leads 16. 32 and 52. The heart IO is shown in a partially cut-away view. illustrating the upper heart chambers, the right airium (RA) and left atriυm (LA), and the lower heart chambers, the right ventricle (RV) and left ventricle (LVX and me coronary sinus (CS) in the right atrium leading into the great cardiac vein 48, which branches to form inferior cardiac veins. Leads 16, 32 and 52 connect ICD 14 with the RA. the RV and the LV, respectively. Each lead has at least one electrical conductor and pace/sense electrode. A remote indifferent can electrode is formed, as part of the outer surface of the ϊCD housing 20- The pace/sense electrodes and the remote indifferent can electrode can he selectively employed

to provide a number of unipolar and bipolar pace/sense electrode combinations for pacing and sensing functions.

RA lead 16 is passed through a vein into the RA chamber and may be attached at its distal end. to the RA wall using a fixation member 17. RA lead 16 is formed with a connector 13 fitting into a connector bore of ICD connector block 12 for electrically coupling RA tip electrode 19 and RA ring electrode 21 to ICD internal circuitry via insulated conductors extending within lead body 15. RA tip electrode 18 and RA ring electrode 21 may be used in a bipolar fashion, or in a unipolar fashion with ICD housing 20, for achieving RA stimulation and sensing of RA electrogram (EGM) signals, RA lead 16 is also provided with a coi! electrode 18 that may be used for delivering high voltage cardioversion/defibrillation pαtees to heart 10 in response to the detection of tachycardia or fibrillation.

RV lead 32 is passed through the RA into the RV -where its dista! end, carrying RV tip electrode 40 and RV riog electrode 38 provided for stimulation, in the RV aad sensing of RV EGM signals, is .fixed in place in the RV apex by a distal fixation member 4.1 , RV lead 32 also carries a high-voltage coil electrode 60 for use in cardioverting and defibrillating heart 10. RV lead 32 is formed with a connector 34 fitting into a corresponding connector bore of LCD connector block 12. Connector 34 is coupled, to electrically insulated conductors wϊthin lead body 36 and connected with distal tip eieeirode 40..ring electrode 38 and coil electrode 60.

Coronary sinus lead 52 is passed through the RA, into the CS and further into a cardiac vein 48 to extend, the distal LV tip electrode 50 and ring electrode 62 alongside the LV chamber to achieve LV stimulation and sensing of LV EGM signals. The IV CS lead 52 is coupled at the proximal end connector 54 into a bore of ICO connector block 12 to provide electrical coupling of conductors extending from electrodes 50 and 62 within lead body 56 to ICD internal circuitry. In some embodi.me.nts, LV CS lead 52 could bear a proximal LA pace/sense electrode 51 positioned along CS lead body 56 such that it is disposed proximate the LA for use in stimulating the LA and/or sensing LA EGM signals. In addition to the lead-mounted electrodes, ICD 14 may include one or more cardiac sensing electrodes 22 formed as uninsulated portions of the ICD housing 20 or included hi the connector block 12. While a particular ϊCD system with associated leads and electrodes is illustrated in FlG. 1 numerous implantable cardiac pacemaker and ICD system configurations are possible including one or more leads, which may be

lra»sy<=røus, subcutaneous, or epicardisl leads, having various electrode arrangements. Embodiments of the invention may also include subcutaneous pacemaker or ICD systems hi which stimulation and sensing electrodes are formed as a part of the device housing and/or carried by subcutaneous leads,

.ICD .14 is shown as a multi-chamber device capable of sensing and stimulation in three or all four heart chambers, it is understood that ϊCD 14 may be modified to operate as a single chamber device for delivering atriai-onl? pacing (with dtiaJ chamber sensing) or a duai chamber device for sensing and stimulation in one xψpvx chamber and one lower chamber. In a single chamber device, ischemia monitoring and monitoring for AV conduction may be performed using subcutaneous electrodes. Furthermore, it is recognized thai embodiments of the present invention may be practiced in a single chamber, dual chamber or .imiiti-cMmber implantable cardiac pacemaker that does not ϊncl ude cardioversion and defibrillation functions.

