Method and apparatus for radiofrequency ablation with increased depth and/or decreased volume of ablated tissue

A method of ablating a tissue site includes at least two stages. A first stage involves conducting bipolar ablation between a first pair of electrodes situated in an opposing arrangement on opposing sides of the tissue site to form a pair of opposing first stage ablation regions extending from respective sides of the tissue towards the center. A second stage involves conducting bipolar ablation between a second pair of electrodes situated in a diametrical arrangement with respect to the first stage ablation regions, which forms a second stage ablation region intermediate the pair of first stage ablation regions. The second stage completes the ablation through the entire depth of the tissue site. Since the overall process can accommodate incomplete ablation during the first stage, lower power, reduced ablation times or both may be used during the first stage, avoiding overheating and with a decrease in ablated tissue volume.

BACKGROUND OF THE INVENTION

a. Field of the Invention

The present disclosure relates generally to ablation systems, and more particularly, to a method and apparatus for radiofrequency ablation with increased depth and/or decreased volume of ablated tissue.

b. Background Art

It is known to deliver radiofrequency (RF) energy to a desired target area through an electrode assembly to ablate tissue at the target site. RF ablation may generate significant heat, which if not controlled can result, generally, in undesired or excessive tissue damage. It is particularly difficult to achieve successful RF ablation of live, relatively thick biological tissue, for a number of reasons. The cross-sectional profile (i.e., depth) of ablation is often too shallow, in part because most of the Ohmic heat is generated near the RF electrodes, i.e., at the surface of the tissue. Furthermore, the rate at which heat diffuses deep into the tissue layer(s) is extremely slow, and may be counteracted by the cooling effects due to blood perfusion, for example, during epicardial ablation. As the tissue surrounding the electrodes is being ablated, its Ohmic resistivity increases. This increase in resistivity further exacerbates the problem of non-uniform heating, because at a given current density, the heating intensity experienced by the tissue is proportional to its then-existing Ohmic resistivity.

Moreover, the goals of thick layer RF cardiac ablation are often at odds. It is often necessary to ablate all the way through the thickness of the tissue, while at the same time avoiding tissue overheating and minimizing the volume of ablated tissue. However, the thicker the tissue layer, the less possible it is to achieve adequate ablation through its thickness without overheating the tissue near the RF electrodes (i.e., damaging the tissue in an unacceptable way) and without ablating an undesirable excess volume of tissue.

There are a number of two-electrode or multi-electrode RF ablating devices known in the art, for example as seen by reference to U.S. Pat. No. 7,241,292 entitled CARDIAC ABLATION DEVICE WITH MOVABLE HINGE issued to Hooven. Hooven discloses a transmural RF ablation device. However, Hooven does not address the problems of inadequate ablation depth or excessive ablated tissue volume described above, particularly for a relatively thick biological tissue layer.

There is therefore a need to minimize or eliminate one or more of the problems set forth above.

BRIEF SUMMARY OF THE INVENTION

One advantage of the methods and apparatus described, depicted and claimed herein relates to increased depth of RF ablation lesions while at the same time reducing the overall volume of ablated tissue as compared to known approaches.

This disclosure is directed to methods of ablating tissue site between diverse electrode pairs and the methods include at least two discrete ablation procedure stages. During a first stage, an embodiment of the inventive method involves conducting radiofrequency (RF) energy (e.g., bipolar or unipolar RF energy) ablation between a first pair of electrodes situated in an opposing arrangement on opposing sides of the tissue site. The first stage ablation forms a pair of opposing first stage ablation regions extending from respective sides of the tissue towards the center of the site. However, the first stage may not completely ablate the site through its depth. During a second stage in one form of the invention, the methods involve conducting bipolar ablation but between a second pair of electrodes situated in a diametrical arrangement with respect to the first stage ablation regions. The second stage ablation forms a second stage ablation region intermediate the first stage ablation regions, effectively bridging the pair of first stage ablation regions. To the extent an un-ablated channel, region or gap exists between the pair of opposing ablation regions after the first stage, the second stage completes the ablation through the entire depth of the tissue site. Since the invention can accommodate incomplete ablation or non-transmural ablation lesions (i.e., a lesion due to ablation that does not penetrate the entire depth of the tissue site) during the first stage, lower power, reduced ablation times or both may be used during such first stage, with a resultant decrease in ablated tissue volume. The second stage completes the ablation process throughout the tissue depth, providing increased depth. As note, unipolar or bipolar electrical configurations can be used, or combinations of each type of energy delivery.

