TECHNICAL FIELD

The present invention relates to embodiments of an ablation catheter suitable for pulsed-field ablation (PF A). In particular, the present invention relates to embodiments of a PFA catheter that may be used for safely performing cardiac ablation procedures, such as, but not limited to, pulmonary vein isolation (PVI), persistent atrial fibrillation ablation, ventricular tachycardiac ablation. The catheter comprises multiple electrodes and delivers pulsed-field energy to achieve irreversible electroporation of cardiac tissue.

BACKGROUND

It is known to use ablation catheters for PVI procedures in the therapy of atrial fibrillation (AF) patients. In such procedures, the pulmonary veins (PV) are electrically isolated from the left atrium by creating contiguous circumferential ablation lesions around the pulmonary vein ostium (PVO) or around their antrum. Thus, irregular atrial contractions can be avoided by hindering undesired perturbing electrical signals generated within the PV from propagating into the left atrium. Ablation catheters may be used to deliver therapy to other tissues, such as, but not limited to: ventricles, right atrium, the body of the left atrium. Additionally, other organs may be treated via use of catheters: lungs, liver, kidneys, etc.

Several types of ablation catheters are available including single point tip electrode catheters, circular multi-electrode loop catheters, and balloon-based ablation catheters using different energy sources. They all lack the ability of producing the required ablations, which safely electrically isolate the arrhythmogenic triggers from the rest of the heart chamber, in a ‘one- shot’ modality, without further repositioning, rotating or moving of the catheter.

It is desirable to further improve ablation treatment by offering catheters and systems which safely achieve a ‘moat’ of electrical isolation in one shot. The concept of a moat of electrical isolation is defined as region of cardiac tissue that surrounds the arrhythmogenic trigger and prevents its propagation to the rest of the heart chamber. For example, without limitation, referring to situations when the arrhythmogenic triggers reside inside a pulmonary vein, an ablation region which completely renders non-viable the tissue located at the vein ostium or antrum, securing transmurality, would represent said moat of electrical isolation. Given that the tissue within the moat is non-viable, excitation originating from triggers within the corresponding pulmonary vein would not conduct to the rest of the left atrium. Such arrhythmogenic excitation would be blocked by the moat and would not capture the body of the left atrium. In the case of atrial fibrillation, if said moat of conduction block was achieved, triggering mechanisms would be eliminated or reduced in frequency of occurrence. Technologies available today achieve said moats of conduction block, or of electrical isolation, by point-by-point (i.e. catheter repositioned in sequential steps which is time consuming and requires skilled physician), by rotation (i.e. catheter active element is rotated to complete the moat) or by repositioning (i.e. catheter active element is repositioned to a neighboring location to complete the moat). In other words, prior-art technologies achieve said moat of conduction block by employing ‘multiple shots.’ Additionally, such technology sometimes requires to redo the procedure as lesion gaps are potentially problematic. While it might be possible to achieve said moat of conduction block in one shot by overpowering the targeted tissue, by doing so collateral organs (e.g. esophagus, lungs, diaphragm, etc.) would be irreversibly damaged. In certain case, these adverse events may pose critical danger to patients. For example, when prior-art technologies overpower structures of the left atrium they may cause atrial-esophagial fistulas. If discovered too late, fistulas may be fatal. Pulsed-field ablation, if designed appropriately, may have the advantage of creating these conduction block/electrical isolation moats in one shot, safely without or with minimal collateral tissue damage.

It is clear that above moat of electrical isolation in one shot additionally requires a good quality. The quality of lesions received by known procedures was previously reduced with regard to rather stiff one-shot balloon catheters which sometimes slipped out of the PV ostium. The disadvantage of such stiff balloon catheters is that they require a larger French size delivery sheath which necessitates larger femoral vein introducer access. Further, it is necessary to provide sufficient quality of the moat for various tissue anatomies, e.g. various PV ostial anatomies. Additionally, a higher quality of the lesions may be achieved if mapping of the progress of the ablation process is improved. Accordingly, one challenge of the ablation procedure is that patient's individual tissue anatomy affects, for example, the tissue contact of the electrodes, if several electrodes are used.

SUMMARY

The above problem is solved by an ablation catheter with the features of claim 1, an operation method of such ablation catheter with the features of claim 14 and an ablation catheter with the features of claim 11.

In particular, an embodiment of an ablation catheter for treatment of a patient's tissue, for example for a PVI procedure at a patient's heart tissue or vein tissue, comprises an elongated catheter shaft and an ablation portion being arranged at a distal end of the catheter shaft with a plurality of electrodes accommodated along the ablation portion, wherein the ablation portion forms a spiral (helix) and comprises a first loop section and a neighboring second loop section, wherein an inner diameter of the first loop section increases towards an inner diameter of the second loop section starting from a first end of the first loop section located opposite the second loop section, wherein the first loop section has a greater stiffness than the second loop section. Catheters employing loop sections or loop segments include, and are not limited to, catheters with continuous or contiguous spirals.

Within the frame of this application the phrase “forms a spiral and comprises a first loop section and a neighboring second loop section” is understood as a structure comprising at least two loop sections arranged in a way that a three-dimensional spiral (helix) or a section of a three-dimensional spiral (helix) is formed. The first loop section and the second loop section could be arranged as a continuous or discontinuous spiral. The first loop section comprises a first end and a second end. Starting from the first end the first loop section extends to the second end that is directly connected to the second loop section. The first end of the first loop section, thus, is opposite of the second loop section. The first end of the first loop section, the second end of the first loop section, a first end of the second loop section and a second end of the second loop section may be arranged either in the same or in a different plane with respect to the central axis of the three-dimensional spiral. In addition, the first loop section and the second loop section itself may be arranged either in the same or in a different plane with respect to the central axis of the three-dimensional spiral. An example of two loop sections forming a continuous spiral is shown in Fig. 1, whereby the first end of the first loop section, the second end of the first loop section and the end of the second loop section is arranged in a different plane with respect to the central axis of the three-dimensional axis and whereby the loops are arranged in different planes with respect to the three-dimensional axis.

The inner diameter of the first loop section and the second loop section increases along these loop sections. In particular, the diameter increases from the first end of the first loop section towards the second end of the first loop section and further along the second loop section, such that the first end of the first loop section has a smaller diameter than the second end of the first loop section. Similarly, the first end of the second loop section has a smaller inner diameter than the second end of the second loop section. Further, the first loop section has a smaller diameter than the second loop section except the connection area of the first loop section and the second loop section where both loops have the same diameter. Depending on which end of the spiral is arranged at the distal end of the catheter shaft, a plunger type ablation catheter or a corkscrew type ablation catheter is formed. The plunger type ablation catheter may be used for ablation in the ventricles or in the atrial area of the posterior left atrium. Alternatively, the diameters of the two neighboring loop section decrease into the direction of the distal end of the ablation portion forming a corkscrew type ablation catheter. The corkscrew type ablation catheter may be used for ablation in the area of the atrial end of the PV.