FIG. 2 is a functional block diagram showing some of the components typically included in an implantable cardiac stimulation device such as the JCB shown in FϊG. L ICD 14 generally includes timing and control circuitry 152 and an operating system that may employ microprocessor 154 or a digital state machine for timing sensing and therapy delivery functions in accordance with a programmed operating mode. Microprocessor 154 and associated memory 156 are coupled to the various components of IMD .10 via a data/address bus .155. ICD 14 includes therapy delivery unit 150 for delivering electrical stimulation therapies, such as cardiac pacing therapies and arrhythmia therapies including cadioversion/defibrillation shocks, under the control of timing and control 152. ^" Therapy delivers' unit 150 is typically coupled to two or more electrode terminals 368 via a switch matrix ϊ5& Switch matrix 158 is used for selecting which electrodes and corresponding polarities are used for delivering electrical stimulation pulses.

Electrode terminals 168 may also be used for receiving electrical signals from the heart or for measuring impedance. Cardiac electrical signals are sensed for determining when an electrical stimulation therapy is needed and in controlling a stimulation mode and the timing of stimulation pulses.

Electrodes used for sensing and electrodes used for stimulation may be selected via switch matrix 158. When used for sensing, electrode terminals 168 are coupled to signal processing circuitry 160 via switch matrix 158. Signal processor 160 includes sense amplifiers and may include other signal conditioning circuitry and an analog to digital

converter. Electrical signals may ihøn be used by microprocessor .154 for delecting physiological events, such as detecting and discriminating cardiac arrhythmias. Signal processing cjrciύuy 160 may include event detection circuitry generally corresponding to R-vvave detection circuitry as disclosed in U.S. Pat No, 5..117.824 (ϋCeitnel., et sl) _; hereby incorporated, herein by reference in its entirety.

Arrhythmia detection algorithms may be implemented for detecting ventricular tachycardia (VT). ventricular fibrillation (VT) as well ss atria! arrhythmias such as atrial fibrillation (A FϊB). Ventricular event intervals (R-R intervals) sensed from the EGM signals are commonly used for detecting ventricular arrhythmias. Additional information obtaiaed mά\ as R-wave morphology slew rate, other event intervals (P-R intervals) or other sensor signal information may be -used in detecting, confirming or discriminating ant arrhythmia. Reference is made to U.S. Pat Nos. 5,354,316 <Kemie ⁾ .X 5,545,186 (Olson et al.) and U.S. Pat, No. 6393,316 (Oillbβrg βt al,) for examples of arrhythmia detection and discrimination using EGM signals, both αf which patents are incorporated herein by reference m their entirety.

In one detection scheme, programmable detection interval ranges designate Che range of sensed event intervals indicative of a tachycardia and may be defined separately for detecting slow tachycardia, fast tachycardia and fibrillation. Sensed event intervals fMing into defined detection interval ranges are counted to pro-vide a count of tachycardia intervals, A programmable number of intervals iø detect (NE)) defines the number of tachycardia intervals occurring consecutively or out of a given number of preceding event intervals thai are required to detect tachycardia. A separately programmed NfD may be defined for detecting slow and fast tachycardia and .fibrillation, In addition to (he interval ranges and MD criteria, rapid onset criterion and rate stability criterion may also be defined for use in tachycardia detection schemes. Furthermore, a combined count of tachycardia and fibrillation intervals may be compared to a combined count threshold and, according to predefined, criteria, used in delecting fibrillation or slow or fast tachycardia. In addition to event interval information, the morphology of the EGM signal may be vseύ in discriminating heart rhythms, for example as described it\ the above-incorporated ^* 3 I 6 Giliberg patents According to one embodiment of fee invention, digitized BGM signals are provided to microprocessor \ 54 for waveform analysis according to an implemented morphology or template matching algorithm. Morphology analysis may be used in. conjunction with event interval analysis to improve fee sensitivity and specificity of

arrhythmia detection .methods. Since the morphology of fee BGM signals is generally altered during ischemia, the waveform analysis used for detecting arrhythmias may be adjusted in response to detecting aα ischemic episode according to one embodiment of Hie invention. For example, the .morphology templates and/or correlation, coefficients used m EGM waveform analysis for detecting arrhythmias may be different during an ischemic episode that? during a nonischemic episode.