A system for RF ablation according to the invention is also presented. In such a system a pair of opposing substantially resilient elongate members couple electrodes for RF energy delivery therebetween. The elongate members can comprise clamping members actuated via a pivot such a scissor-like unit or a pair of members slideably coupled so that one is fixed and the other moveable toward the fixed member. Suitable elongate conductors, switches couple the electrodes to an RF generator. In one form the members are designed and fabricated of biocompatible material and of dimension providing ease of pericardial access via, for example, a sub-xiophoid incision to access one or more pulmonary veins of a subject. In another form, the elongate members comprise at least two pairs of opposing members so that both pairs can engage a target tissue site in a compressive state and at least one can be relaxed in a non-compressive state thereby not engaging the target tissue site. In another form an insulative sheath can be advanced over one or more of the electrodes to selectively insulate same from the target tissue site during subsequent ablation procedures. The electrodes can be coupled to only an RF generator or one or more electrodes can also couple to a cardiac pacing circuit and/or a cardiac sensing circuit or impedance measuring circuit according to various forms of the invention. The pacing and/or sensing circuitry can be used to confirm transmurality of a particular lesion set (e.g., a continuous transmural lesion around a plurality of pulmonary veins) whether or not the set was produced by a system according to the foregoing or in part by another apparatus. That is, pacing or sensing “above” the lesion set (above natural sinus rhythm) should not conduct or capture the myocardium thus indicating continuity of the lesion set. Likewise, sensing above the lesion set should not detect cardiac activity (or at least not at typical amplitude(s)).

These and other benefits, features, and capabilities are provided according to the structures, systems, and methods depicted, described and claimed herein.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views,FIG. 1is a simplified block and diagrammatic view of a system10for conducting RF ablation of a live, biological tissue site12. The system10is configured to increase the depth of lesions created through RF ablation as well as reduce the volume of ablated tissue for a given tissue thickness. The method divides the ablation process into two main stages, which, without loss of generality, will be described in connection with a number of bipolar ablation embodiments. Although there are many possible uses of the present invention, one in particular may involve epicardial ablation, such as, for example, as therapy in connection with the treatment of atrial fibrillation. In this regard, the tissue site12may comprise either a single layer of tissue (e.g., tissue wall), or may alternately comprise a pinched or clamped pair of tissue layers such as described and illustrated in U.S. Pat. No. 7,231,292 to Hooven (noted in the Background), herein incorporated by reference in its entirety. Of course, other applications and purposes are possible, as understood in the art.

With continued reference toFIG. 1, the system10may include an energy source, such as an RF ablation generator14. The RF ablation generator14may serve to facilitate the operation of an ablation procedure and may involve monitoring any number of chosen variables (e.g., temperature of the ablation electrode, ablation energy, duration) and providing the requisite energy source to electrodes, shown in block form as electrode blocks161and162.

The ablation generator14may include a main controller18, a user interface20, an RF power source22and optionally an impedance detection circuit24. The main controller18may be configured to control the operation of the RF power source22in accordance with various inputs (e.g., power setting, ablation duration (time), etc.) received from a user thereof through the user interface20. The controller18may be further configured interact with the impedance detection circuit24to obtain data indicative of an impedance value measured through or using one or more of the electrodes within electrode blocks161,162and to communicate and/or control the user interface20to display or otherwise communicate the measured impedance to a user. The RF power source22may typically produce RF signals in the frequency range of between about 50 kHz and 500 kHz. The controller18may be still further configured to control a switching function block26to make the connections between the RF power source22and the plurality of electrodes contained in the electrode blocks161,162, in a manner that will become more apparent from the detailed description below. The controller18, user interface20, RF power source22, impedance detection circuit24and switching function block26may each comprise conventional components known to those of ordinary skill in the art, except as may be otherwise noted herein. For example, the RF ablation generator14may comprise conventional apparatus, such as a commercially available unit sold under the model number IBI-1500T RF Cardiac Ablation Generator, available from Irvine Biomedical, Inc. Of course, the RF ablation generator14can also comprise other known energy sources. The art is replete with RF ablation generator configurations, designs, implementations and the like, and will therefore not be described in any further detail.