The diameters of loop sections may be, for example, between 10 mm and 40 mm. More specifically, if used in the left atrium, the widest loop section may have a diameter between 20 - 35 mm, preferably between 25 - 32 mm. The smallest diameter can be 12 - 22 mm, preferably 15 - 20 mm. The diameter is measured from both inner surfaces of opposite loop sections. Depending on the area of application, the form of the ablation portion is adapted to the specific form of the respective area to be ablated. The stiffness of the first loop section having the smaller diameter is greater than the stiffness of the second loop section having a greater diameter. The stiffness may be, but is not limited to, a torsional stiffness, a rotational stiffness, a bending stiffness or a combination of named stiffnesses. The degree of stiffness determines the compression rate of the respective loop section, such that due to application of force along the central axis of the three-dimensional spiral formed by the ablation portion, a loop section with a lower stiffness may be more flexible than a loop section with a greater stiffness and, hence, deforms more severely. Accordingly, the stiffer first loop section may transfer any mechanical force/strain provided by the catheter shaft to the more flexible section of the ablation portion when the ablation portion is pushed into the direction of the tissue. Thereby the more flexible second loop section closely adapts to the anatomy of the tissue to be treated thereby providing a good contact of the electrodes accommodated along the loop sections. In this way, good electrode- to-tissue contact may be provided and maintained in all electrodes during ablation energy delivery and ablation quality is enhanced.

In one embodiment, ablation portion is connected to the catheter shaft by a transition joint. For example, the transition joint may comprise flexible material. In one embodiment said joint may resemble the shape of an elbow- or knee joint.

In one embodiment the ablation catheter further comprises a third loop section, which is a neighboring section to the second loop section, wherein an inner diameter of the third loop section increases along the third loop section starting with a first end that is closest to the second loop section such that all three loop sections form a spiral. Accordingly, the third loop section has a greater diameter than the second loop section except in the area where the first end of the third loop section is directly connected to the second end of the second loop section. The third loop section has a greater stiffness than the second loop section. The stiffness of the first loop section and the third loop section may be equal or similar. The stiffer third loop section having the greatest diameter compared to the first and the second loop section provides a defined position of the ablation portion with regard to the tissue area to be treated. This loop provides a kind of a stop surface for the ablation portion thereby ensuring correct positioning of the ablation portion for treatment. In one embodiment, the ablation portion may comprise at least one further loop section. In an embodiment the respective stiffness of the first loop section, the second loop section and, if applicable, the third loop section is provided by the form and/or the material of a support structure, in particular a wire, of the respective loop section. Within the frame of this specification the support structure of the ablation portion or its respective loop sections comprises the wire-like support structure within the tubing of the ablation portion, the electrodes located at the surface of this tubing and, if applicable, a conductor cable or any similar electrically conducting structure which provides the electrical contact of the catheter's proximal end and the electrodes mounted at the ablation portion. A first support structure, in particular a wire, provided at the first loop section and, if applicable a third support structure, in particular a wire, provided at the third loop section may be of greater stiffness than a second support structure, in particular wire provided at the second loop section. Alternatively, or in addition to that, the form of the support structure, in particular the wire, of the loop sections may be varied. This may include, for example, diameter variation or the introduction of indentations or elevations forming stiffening structures. In an embodiment a first diameter of a wire of the first loop section is greater than a second diameter of a wire of the second loop section, for example the first diameter is between 350 pm and 700 pm and the second diameter is between 200 pm and 349 pm. In that way, the stiffness of the wire may easily be varied. Accordingly, in an embodiment, if applicable, a third diameter of a wire of the third loop section is greater than the second diameter of the wire of the second loop section, for example, the third diameter is between 350 pm and 700 pm and the second diameter is between 200 pm and 349 pm.

Alternatively, or in addition to that, in an embodiment which can be realized cost-effectively the support structure, in particular the wire, of the second loop section comprises at least one indentation and/or the support structure, in particular the wire, of the first loop section and/or, if applicable, the support structure, in particular the wire, of the third loop section comprises at least one elevation at its surface, wherein the elevation may form a stiffening structure. Said indentation may be provided in the form of a slit-shaped indentation which may, for example, run transversely to the axis of the support structure of the second loop section and said elevation may be formed as a rib which may, for example, run parallel to the axis of the supports structure of the first loop section and/or, if applicable, the third loop section. In an embodiment a pitch and/or clearance is provided between the first loop section and the second loop section and, if applicable, between the second loop section and the third loop section. Within the frame of this application the pitch of two neighboring loop sections (or loop/spiral arms in the case of a continuous loop/spiral) is defined as the distance of the outer opposite surfaces of the loop section of each of the two neighboring loop sections, wherein the distance is measured perpendicular to the direction of the tangents local to the respective section between which the distance is measured. The pitch is determined in a stage of the catheter, wherein the three-dimensional form of the ablation portion comprising the at least two loop sections is not restricted by any external force. Within the frame of this application the clearance of two neighboring loop sections is defined in the same way as the pitch measured in a stage of the catheter, wherein the three-dimensional form of the ablation portion comprising the least two loop sections is flattened or almost flattened by external force, e.g. when the catheter is compressed against tissue as shown in Fig. 21b. The first loop section, second loop section and, if applicable, the third loop section are in the same plane with respect to the central axis of the three-dimensional spiral, if the ablation portion is flattened by external forces. In one embodiment the pitch and/or clearance between the first loop section and the second loop section and, if applicable, the second loop section and the third loop section is between 2 mm and 8 mm, in particular between 4 mm and 6 mm.

With regard to above embodiments, the deformation of the ablation portion prior and/or during ablation procedure may be in the way that the three-dimensional form of the spiral is everted/inverted compared with the form which is taken by the ablation portion if a longitudinal force (i.e. a force in longitudinal/axial direction) is provided by a distal movement of the catheter shaft as explained below in detail. By such eversion/inversion of the ablation portion form, a particularly good contact of the electrode accommodated along the ablation portion with the tissue to be treated may be achieved and thereby ablation quality may significantly be increased.