In response to an arrhythmia detection, a programmed arrhythmia therapy is delivered by therapy deliver module .150 under the control of timing and control 152. A description of high-voitage output circuitry and control of high-voltage shock pulse delivery is provided in the above-incorporated ^V 186 Olson patent. Typically, a tiered m&im of arrhythmia therapies are programmed iaio the device ahead of lime by the physician and stored in memory 156. For example, on initial detection of an atrial or ventricular tachycardia, an anii-tachycardia pacing therapy may be selected and delivered to the chamber in which the tachycardia is diagnosed or. to both chambers. On redetection of tachycardia, a more aggressive anti-tachycardia pacing therapy may be scheduled, If repeated attempts at anti-tachycardia pacing therapies fail. a. higher level cardioversion pulse may be selected thereafter. Therapies for tachycardia termination may also vary with the rate of the detected tachycardia, with the therapies increasing in aggressiveness as the rate of the detected tachycardia increases. For example, fewer attempts at anti-tachycardia pacing may be undertaken prior to delivery of cardioversion pulses if the rate of the detected tachycardia is above a preset threshold,

Ih the event that fibrillation is identified, high frequency burst stimulation may be employed as the initial attempted therapy. Subsequent therapies may be delivery of high amplitude defibrillation pulses, typically in excess of 5 joules. Lower energy levels may be employed for cardioversion. The defibrination pulse energy may be incremented in response to failure of an initial pulse or pulses to terminate fibrillation.

The occurrence of arrhythmias may be associated with a variety of pathological conditions. The ischemic myocardium is generally more vulnerable to arrhythmias and may be less responsive to arrhythmia therapies than non-ischemic myocardium. Accordingly, as will be described in greater detail below, one embodiment of the invention includes altering programmed arrhythmia therapies in response to a detected ischemic episode.

Cardiac electrical signals may also used in detecting aα ischemic episode. For example, electrical signals may be used for detecting changes in an EOM/ECG signals, such as changes in the S-T segment, indicative of myocardial ischemia Methods for detecting ischemia using ST seg.rae.nt changes are generally described in U.S. Pat. Mo. 6,128,526 (Stadle-v βt al.) and U.S. Pat. No. 5.199,428 COhel et al.) ₅ both of which patents are incorporated herein by reference in their entireties. Electrical signals received at terminals 1<>8 may be provided to dedicated ischemia detector circuitry 172 for detecting changes in the signals indicative of an ischemic state. Alternatively, microprocessor 154 may execute ischemia detection algorithms using sensed cardiac electrical signals for detecting m ischemic episode.

ϊ€D .14 may additionally or alternatively be coupled to one or more physiological sensors via physiological sensor terminals I 70. Physiological sensors .may include pressure sensors, accelerometers., flow sensors, blood chemistry sensors, activity sensors or other physiological sensors known for use with implantable cardiac stimulation devices. Physiological sensors may be carried by leads extending from ϊCD .14 or incorporated in or on the ICD housing.

Signals received at sensor terminals 170 are received by a sensor interface 162 which provides sensor signals to signal processing circuitry 160. Sensor signals are used by microprocessor .154 for detecting physiological events or conditions. For example, ICD 14 may monitor heart wall motion, blood pressure, blood chemistry, respiration, or patient activity. Monitored signals may he used for sensing the need for delivering a therapy under control of the operating system, hi some embodiments, microprocessor 154 or ischemia detector 172 use physiological signals received from sensor terminals .170 for use in detecting myocardial ischemia. For example, an acceleronaeter signal, a pressure sensor signal, an impedance signal, or an oxygen saturation, pU or other blood chemistry sensor signal may be used alone or in combination with cardiac electrical signals for detecting myocardial ischemia.