In accordance with an embodiment, the ablation catheter is configured for delivering pulsed- field ablating (PF A) energy to the tissue of a treatment region, in particular atrial or ventricular tissue, via the ablation electrodes. In other words, the ablation catheter may be configured for carrying out PF A. In particular, the ablation catheter may be used to provide cardiac catheter ablation to treat a variety of cardiac arrhythmias including AF. For example, the ablation catheter may be configured for being connected to a multi-channel PF energy generator which is configured for delivering PF energy. The inventive catheter may also be used for different type of tissue, for example veins, lungs, liver, kidneys. It may be used for pulmonary vein isolation (PVI), persistent atrial fibrillation ablation, ventricular tachycardiac ablation and other ablation procedures.

In another embodiment, the ablation catheter is configured for delivering radio frequency ablating (RF ablation) energy to the tissue of a treatment region, in particular atrial or ventricular tissue, via the ablation electrodes.

In an embodiment the first end of the first loop section is directly connected or attached to the distal end of the catheter shaft. Alternatively, a second end of the second loop section or, if applicable, a second end of the third loop section is directly attached to the distal end of the catheter shaft, wherein as indicated above, the second end of the second loop section and, if applicable, the second end of the third loop section is located opposite the first loop section. In other words, the distal end of the catheter shaft is either directly attached to an ablation portion section with the largest diameter or directly attached to an ablation portion section with the smallest diameter, respectively. In particularly, if the distal end of the catheter shaft is directly attached to the first end of the first loop section, the above mentioned inversion of the spiral form may be achieved by moving the catheter shaft longitudinally into the distal direction.

In an embodiment the catheter shaft is generally located at or close to an axis of the spiral form of the ablation portion. The axis of the spiral form of the ablation portion is formed by the longitudinal axis of the spiral that approximately runs through the (imaginary) center point or (imaginary) center axis of the spiral. Locating the catheter shaft at or close to said axis enables to apply a central force, for example in distal direction, that is approximately uniformly distributed over the whole ablation portion. In one embodiment, the ablation portion, and in particular the support structure, in particular the wire, of the first, second and/or third loop section, may comprise a shape memory material. Preferably, the shape memory material is a super-elastic material (such as a superelastic alloy), which is to say that the material is elastic and has a shape memory property. For example, Nitinol is a biocompatible super-elastic alloy that is suitable for the present purpose.

In one embodiment the catheter shaft may have an overall length greater than 1 m from a handle (accommodated at the proximal end of the catheter shaft) to the distal tip of the ablation portion.

In an embodiment, the ablation catheter may further comprise a steerable delivery sheath. Thus, in operation, a position of the ablation portion may be easily adjusted at the target visceral tissue until the contact of each ablation electrode is satisfied.

According to another aspect of the present invention the above problem is solved by an ablation catheter for treatment of a patient's tissue by delivery of high-voltage pulses comprising a catheter shaft and an ablation portion being arranged at a distal end of the catheter shaft with a plurality of electrodes accommodated spaced from one another along the ablation portion, wherein the plurality of electrodes comprises at least one first electrode of a first electrode type and at least one second electrode of a second electrode type, wherein a first surface area of the first electrode type is smaller than a second surface area of the second electrode type. The electrodes may have a length along the longitudinal direction of the respective loop section of 1 mm to 10 mm, in particular 0.5 mm to 2 mm for the mapping electrodes (first electrode type) and, in particular 2.5 mm to 5 mm for the ablation electrodes (second electrode type). The width between neighboring electrodes along the respective loop section may be chosen between 1 mm and 10 mm, preferably between 3 and 6 mm, in order to provide a contiguous ablated area at the patient's tissue. For example, the first electrodes may be used as mapping electrodes measuring electrical signals from the tissue to monitor ablation progress and the second electrodes may be used as ablation electrodes. Due to their smaller surface area the smaller first electrodes provide a better resolution of the electrogram signals thereby increasing the accuracy of the ablation monitoring. Accordingly, in this embodiment, mapping electrodes (first electrodes) may have a similar structure compared with the ablation electrodes (second electrodes) but may have dimensions slightly smaller than the ablation electrodes in order to provide a higher electrical signal resolution.

Each electrode at the ablation portion is electrically connected via one electrode lead to a power supply and a pulse generator provided at the proximal end of the catheter shaft. Further, the catheter may comprise an electronic control unit (ECU) for controlling ablation procedure and/or processing measurement data. In another embodiment, there are two electrode leads provided at the proximal end and the middle section of the catheter shaft. At the proximal end the first electrode lead is connected to the first group of electrodes and the second electrode lead is connected to the second group of electrodes in order to reduce the diameter of the catheter shaft. The catheter shaft size may be compatible with a 7 F to 14 F ID introducer sheath, preferable with an 8.5 F ID sheath.

In one embodiment the mapping electrodes, for example the first electrodes, being configured for receiving electrical signals, e.g. electrical or biopotential, from vascular or atrial tissue. Alternatively, the electrodes, for example the second electrodes, used for ablation in the ablation mode may be used for mapping, namely receiving electrical biosignals, e.g. acquiring electrical or biopotential, from vascular or atrial tissue. During ablation these electrodes are in the ablation mode. This may enable mapping and ablation with a single ablation catheter for PVI as well as ablating some non-PV triggers for AF patients.

Welding may be used to attach a conductor wire to one electrode. In one embodiment a sequence of accommodation of the electrodes along the whole or a part of the ablation portion is such that one first electrode and one second electrode alternate. Alternatively, the ablation portion forms a three-dimensional spiral, wherein a sequence of accommodation of the electrodes along the ablation portion is such that a plurality of second electrodes is accommodated along a spiral section having a greater inner diameter and that a plurality of first electrodes is accommodated along a spiral section having a smaller inner diameter. Accordingly, the mapping is provided at a tissue area which is spaced apart from the ablation portion so that the measurement of the electrical signals is less interfered by the ablation. The detected electrical voltage signals are transmitted via the respective electrode lead to an electronic control unit accommodated at the proximal end of the catheter shaft. Additionally or alternatively, the mapping electrodes may be used to acquire an electrical current. For example, mapping electrodes may be used to also measure local tissue impedance. This can be useful in order to monitor the degree of tissue contact or the progress of PFA effects. During the treatment of the patient, mapping may be conducted prior ablation and after one ablation step or after more than one ablation step in order to observe the ablation result and progress in ablation. In order to ease and improve assessment the received mapping signals of the mapping electrodes or electrodes operating in mapping mode, e.g. electrical potential signals, may be visualized using standard mapping or navigation technology. Thereby, the local conduction properties of the surrounding tissue may be mapped.

As a suitable material, the ablation electrodes may comprise, for example, at least one of gold and a platinum/iridium alloy. Each electrode may additionally contain a temperature sensor to monitor the temperature of the respective electrode.

In one embodiment, the ablation electrodes may be sleeve-shaped or tubular. For example, a diameter of such a sleeve-shaped or tubular ablation electrode may be in the range of 2 - 2.5 mm. In one embodiment, a split electrode design may be used. In this embodiment two electrodes in form of half-shells separated by a gap are arranged at the inner side (facing the body lumen) and the outer side (facing the tissue) of the catheter. The gap may be 0.2 - 1 mm wide, preferably 0.5 mm wide. Such an embodiment is shown in Fig. 18c. Alternatively, electrodes may be solid but coated with insulating material on the inner side facing blood (the body lumen). Parylene, Polyimide or Teflon are examples of a suitable coating. The coating material should be an electrical insulator with high dielectric strength, in excess of 200 kV/mm.

Another aspect of the present invention refers to a method to operate an above described ablation catheter, wherein in a first step the catheter shaft is moved distally within the patient's body until the second loop section or, if applicable, the third loop section is brought into contact with tissue at a pre-defined treatment area surrounding a recess, wherein in a second, consecutive step distal movement of the catheter shaft or of a wire connected to the distal end of the ablation portion forces the first loop section to advance into the recess and relatively to the second loop section and, if applicable, to the third loop section.

For example in the case of a plunger type ablation portion, in a first step the catheter and the ablation portion is moved distally within the patient's body until the second loop section or, if applicable, the third loop section is brought into contact with tissue at a pre-defined treatment area surrounding a recess, for example the outer rim of the pulmonary vein (PV). Further distal movement of the catheter shaft transmits force through the spiral of the ablation to the loop section that is in contact with the tissue leading to a compression of the spiral so that it eventually forms a flattened spiral. Due to further distal movement of the catheter shaft the first loop section is advanced into the recess. In other words, the initial conical shape of the spiral was first compressed and then everted, resulting in an inverted conical shape of the spiral similar to the corkscrew type thereby ensuring good contact to the tissue forming the recess of the treatment area. Further, the distal movement of the catheter shaft may allow exact positioning of the everted spiral in the recess thereby providing good contact of the loop sections with the tissue at the inner wall of the PV ostium or antrum.

In one embodiment, the distal tip of the ablation portion may be connected with a steering wire or center wire which may be manipulated by the handle. This embodiment may particularly be used for the corkscrew-type ablation portion. Accordingly, the center wire may be connected to an actuation mechanism within the handle element. Along the ablation portion, the center wire approximately run along a longitudinal axis of the catheter shaft. A steering plate, steering ring, or other known steering structures may be placed at the distal end of the catheter shaft, which connects to the distal end of the ablation section. The center wire may connect to said steering structure. The center wire may be manipulated such that a longitudinal length of the ablation portion (i.e. its length along the longitudinal axis of the three-dimensional spiral/multiple loop structure) or the loop sections may be steered towards tissue targets, according to the therapeutic needs.

In another embodiment, the distal tip of the ablation portion may be connected with at least two steering wires which may be manipulated by the handle. The steering wires may be manipulated by a pull mechanism resulting in a steering into the direction of the pulled wire. The at least two pull wires may run in separate lumens parallel and opposite to each other along the longitudinal axis of the catheter shaft.

Within the frame of this application in one embodiment in a third step provided after the second step the plurality of electrodes is energized with high-voltage charge-balanced pulsed electric fields which are delivered in a monopolar arrangement or in a bipolar arrangement or in a combination of a monopolar arrangement and a bipolar arrangement. Some examples of applicable waveforms are shown in Fig. 15a and b. Such waveforms, in combination with the loop structures described above, ensure one-shot application of electrical fields that are high enough to generate therapeutic effects capable of creating moats of conduction block, yet lower than ionization thresholds so to avoid arcing. The PFA pulses can be delivered gated by the QRS complex of the cardiac cycle. Alternatively, when ablation targets regions remote from ventricles, PFA pulses may be delivered asynchronously, without QRS gating. The electronic control unit is adapted to switch between monopolar PF energy and bipolar PF energy supply mode.

DESCRIPTION OF THE DRAWINGS

The present invention will now be described in further detail with reference to the accompanying schematic drawing, wherein

Fig. 1 depicts a distal end of a first embodiment of an ablation catheter in a perspective side view;

Fig. 2 illustrates a delivery path for an ablation catheter leading to a pulmonary vein ostium of a human heart;

Fig. 3 and 4 show the distal end of the embodiment of Fig. 1 in a perspective front view and in a side view;

Fig. 5 to 7 depict a distal end of a second embodiment of an ablation catheter in a side view, a front view and in a perspective front view; Fig. 8 to 11 depict a distal end of a third embodiment of an ablation catheter in side views and a front view;

Fig. 12 shows the distal end of the embodiment of Fig. 1 in a perspective side view with some marked dimensions;

Fig. 13 shows part of the electric control of the electrode leads for the embodiment of Fig. 1;

Fig. 14 shows electrical field vector distribution electronically steered to achieve a moat of conduction block;

Figs. 15a - b illustrate exemplary waveforms that are charge balanced (Waveform in Figs. 15 a is of exponential decay type. Waveform in Fig. 15b is of rectangular type.);

Figs 16a - c illustrate the concept of QRS gating (Fig. 16a shows the QRS detector signal (top trace), the PFA trigger signal (middle trace) and the ECG (bottom trace) over several heart beats. Fig. 16b shows a detail into one heartbeat. The PFA trigger signal (middle trace) falls within the refractory period of the cardiac cycle. Fig. 16c shows the PFA pulse artifact, as recorded during a preclinical study.);

Fig. 17 shows an actual histology slide identifying the moat of conduction block (or electrical isolation) around the right superior pulmonary vein (RSPV);

Fig. 18a - c further exemplify a possible electrode distribution on a spiral distal section. Fig. 18a shows the catheter of this invention facing a PV. Fig. 18b shows the catheter of this invention deployed when pressed against PV wall. Note the clearance between spiral arms. Fig. 18c shows an alternative split-tip electrode construction; Fig. 19 shows three schematic impedance curves measured over frequency between two electrodes;

Fig. 20a, b show two examples of measured impedance over frequency;

Fig. 21a to c depict the distal end of the second embodiment of Fig. 5 in a perspective side view in three different steps during positioning at an outer rim of a PV;

Fig. 22 shows thedistal end of the embodiment of Fig. 5 in a different perspective side view;

Fig. 23 shows a possible movement of the distal end of the embodiment of Fig. 5 in a perspective side view;

Fig. 24 depicts a distal end of a fourth embodiment of an ablation catheter in a perspective side view;

Fig. 25 depicts a distal end of a fifth embodiment of an ablation catheter in a front view;

Fig. 26 depicts a distal end of a sixth embodiment of an ablation catheter in a front view; and

Fig. 27a-c depict a distal end of a seventh embodiment in a perspective side view in three different steps during positioning at an outer rim of a PV.