The operating system includes associated memory 156 for storing a variety of programmed-in operating mode and parameter values that are used by microprocessor 154. The memory 156 may also he used fbr storing data compiled .from sensed physiological signals and/or relating to device operating history for telemetry out on receipt of a retrieval or interrogation instruction. All of these functions and operations are known in. the art, and many are generally employed to store operating commands and data

.for controlling device operation and for later retrieval Io diagnose device function or patient condition. An ischemia detection algorithm may be stored in memory 156 and executed by microprocessor 154 with input received from electrode terminals 16$ and/or sensor terminals 170 for detecting ischemia Alternatively, ischemia detector 172 may be embodied »s dedicated circuitry for receiving cardiac EGM/ECG or other physiologies! signals and for generating a signal indicating detection of myocardial ischemia. As will be described below, timing and control 152 responds to the detection of ischemia by altering a pacing mode, pacing parameter, arrhythmia detection parameter, and/or arrhythmia therapy according to ischemia response data stored in memory 156. Data relating to ischemia detection may be stored in .memoϊy 156 for later retrieval.

ICD .14 further includes telemetry circuitry .164 and antenna 128. Programming commands or data are transmitted during uplink or downlink telemetry between ICD telemetry circuitry 164 and external telemetry circuitry included in a programmer or monitoring unit Telemetry circuitry 164 and antenna 128 may correspond to telemetry systems known in the art, In one embodiment _* telemetry circuitry 164 is adapted to receive signals from an external patient activator, home monitor or other external device operable by a patient or caregiver. The patient of caregiver, ysing the external device, may manually generate a telemetry signal indicating the patient is experiencing symptoms relating to myocardial ischemia Upon receipt of the external signal, microprocessor 154 causes timing and control 152 to provide an ischemia . response.

ϊCD 14 may optionally be equipped with patient alarm circuitry 166 for generating audible tones, a perceptible vibration, muscle stimulation or other sensory stimulation for notifying the patient that a patient alert condition has been detected by ϊCD 14. In some embodiments, an alarm signal may be generated upon detection of ischemia or upon altering a pacing mode of operation in response to ischemia detection.

FfG. 3 illustrates the general physical configuration of one externa! device that may be used to manually indicate an ischemic episode. The external device shown in FlG. 3 is embodied as a handheld device known as a patient activator IDO, Activator 100 generally takes the form of a plastic enclosure provided with a push button 102 by which the patient or a caregiver may generate an ischemia signal upon presenting symptoms associated with myocardial ischemia, such as angina. The ischemia signal is transmitted to an implanted device, such as ICD 14 shown in FiG. 2, adapted to receive such a signal and provide an ischemia response thereto. Activator .100 is battery powered, employing

batteries accessible by means of the batt ery cover 104. LED indicator lights 116 and a speaker 112 may be provided to communicate to the patient the status and functioning of telemetry communication with the implanted device and the status of a patient-initiated ischemia response. It is to be understood that other types of external devices enabled for telemetric communication with ICD 14 may be used for manually generating an ischemia signal Such devices include programmers and home monitors.

FIG. 4 is a block functional diagram of a patient activator. Control functions are provided by microprocessor 108, based upon programming stored in its associated read- only memory located therein. Microprocessor 108 provides output signals for producing audible patient alert signals by means of driver 110 and speaker 112. Microprocessor 108 also provides control signals to LED driver 114 to power associated colored LED indicator lights 116, referred to above. The device is powered by a battery 118 which is coupled Io the microprocessor 108 by means of power/switching/battery monitor circuitry 120, which, also provides the microprocessor with an indication that push button 122 has been pressed.

Communication with microprocessor 108 is accomplished by means of the antenna driver/switching circuit 124, the receiver demodulator 126 and RF antenna 128. Transmissions from the implanted device are received by antenna 128, and are demodulated by receiver demodulator 126 to be provided to the microprocessor. In response to received transmissions from the implanted device, the microprocessor controls operation of tine audio and light drivers 110 and 114 to indicate the nature of the communication received. Transmissions to the implanted device, for example, in response to activation of the push button 102 are provided by microprocessor 108 to the antenna drive/switching circuit, which then communicates with the implanted device by means of antenna 128.

FIG. 5 is a flow chart providing an overview of a method for monitoring for myocardial ischemia and providing an ischemia response. Flow chart 180 is intended to illustrate functional operations of a pacesmaker or ICD, generally referred to hereafter as a "device*. Flow chart 180 should not be construed as reflective of a specific form of software or hardware necessary to practice the invention. It is believed that the particular form, of software will be determined primarily by the particular system architecture employed in the device and by the particular detection and therapy delivery methodologies employed hy the device. Providing software to accomplish the present invention in the

eontext of any modem ICD, given the disclosure herein, is within the abilities of one of skill in the art.

At block 182, fhe device operates in a normal minimum ventricular pacing mode. which includes delivery of cardiac pacing pulses as needed and may include monitoring for arrhythmias and delivering programmed arrhythmia therapies. Ii should be appreciated that the description herein is provided as an overview to illustrate various embodiments for practicing the invention and k not intended to be limiting. For example., reference to a minimum ventricular pacing mode may refer to an operational state that includes mode switches from one pacing mode io another _* Alternatively, a minimum ventricular pacing mode may reier to a single operational mode that effectuates both atrial based aid dual chamber based functionality aimed at allowing intrinsic AV conduction whenever possible. Thus, for purposes of description, references to s pacing mode and to pacing control parameters with regard to minimum ventricular pacing and an ischemia response are indicative of the functionality imparted and may include on actual mode status switch or a change ir» the functional status of a single mode inclusive of the altered functions provided in response to ischemia.

Furthermore,, it is recognized that the normal minimum ventricular pacing mode may include delivery of other cardiac pacing therapies when needed as determined based on evaluation® of the heart .rhythm or other physiological sensors. Other therapies that may be delivered include, for example,, cardiac ;røsytichroni;?.atϊo.n therapy, extra systolic stimulation, arrhythmia prevention therapies, and overdrive pacing,

At block 184, the device determines if ischemia is detected. The device may rely on any ischemia detection method available, including but not limited to ECG'EGM signal analysis, physiological sensor data, and patient feedback provided, with the use of an external device such as the activator shown in FIG. 3 or other feedback method such as physical tapping on the device. During automatic ischemia detection, intermittent ventricular paced cycles that may occur during minimum ventricular pacing mode may be ignored, particularly when the ischemia detection is based on an analysis of ECO/EGM morphology. If ischemia is not detected, the device continues operating according to the normal programmed operating parameters and operating mode (block 182).

Xa response to detecting an Ischemic episode, the frequency of ischemia monitoring using ECG/EC5M signals and/or other physiological signals may be increased as indicated at block 186. During normal operation, ischemia monitoring may be programmed to occur

iit a periodic interval, for example even" 4 hours. Ischemia mom taring may also occur on a triggered basis, for example in response to a high heart rate, change in blood pressure, change in activity level, or other detected physiological condition, If ischemia is detected, either by the implanted device or upon receipt of an external, manually generated signal, the frequency of ischemia monitoring may be increased, for example to even' hour In addition or alternatively so adjusting the ischemia monitoring frequency at block IB6, the sampling rate used in acquiring the ECQfEGM signals and/or other physiological signals αsed in monitoring for ischemia may be adjusted to obtain a higher resolution signa I(s). A higher signal resolution may allow more detailed signal analysis ma more reliable detection of ischemia,

At block 188, an ischemia, response is provided. As wiil be described m greater detail below, the ischemia response may include altering the current pacing mod® by performing a mode switch or altering a pacing control parameter. Facing control parameters that may be altered include, but are not limited to. timing intervals, rates, and pacing electrode configurations. The ischemia response may alternatively or additionally include altering arrhythmia detection methods and/or programmed arrhythmia therapies. The ischemia response may include alerting the patient using the patient alarm circuitry 166 shown in FIG. 2 to notify the patient of the ischemia episode. The patient can respond to an ischemia notification according to clinician instructions, .for example by modifying their activity or by seeking medical attention.