DETAILED DESCRIPTION

Fig. 1, 3, 4 and 12 schematically and exemplarily illustrate a distal portion of an ablation catheter 1 in accordance with a first embodiment. The ablation catheter may be used for PF A, when used with the PFA generator and accessories, and is indicated for use in cardiac electrophysiological mapping (stimulation and recording) and in high-voltage, pulsed-field cardiac ablation. Peak voltages are, for example, without limitation, +/- 1 kV to 3 kV with a pulse width of up to 30 ps. Higher peak voltages (e.g. up to 10 kV) may be used provided the pulse duration is correspondingly shorter (e.g 0.5 us). The catheter 1 has an elongated circular catheter shaft 10, which may connect with a handle comprising a steering mechanism at a proximal end (not illustrated). As a result, the catheter may control deflections of the depicted distal section carrying the ablation electrodes.

At the illustrated distal end of the catheter shaft 10 an ablation portion 12 is arranged, which comprises a plurality of loop sections. The concept of loop sections includes embodiments that use continuous loops or spirals configurations. In this case, the ablation portion has a first loop section 122 at the distal end of the ablation portion 12 (with inner diameter between D3 and D2, see Fig. 12) and an adjacent second loop section 121 (with inner diameter between D2 and DI). The catheter shaft may have an effective length of approximately 115 cm from the distal tip of the ablation portion 12. Each of the second loop section 121 and the neighboring first loop section 122 exhibits ablation electrodes 120 (altogether, for example, 14 electrodes), which are configured for delivering energy to tissue. Although two loops are illustrated in Fig. 1, more can be used. In one embodiment at least a partial third loop section is used to provide sufficient overlap among resulting ablation zones. Said overlap would increase chances of achieving a conduction block moat without drops in lesion continuity, contiguity or transmurality. As an example, see catheter illustration in Fig. 14. The distal section comprises at least 45° of overlap of a 3 ^rd loop section with the previous two sections. In particular, the ablation catheter 1 may be configured for delivering an electrical high voltage PFA signal to tissue via the ablation electrodes 120. For example, the ablation electrodes 120 may consist of or comprise gold and/or a platinum/iridium alloy. Alternatively, electrodes 120 from different loop sections may be positioned so that electrodes of same polarity are aligned. Either the staggering or the polarity -based approach ensures that electrodes of opposite polarities would not collide when the spiral catheter is compressed. In the exemplary embodiment illustrated in Fig. 1, the ablation electrodes 120 of the second loop section 122 are arranged partly in a staggered manner with respect to the ablation electrodes 120 of the first loop section 121.

The loop sections 121, 122 may further exhibit a plurality of mapping electrodes, which are configured for receiving electrical signals from tissue.

Together, the loop sections 121, 122 form a three-dimensional spiral, which form a corkscrew-similar form. Alternatively, they may form a plunger-like configuration, as shown in the second embodiment, for example depicted Figs. 5 - 7. It should be noted that respective diameters of the loop sections 121, 122 are such that the second, more proximal loop section 121 has a greater inner diameter DI (for example 30 mm, see Fig. 12) than the first, more distal loop section 122 (inner diameter D2, for example 24 mm). At the furthest distal tip of the ablation portion 12 the inner diameter D3 is even lower (for example 18 mm). In general, the diameters of loop sections may be, for example, between 10 mm and 40 mm. More specifically, if used in the left atrium, the widest (second) loop section may have a diameter between 20 - 35 mm, preferably between 25 - 32 mm. The smallest diameter can be 12 - 22 mm, preferably 15 - 20 mm. In this embodiment, the second loop section 121 with the greatest diameter is directly attached to the distal end of the catheter shaft 10.

The loop sections 121, 122 may comprise a shape memory material, for example, in the form of an inner support structure, in particular a wire (not illustrated), for example a Nitinol wire as described above. In particular, the loop sections 121, 122 may have super-elastic properties.

The ablation portion 12 may be constrained into an essentially elongate shape for the purpose of delivery to a target region in the human body by means of a (fixed or steerable) delivery sheath 15, which may also be referred to as an introducer sheath. At the target position, upon exiting a distal end of the delivery sheath 15, the ablation portion 12 may then recoil to its original (biased) shape. The length of each electrode 120 in longitudinal direction along the respective loop section 121, 122 is, for example, 4 mm. In general, the electrode length is in the range 1 - 10 mm, preferably 3 - 5 mm. The catheter shaft 10 size may be compatible with an 8.5 F ID sheath and may consist of radiopaque extrudable polymer and, if applicable, a polymer-reinforcing braid. In general, the size of the catheter shaft 10 may be compatible with a 7 F to 14 F ID sheath. The width between neighboring electrodes along the respective loop section may be chosen between 1 mm and 10 mm, preferably 3 - 6 mm, in order to provide a contiguous ablated area at the patient's tissue.

The first loop section 122 has a greater stiffness than the second loop section 121. Accordingly, the second loop section 121 comprises slits or small cuts/indentations at the surface of the support structure, in particular the wire, forming the loop. The indentations or slits can be short and perpendicular segments to the longitudinal axis of the loop of the wire.

Alternatively, the greater stiffness of the first loop section 122 may be provided by ribs at the surface of its support structure, wherein the second loop section does or does not comprise the indentations described above.

The above stiffness variation of the loop sections 121, 122 leads to a better adaption of the ablation portion 12 to the anatomy of the tissue to be treated and thereby to a better electrode contact.

Fig. 2 schematically and exemplarily illustrates a delivery path for an ablation catheter 1 leading to a pulmonary vein ostium (PVO) of a human heart. For orientation, the inferior vena cava (IVC), the right atrium (RA), the right ventricle (RV), the left atrium (LA), the left ventricle (LV), as well as pulmonary veins (PV), each with a PVO, are shown. The large black arrows indicate a delivery path passing through the IVC, the RA, transseptally through the septal wall (SW), and into the LA. Finally, using appropriate deflection means, catheter 1 is steered to PVO regions. There, the corkscrew type ablation catheter may be used for ablation in the area of the atrial end of the pulmonary vein close to PVO. The form of the ablation portion 12 is configured such that it fits to the dimensions of the targeted PVO. Alternatively, corkscrew-type catheters may be used to ablate at the SVC or at Appendages, such as the left or right atrial appendages (LAA or RAA).