At block 190» the device continues to monitor ischemia, at the adjusted monitoring frequency. If ischemia is still being detected, therapy alterations provided by the ischemia response may be maintained at block 194, Alternatively, the ischemia response provided at block 194 in response to a sustained or worsening state of ischemia, as detected at block 190, may include a different response than the initial ischemia response provided at block 188.

A response to a prolonged or worsening ischemic state provided at block \ 94 may include further alteration of the pacing mode and/or arrhythmia detection criteria and therapies aid may include delivering a higher level physician or patient warning. Further alterations in the pacing mode nmy include switching to a non-rate responsive mode, adjusting the lower pacing rate, adjusting tiϊBϊng parameters, changing pacing electrode configuration, or attempting other available pacing modes or therapies. A pacing mode, control parameter settings, or other data relating to the ICD operating status at the time of

the alleviation of the ischemia episode may be stored at block 194 Such data may be used by the ICD in providing the first response (block 188) to later ischemia episode detections. When ischemia is no longer detected at block 190, the frequency of ischemia monitoring and/or signal sampling rate may be decreased at block 198, The reduction in ischemia monitoring frequency or sampling rate may occur aft er a delay following the end of the ischemic episode to ensure that an early recurrence of ischemia does not go undetected. Aft er ischemia is no longer detected, the device returns to the normal programmed operating mode at block 182.

FiG. 6 is a flow chart summarizing a method for responding to myocardial ischemia detection according to one embodiment of iha invention. Flow chart 200 is intended to illustrate a number of responses thai may he provided by a device programmed to operate in a minimum ventricular pacing mode. While .flow chart 200 is shown with a particular operational flow indicated by the arrows joining the various operational blocks, it is to be understood that numerous protocols that include one or more of the ischemia response operations shown and described herein may he developed for providing an appropriate response to myocardial ischemia Accordingly, the particular order of the blocks shown in flow chart 200 may be rearranged, with, additions or deletions, to achieve a desired response to myocardial ischemia in a device adapted for operating in a minimum ventricular pacing mode.

In general, the device .monitors cardiac activity and delivers cardiac stimulation in the form of pacittg or cardioversion/defibrillation shocks as needed in accordance with a programmed operating mode. During normal device operation, ischemia monitoring occurs, either on a periodic, triggered or continuous basis. Io response to detecting ao ischemic episode, device operation is modified. Numerous modifications, as will be described below, may be made, including modifications to pacing control parameters., pacing mode, arrhythmia detection parameters, and/or arrhythmia therapy delivery.

Generally, .modifications made in response to .ischemia detection are .made to alleviate the ischemia by reducing the demand placed on the heart and/or improve perfusion of the heart. Other modifications may be made to alter arrhythmia detection and therapies during an ischemic episode.

At block 210, an AV conduction time limit or a VA time interval limit may be programmed by a user or stored as nominal values in device memory. It is generally assumed that the device will operate in a minimum ventricular pacing mode of operation

as indicated at block 215. A minimum ventricular pacing mode generally includes operating in an atrial pacing mode, for example AAl, ADi, AAIR, or ADIR, in order to allow intrinsic conduction from the atda to the ventricles to occur and subsequent intrinsic ventricular depolarization. The atrial pacing mode is maintained as long as atrial- ventricular conduction remains intact During the atrial pacing mode, if a ventricular sense event does not occur between two consecutive atrial events, i.e. if AV conduction becomes blocked, the device will switch to a pacing mode that includes ventricular pacing, typically a dual chamber mode such as DDD. BDI, DDDR, or DDIR. Apparatus and methods for delivering pacing to ensure AV conduction whenever possible are generally described in U.S. Pat. No. 6,772,005 (Casavant et al.} and U.S. Pat. Pub. No. 2005/0237539 (Betzold et al.), both of which are hereby incorporated herein by reference i» their entirely.