The second embodiment of an ablation catheter 2 shown in Fig. 5 to 7, 21a to c, 22, and 23 is adapted to the use for ablation in the atrial area of the left atrium LA surrounding the PVOs, or located between PVOs (e.g. posterior LA wall). Alternatively, catheter 2 may be well suited for ablations on ventricular (RV or LV) walls, or in the RA (e.g. free RA wall, Tricuspid Valve annulus, etc.). The ablation portion 22 comprises three loop sections 221, 222 and 223 with a plurality of ablation electrodes 220 (and, if applicable also with mapping electrodes). The form and positioning of the ablation electrodes 220 is similar to the electrodes 120 of the first embodiment. The ablation portion 22 is formed like a three- dimensional spiral having the form of a plunger (if no external mechanical force is exerted onto the ablation portion 22). The ablation portion comprises a first loop section 222 having the smallest inner diameter, a second loop section 221 having a diameter greater than the one of the first loop section 222 and a third loop section 223 having the greatest inner diameter. The first, second and third loop sections 222, 221, 223 are adjacent to each other as shown in the figures and a first end of the first loop section 222 is directly attached to the catheter shaft 20 at a transition joint 21 (see Fig. 21a). The first loop section 222 forms the proximal end of the ablation portion 22 and the third loop section 223 forms its distal end (if no external mechanical force is exerted onto the ablation portion).

As shown in Fig. 22 (not showing the electrodes for more clarity) the first diameter dl of the wire of the first loop section 222, for example dl is between 350 pm and 700 pm, is greater than the second diameter d2 of the wire of the second loop section 221. For example, the second diameter d2 is between 200 pm and 349 pm. Further, the diameter d3 of the wire of the third loop section 223 is greater than the diameter of the wire of the second loop section 221, for example d3 is between 350 pm and 700 pm. Accordingly, the first and the third loop sections 222, 223 have a greater stiffness than the second loop section 221. The different flexibility of the different loop sections provided by the different wire diameter.

The helical/spiral, expanded structure of the ablation portion 22 of this embodiment may be placed inside the chamber of the heart and e.g. over the opening of the PV as shown in Fig. 21a. By pushing the catheter shaft 20 in distal direction, the third loop section 223 contacts the outer rim 250 of the PV. Contact of the ablation electrodes 120 may be confirmed by indicators such as impedance and electric signals provided by ablation electrodes, if applicable or a force sensor of the catheter. As shown in Fig. 21b, upon further distal movement of the catheter shaft 20, the ablation portion 22 is compressed, such that the first loop section 222, the second loop section 221 and the third loop section 223 form a flat spiral. Upon further distal movement of the catheter shaft 20 the catheter shaft further pushes the stiff first loop section 222 and advances into the PV together with the second loop section 221 until it reaches the position shown in Fig. 21c. The distal tip of the catheter shaft 20 that was initially (Fig. 21a) located proximal from the ablation portion 22 penetrates into the PV and in that moves past the third loop section 223 and the second loop section 221. The ablation portion 22 that was initial formed plunger-like (Fig. 21a) has, after insertion into the PV (Fig. 21c), a corkscrew-like form. In this way, caused by the reduced stiffness of the second loop section 221 an eversion of the helical/spiral loops is facilitated and sustained which leads to a better adaptation to the different individual anatomical shapes of the PVs for better tissue contact of the electrodes. Specifically, the outermost loop (third loop section 223) is sufficiently firm to sit on the PV antrum/ostium, while the middle loop (second loop section 221) is softer and more flexible to allow the helix to evert. The inner most loop (first loop section 222) forms a stiffer section that gives the everted helix stability.

As shown in Fig. 23 with regard to the second embodiment, while helical loops are in the everted state or in the inverted position, the catheter shaft 20 which is attached to the innermost loop (first loop section 222), may be steered to position the axis of the ablation portion 22 in alignment with the longitudinal axis of the PV. Since the steering or deflection point of the catheter shaft 20 in this everted state or inverted position is closer to or within the ablation portion 22 of the helical loops, finer or smaller movement of the first loop section 222 and the second loop section 221 can be made to achieve improved tissue contact for various anatomical topography.

There is a third embodiment shown in Fig. 8 to 11 similar to the first embodiment. Without limitations though, elements of this embodiment (e.g. center wire 31, for spiral expandability or compression) may be used with other type of spiral catheter. Additional to the construction of the first embodiment the third embodiment of an ablation catheter 3 comprises a center wire 31, used to facilitate expandability or compression of the distal section. Center wire 31 is connected with the distal tip of the ablation portion 32. Ablation portion 32 carries ablation electrodes 320. Center wire 31 is running approximately along the longitudinal axis of the spiral formed by the ablation portion and its two loop sections 321, 322. Center wire 31 enters and runs inside catheter shaft 30. At the proximal end of the catheter, center wire 31 connects with actuating element associated with or integrated in the catheter handle. The center wire 31 may be manipulated such that a longitudinal length of the ablation portion 32 (i.e. its length along the longitudinal axis of the three-dimensional spiral of the ablation portion 32) and, accordingly the diameters of the loop sections 321, 322 may be changed and adapted to the therapeutic needs and the local situation. In the drawing of Fig. 8 the longitudinal length of the ablation portion is greatest compared with the drawings of Fig. 9 and 10 because the center wire pushes the distal tip of the ablation portion 32 in distal direction. Accordingly, the diameter of the loop sections 321, 322 is smallest. Fig. 10 shows the shortest longitudinal length of the ablation portion 32 of the ablation catheter 3. This is achieved by pulling the center wire 31. The ablation catheter 3 shown in Fig. 9 has a nominal longitudinal length of the ablation portion 32, which is between those of Figs. 8 and 10. Hence, the diameter of the loop sections 321, 322 is greatest in Fig. 10 and smallest in Fig. 8.

Fig. 24 shows an ablation catheter 4 according to a fourth embodiment that comprises a first loop section 422, a second loop section 421 and a third loop section 423. The ablation catheter 4 is similar to the ablation catheter 2 of the second embodiment (also with regard to the electrodes which are not shown in Fig. 24) but compared to the second embodiment the pitch of the loop sections 422, 421, 423 is so small, that a flat spiral is formed. As one can derive from Fig. 24, the first diameter of the wire of the first loop section 422 is greater than the second diameter of the wire of the second loop section 421. Additionally, the third diameter of the wire of the third loop section 423 is greater than the second diameter of the wire of the second loop section 421. Accordingly, the ablation portion 42 of the ablation catheter 4 may be everted similar to the second embodiment so that steps similar to Fig. 21b and 21c may be realized.