During an atrial pacing mode, AV conduction may remain intact but AV conduction time may become relatively long. At long AV conduction times, the veairicle contracts late in the cardiac cycle, A subsequent atrial contraction may occur prior to the end of passive ventricular filling. The force available from the contracting atria is under utilized when blood is forced into an empty or only partially filled ventricle. Overall tilling is reduced The atrial force, often referred to as the atrial "kick", is better utilised after passive filling is complete to force additional Mood into the ventricle, which, promotes a strong ventricular contraction on the next cardiac cycle according to Starling's Law.

On the other hand, if the AV conduction time is short, the ventricles may contract early in the cardiac cycle, before filling is complete. The atrial may contract against a closed or partially closed valve causing a reversal in blood flow, Improper atrial- ventricular coupling can decrease cardiac output and myocardial perfusion, causing or exacerbating an ischemic state. During an atrial pacing mode that is intended to allow intrinsic AV conduction to occur whenever possible, improper AV coupling may lead to or worsen myocardial ischemia As such, limits to the AV conduction time may be set to ensure that ventricular depolarization occurs after atrial depolarization at sn acceptable coupling interval. Additionally or alternatively, VA time interval limits may be set to ensure that the relative timing of an intrinsic ventricular depolarization and the subsequent atrial pace or sense event is consistent with an acceptable coupling interval Maximum and/or minimum AV conduction lime and/or VA time interval limits may be set at block

21.0. As w ^' ύ) be described below, if ischemia is detected during arc atrial pacing roods, an ischemia response may include measuring t ie AV conduction time and/or VA time interval, If the AV conduction time measurement or a VA time interval measurement crosses a predefined UmU ₅ the ischemia response may furt her include a modification of the pacing mode snd/or pacing control, parameters. For example, the pacing mode may be altered to include LV pacing at a short AV interval to improve AV coupling, particularly when a long AV or short VA interval is observed during atrial-only pacing. At step 215, the device operates in a minimum ventricular pacing mode. Hie minimum ventricular pacing mode includes at least one aimύ pacing mode intended to allow intrinsic AV conduction to occur whenever possible and at least one pacing mode that includes ventricular pacing to ensure ventricular depolarteation when AV block occurs. Accordingly, the pacing mode may switch between an avrial-ønly pacing mode and a ventricular pacing mode (which includes dual chamber modes) as needed during the miaiiaura ventricular pacing mode of operation. However, it is recognised that the device may j at times, deliver other pacing therapies when needed such a-? anli-tachycsrdia pacing therapies, cardiac resynchrønizatioπ therapy, extra systolic stimulation therapy _> arrhythmia prevention therapies,, or other pacing therapies,

At block 220.. the device determines if ischemia is detected- If ischemia is delected, an ischemia response may include adjusting arrhythmia detection parameters and/or programmed arrhythmia therapies as indicated by block 223. A clinician .may program ischemia response arrhythmia detection requirements and/or therapies to he different than those applied during periods of non-ischemic. A clinician may select more aggressive or less aggressive tachycardia detection and therapies depending on patient history and additional physiological sensor data. For example, more aggressive ardiyihrnia detection requirements, such as a lower MD, may be relied upon during a delected ischemic episode since tbs ischemic myocardium may be more vulnerable to arrhythmias, In another example, a programmed therapy may be cancelled or delayed since the ischemia may be the cause of {he arrhythmia. The ischemic myocardium may not respond well to a particular programmed therapy.

Following m ischemia detection at block 22O ₅ an ischemic response may include determining the current pacing mode as indicated at block 225. The pacing mode may be altered in an attempt to alleviate the ischemia, if the device is not currently operating in an atrial-only pacing mode, as determined at block 225, the device may swiich to an atrial-

only pacing mode as indicated at block 230. Myocardial ischemia .may be more prevalent when ventricular pacing is delivered. As such, the ventricular pacing may be minimized or reduced by switching to aa atrial-only pacing mode while continuing to monitor for AV conduction such that ventricular pacing may be restored if needed to ensure ventricular depolarization-

In some embodiments, atrial overdrive pacing may be delivered in response to ischemia detection, as indicated at block 235. AV conduction block may be induced by overdrive pacing in the atrium The pacing .mode may then be switched to a .non-tracking ventricular pacing mode at block 240 to control the ventricular rate so as Io reduce demand in an attempt to alleviate the ischemia. The response of converting to an overdrive atrial pacing rate may be performed independent of the initial pacing mode, determined at block 225, at the time of ischemia detection. In a single chamber device, operating in an. AAl or AAlR mode, ventricular sensing would be performed to ensure ventricular asystole does not occw during atrial overdrive pacing. The atrial overdrive pacing would be suspended in the presence of complete AV block.

If the current pacing mode at the time of ischemia detection is an. atrial pacing mode, as determined at block 225, the ischemia response may include switching to a pacing mode that includes ventricular pacing at block 265. A "ventricular pacing mode " as referred to herein refers to pacing modes that include ventricular pacing and may or may not include atrial pacing. Furthermore, ventricular pacing modes may include pacing therapies such as cardiac resynchronization therapy or extra systolic stimulation which include the delivery of ventricular pacing pulses. Ventricular pacing modes, in particular dual or multi-chamber modes, may act to alleviate ischemia by improving AV coupling or improving cardiac output. Ventricular pacing modes that include LV or biventricular pacing may be more beaeft cial than, ventricular pacing modes that are limited to RV pacing.

In another embodiment, the ischemia response may include measuring an AV conduction time and/or VA time interval during atrial pacing. AV conduction time and VA lime intervals may be measured according to methods known in the art, for example by measuring the time interval between an atrial sensed or paced event and a sensed R- wave. If an AV conduction time or VA time interval limit its crossed, as determined at block 255. fee device converts to a pacing mode that includes ventricular pacing at block 265. If the AV conduction time and VA time interval do not cross pre-sei limits (block

2H ⁾ )j atrial pacing may be maintained without an alteration to the pacing mode. Alternatively, an. atrial pacing rate may be reduced at block 260 to reduce myocardial demand in an attempt to alleviate ischemia, it is recognised that a reduction in pacing rate at block 260 may be made directly in response to an ischemia detection, regardless of the initial pacing mode or other criteria such <& an AV conduction time limit.

Ia another embodiment, an ischemic response includes determining if the atrial rate is high., e.g. during sinus tachycardia, as indicated at block 245. If the atrial rate is high and an ischemia detection is made, pacing control parameters may be altered by reducing a maximum (racking rate and/or reducing a maximum rate responsive sensor-indicated rate as indicated at block 250, Reduction of a maximum ventricular rate may be followed by switching to a ventricular pacing mode at Mock 265 if the initial pacing mode was an atrial-only pacing mode (block 225) and a high atrial rate was detected (block 245). it h recognized that if the initial pacing mode included ventricular pacing at the time of ischemia detection, the adjustment of a maximum tracking rate and/or maximum sensor- indicated rate rø response to the ischemia detection may be made without a pacing mode switch.

If the initial pacing mode includes ventricular pacing, or if the ischemia response includes a mode switch to a ventricular pacing mode at block 265 the ischemia response may include altering the ventricular pacing mode. For example, the ischemia response may include altering a timing control parameter, such as an AV delay or a VV delay, as indicated at block 270. Adjusting a timing interval may include searching for an optimal lime interval which results in an alleviation of the ischemia.

Ia other embodiments the ischemia response may include switching to a non- tracking mode as indicated at block 275. Additionally or alternatively, the ischemia response may include changing s ventricular pacing configuration, for example by switching from Mi ventricular pacing to biventricular pacing or vice versa, as indicated at block 280. Changing a timing parameter at block 270 and/or changing the pacing configuration at block 280 may have a beneficial effect on AV coupling or the synchronization between the right and left ventricles, thereby improving cardiac output and myocardial perfusion. Converting to a .non-tracking mode at block 275 will act Io reduce demand by maintaining the ventricular rate at a lower rale. Thus, apparatus and associated methods have been presented in the foregoing description with reference to specific embodiments for providing a response to the detection of

myocardial ischemia . It is appreciated that various modifications to the referenced embodiments may be made without departing from the scope of the invention as set forth in the following claims.