A fifth and a seventh embodiment of an ablation catheter 5, 7 shown in Fig. 25 and 27a to c are similar to the second embodiment described above except the electrode structure. Accordingly, the reference numbers with similar last two numbers correspond to the second embodiment having the same structure and function. The fifth and seventh embodiments also comprise the same wire diameter variation as the second embodiment. The ablation portion 52, 72 comprises a plurality of ablation electrodes 520, 720 having a greater surface area than a plurality of mapping electrodes 530, 730 located at the first loop section 522, 722. For example, the mapping electrodes 530, 730 have a length of, for example, 0.5 to 1.0 mm in longitudinal direction along the ablation portion 52, 72 whereas the ablation electrodes 520, 720 have a length of, for example, 3 to 5 mm in longitudinal direction along the ablation portion 52, 72. The mapping electrodes 730 of the seventh embodiment shown in Fig. 27a to c are grouped so that every two electrodes are positioned closer to each other. In contrast, the mapping electrodes 530 are equidistantly spaced to each other. When everting the spiral structure of the ablation portion 52, 72 as shown in Fig. 27c, the mapping electrodes 530, 730 are located at the distal end of the catheter so that above explained mapping is provided with some distance from the ablation area formed by the ablation electrodes 520, 720. This enables mapping simultaneously with ablation.

In a sixth embodiment which is also similar to the second embodiment, the plurality of mapping electrodes 630 are arranged such with the plurality of ablation electrodes 620 along the ablation portion 62 that one mapping electrode 630 alternates with one ablation electrode 620. In this embodiment, mapping is provided with ablation in subsequent steps.

Due to their small size the mapping electrodes 530, 630, 703 may be used for localized, high density mapping.

Reliable full ablation along a whole circumference is achieved with the first to seventh embodiment at their respective position within the heart or the vein to which the form is adapted.

In order to spare adjacent tissue and shorten ablation time, the pitch of neighboring loop sections is chosen between the ionization threshold and the therapeutic threshold when they are correctly positioned at the treatment area. Referring to the first embodiment shown in Fig. 12, the first pitch, or clearance, si of the second loop section 121 and the first loop section 122 is approximately 5 mm and the second pitch, or clearance, s2 of the first loop section 122 and the furthest distal end of the ablation portion 12 is approximately 5 mm, as well. In general, the pitch, or clearance, should be between the ionization (2 mm) and therapeutic thresholds (up to 8 mm). It is important that the angular offset between most distal and most proximal electrodes on any of the above described catheters exceeds 2*360°, preferably it is 2*360°+45° (i.e. two full loops plus 1/8 ¹¹¹ of a third loop).

The ablation procedure using one of the ablation catheters 1 to 7 may start after the ablation portion 12, 22, 32, 42, 52, 62, 72 is in the correct position relative to the targeted tissue, for example at a PVO. The ablation electrodes 120, 220, 320, 420, 520, 620 will provide pulsed electric RF field in a monopolar or bipolar arrangement. Peak voltages are, for example, without limitation, +/- 1 kV to 3 kV with a pulse width of up to 30 ps. Higher peak voltages (e.g. up to 10 kV) may be used provided the pulse duration is correspondingly shorter (e.g 0.5 us). The pulse width may be 12 ps (between 0.5 - 30 ps) forming a pulse train comprising up to 500 pulses/train. Any of the waveforms illustrated in Figs. 15 may be used.

Without limitations, as an example, waveform in Fig. 15a shows biphasic exponentially decaying voltage pulses suitable for PFA treatment. Over the entire waveform complex, the exponential decays achieve a charge-balanced goal, needed to minimize chances of bubbling, arcing or undesired tissue stimulation. Such waveforms may be achieved by using high-voltage output stages which are AC-coupled to the ablation electrodes 120, 220, 320, 420, 520, 620, 720. The two biphasic pulses shown in Fig. 15a form a pulse train, which could be repeated N-times. The biphasic pulses consist of a positive section PP and a negative section PN. As shown in Fig. 15a a positive biphasic pulse is followed by an inverse negative biphasic pulse. The interphase delay Ii is the time between the end of the negative section PN of the first biphasic pulse and the start of the positive section of the following pulse. As defined above the pulse width P corresponds to the length of the positive/negative section PP/PN, if biphasic pulses are used. The next pulse train starts after the interpulse delay b.

Similarly, Fig. 15b shows an example of suitable PFA waveforms which have rectangular shapes. The rectangular pulses as shown in Fig. 15b are characterized by the voltage peak V and the pulse width P. A positive rectangular pulse is followed by a negative rectangular pulse after the interphase delay Ii. The two pulses shown in Fig. 15b form a pulse train, which is repeated N-times. The next pulse train starts after the interpulse delay h. These waveforms are also charge balanced. Such charge-balanced rectangular waveforms may be achieved by using DC-coupled high-voltage output stages with reasonably accurate control of the positive and negative phase amplitude and duration. As a result, the net charge (current amplitude*pulse width) can be controlled to achieve net balancing.

Figures 16 show QRS-gated output waveforms. A typical lead-I ECG waveform 1601a is shown in Fig. 16a. The output of the QRS detector is illustrated as signal 1602a. The trigger of the PFA waveform is shown as signal 1603a. Fig. 16b provides a zoomed-in view of Fig. 16a. The ECG waveform 1601b is represented over one cardiac cycle. Its R-wave 1604 is detected by the QRS detector output 1602b. After a programmed delay 1605, the PFA waveform trigger 1603b is turned on. In this embodiment, delay 1605 is shown to be about 70 ms. Delay 1605 may be between 20 - 150 ms, depending on the heart rate. It is important to make sure PFA pulses are applied within the refractory period of the heart. As Fig. 16b shows, in this particular example the train of pulse ends before the beginning of T wave 1606. Figure 16c shows an example of PFA pulses artifacts, as recorded with a standard cardiac recording system. R wave 1604c is seen being followed by artifacts 1607 caused by delivery of PFA pulses. Artifacts 1607 safely end before the beginning of T waves 1606c. The process described above delivers one train of pulses within one cardiac cycle. In the above example, 10 pulses/train were delivered using waveform 1501 from Fig. 15a. Persons of skill in the art may modify the above approach by using other known parameters without departing from the essence of this invention. For example up to 500 pulse trains may be provided. However, although not required, it is preferable to select a number of trains so that to keep the PFA application time to greater than 1 s (to allow of cell membrane poration) but less than 2 min (to avoid long procedures). The interphase delay may be 1 - 100 ps. The interpulse delay may be 0.1 ms or 100 ms.

The electric field generation (in particular voltage, current and impedance) is monitored by an electronic control unit (ECU) 70 which is connected to the leads 61 of the electrodes 120, 220, 320, 420, 520, 620, 720 and produced by a waveform generator 50 (see Fig. 13). Figure 14 also shows connectivity that can be used to generate monopolar or bipolar electric fields. ECUs in Figs. 13 and 14 may control application of PFA fields with a goal of achieving wide coverage of the tissue space in between catheter loops or spirals. Figure 14 illustrates a catheter 1401 (such was #1, #2 or #3 from Figs. 1 - 10) with its electrodes driven by ECU 1403. ECU 1403 can be controlled to deliver field vectors 1402 that cover the tissue zone in between catheter 1401 spiral arms/loops. By doing so, a moat of conduction block/electrical isolation is more likely to be achieved.

In the bipolar arrangement neighboring (adjacent) electrodes 120, 220, 320, 420, 520, 620, 720 may be paired along the loop sections, for example 121, 122, 212, 222, 321, 322 or across two neighboring loop sections, for example 121 and 122; 221 and 222; 321 and 322. Further, the electrodes 120, 220, 320, 420, 520, 620, 720 may be used in monopolar arrangement. In this case, a ground pad 1404 may be provided at the surface of the patient's body. Alternatively, reference electrodes associated with the catheter shaft may be used.

In order to switch between different bipolar arrangements or between monopolar and bipolar arrangement, the ablation catheter 1, 2, 3, 4, 5, 6, 7 may comprise a switch unit 60 connected to and controlled by the ECU 70. The switch unit 60 provides the respective phase of the pulsed electric field provided by the waveform generator 50 to the predefined electrode lead 61 and thereby to the predefined electrode 120, 220, 320, 420, 520, 620, 720, wherein each electrode lead 61 is electrically connected to one particular electrode 120, 220, 320, 420, 520, 620, 720 at the ablation portion 12, 22, 32, 42, 52, 62, 72. The switch unit 60 comprises a switch matrix and may realize any configuration of phase distribution, for example, such that two neighboring electrodes along the loop sections or across the loop sections are paired to achieve the aforementioned uniform moat of conduction block. Any other configuration is possible. The switching signal and configuration information is provided by the ECU 70. ECU 70 further may provide data processing of electrical or biopotential data or impedance data acquired by mapping electrodes of ablation catheters Ito 7. As indicated above mapping electrodes 530, 630 , 730 located in the ablation portions 52, 62, 72 may comprise mapping electrodes for determining the electrical potential of the surrounding tissue in order to observe the ablation progress at pre-defined time points during ablation procedure. Alternatively, the ablation electrodes 120, 220, 320, 420 may be switched into the mapping mode and back into the ablation mode. Further, the impedance between neighboring electrodes or across two different, neighboring loop segments may be determined prior to delivery of PFA energy. Thereby impedance (monopolar or bipolar) is monitored whether the neighboring loop segments and hence the electrodes of these segments are located in a sufficient distance to the other loop segment or electrode, respectively. By monitoring impedance, ECU 70 or 1403 may alert the user when any two electrodes are too close, with respective inter electrode distance falling below the ionization threshold. Conversely, users may be alerted when impedance measurements indicate that the inter electrode distance exceeds the therapeutic threshold.

As indicated above, the catheter shaft 10, 20, 30, 40, 50, 60, 70 may comprise two lumens separated by a material, e.g. Kapton®, with a dielectric strength greater than a dielectric threshold for high-voltage PFA pulses. The first lumen may retain, for example, 7 electrode leads 61 providing the first polarity and the second lumen may retain, for example, 7 electrode leads 61 providing a second polarity thereby reducing the overall diameter of the catheter shaft.

The above explained embodiments of ablation catheters realize IRE in order to prevent spread of electrical signals (i.e. achieve conduction block) that gives rise to the cardiac arrhythmia along a contiguous area with improved safety, as it is believed to spare adjacent tissues (e.g. nerves, vessels, esophagus), and with shorter ablation time. Figure 17 illustrates such moat of conduction block, or electrical isolation. The right superior pulmonary vein 1701 is seen at the center of the picture. After application of PFA pulses according to this invention (totaling cumulative PFA application time of about 90 s/PV), a continuous, contiguous and transmural lesion was achieved. The lesion perimeter 1402 is illustrated. The moat of conduction block, or electrical isolation, 1403 completely covers the cardiac tissue zone between RSPV 1401 and the lesion border 1402. Electroanatomic mapping confirmed lasting chronic isolation of pulmonary veins. Fig. 18a shows the catheter of this invention facing a pulmonary vein atrium. Fig. 18b shows the catheter of this invention deployed when pressed against pulmonary vein wall. As indicated by line cl and c2 the angular separation between the most distal electrode 1802 and the most proximal electrode 1801 exceeds 2*360°, or 720°. Fig. 18c shows an alternative split-tip electrode construction with an inner electrode facing the blood and an outer electrode facing the tissue.

Fig. 19 shows three schematic impedance curves measured over frequency between two electrodes. The impedance could be measured starting with a low frequency flow of 10 kHz up to a frequency fhigh of 500 kHz. A pronounced impedance curve as the topmost curve ranging from Zi to Z4 indicates a good tissue contact between the two electrodes. A flat lowermost impedance curve in the lower range of Z3 to Z5 indicates contact between the two electrodes. The flat impedance curve in the middle ranging from Z2 to Z4 indicates bad tissue contact between the two electrodes. For example, without limitation, following thresholds may be used:

1. Good tissue contact - at frow (e.g. 10 kHz) Zi is in the range 100 - 500 ohm, depending on electrode size and tissue properties. At 1'HIGH (e.g. 500 kHz) Z4 is at least 20% lower than Zi (S-curve).

2. Poor contact - at frow (e.g. 10 kHz) Z2 is in the range of 80-400 ohm, depending on electrode size and blood properties. At 1'HIGH (e.g. 500 kHz) Z4 is at most 20 % lower than Z2, typically only 10% or less lower (flat curve). As shown in Fig. 20a, under poor electrical contact conditions, the bipolar impedance decreases from about 113 ohm at 10 kHz to about 110 ohm at 500 kHz. The phase varies only slightly, increasing from about -4° to 2°.

3. Electrodes in contact - at frow (e.g. 10 kHz) Z3 is in the range 0 - 300 ohm, depending on the amount of contact, electrode size, blood properties. At IHIGH (e.g. 500 kHz) Z5 is at most 20% lower than Z3, typically only 10% or less lower (flat curve). As shown in Fig. 20b, when electrodes collide and make good electrical contact, Z5 is low, between 4 - 9 ohm, while phase increases with frequency. At 500 kHz, the phase is approximately 66°, indicating a mostly inductive electrical characteristic given by the electrode wires. It will be apparent to those skilled in the art that numerous modifications and variations of the described examples and embodiments are possible in light of the above teaching. The disclosed examples and embodiments are presented for purposes of illustration only. Other alternate embodiments may include some or all of the features disclosed herein. Therefore, it is the intent to cover all such modifications and alternate embodiments as may come within the true scope of this invention.