Additional components (not shown) may also be integrated into the system10, such as visualization, mapping and navigation components known in the art, including among others, for example, an EnSite™ Electro Anatomical Mapping System commercially available from St. Jude Medical, Inc., and as also seen generally by reference to U.S. Pat. No. 7,263,397 entitled “METHOD AND APPARATUS FOR CATHETER NAVIGATION AND LOCATION AND MAPPING IN THE HEART” to Hauck et al., owned by the common assignee of the present invention, and hereby incorporated by reference in its entirety. Additionally, an electrophysiological (EP) monitor or display such as an electrogram signal display, or other systems conventional in the art may also be integrated into the system10. It should be understood that embodiments consistent with the present invention may, and typically will, include other features not shown or described herein inFIG. 1for the sake of brevity and clarity.

FIG. 2is a flow chart showing steps of a method for two-stage RF ablation according to an embodiment of the invention. The method begins in step28.

In step28, the method involves conducting, during a first stage, bipolar ablation between a first pair of electrodes situated in an opposing arrangement on opposing sides of the tissue site so as to form a pair of opposing first stage ablation regions. This step may or may not achieve complete transmural ablation (i.e., complete through the depth of the tissue site), and accordingly may leave a channel of un-ablated tissue between the first stage ablation regions. Since the two stage process can accommodate ablation that does not, during the first stage alone, achieve ablation through the complete depth of the tissue, the method contemplates that reduced power levels, reduced ablation durations (e.g., time) or other variations may be used, which has the desirable result of avoiding overheating and yielding a reduced volume of ablated tissue. The method proceeds to step30.

In step30, the method involves determining whether first stage termination conditions have been met. This step may be expressed in a number of different embodiments. In one embodiment, the user (e.g., practitioner) may set an initial power level (e.g., constant power level in watts) and duration, based on, for example, experience, patient/tissue site assessment and/or other factors known in the art. Accordingly, when the manually-set duration expires, the first stage ablation is complete. Alternatively, the termination conditions may involve assessment of the impedance between different pairs of electrodes, for example, when an impedance value is determined to saturate (i.e., increase to a certain level or by a certain amount or by a certain factor and then substantially level off). This impedance assessment may be made manually (i.e., by the user), or may be automated, in which case the main controller18is configured to monitor the impedance, as determined by the impedance detection circuit24, and when the impedance saturation condition is detected, generating a control signal to the RF power source to automatically discontinue power (i.e., or otherwise discontinue application of power, such as by disconnecting the power source via opening of the switching devices in the switching function block26, or in other ways known in the art). This impedance assessment feature will be described in greater detail below with respect to specific electrodes in specific embodiments. In any event, if the termination conditions have not been met (“NO”), then the method branches back to step28, where the first stage ablation continues. However, if the first stage termination conditions are met (“YES”), then the method proceeds to step32.

In step32, the method involves further bipolar ablation, during a second stage, but now between a second pair of electrodes situated in a diametrical arrangement with respect to the first stage ablation regions (i.e., not the opposing arrangement described above). Because of the increased impedance exhibited by the first stage ablation regions, the tissue that may remain unablated after the first stage (i.e., the region between the first stage ablation regions) constitutes the proverbial “path of least resistance” for the ablation energy to be delivered and focused into during this second stage, given the diametrical arrangement of electrodes. The second stage is thus operative to form a second stage ablation region intermediate the opposing first stage ablation regions. The second stage ablation completes the ablation process, ensuring that the tissue site is ablated completely through its depth, without undesired, excessive volume of tissue being ablated.

FIG. 3is a cross-sectional view of a biological tissue site12showing an RF ablation electrode configuration in a first embodiment. As described above, the ablation process is conceptually divided into two stages, and without loss of generality, such process will be described for specific embodiments involving bipolar ablation under the following conditions: (1) the tissue-electrode interface is substantially flat; (2) the electrode width is small relative to its length (e.g., length≧width; preferably, length≧2*width, more preferably, length≧3*width); (3) the heating intensity produced by the electrode extends generally in a direction perpendicular to the electrode (when viewing the cross-sectional profile, e.g., as inFIG. 3), and moreover that the cross-sections shown in the various Figures are removed by at least several electrode-widths from the electrodes tips/forward edges; and (4) the RF electrodes that are not powered during a given ablation stage may be physically removed from tissue electrical contact or alternatively electrically isolated from the tissue, even though such electrodes may continue to be shown positioned on the tissue. With this understanding, a number of embodiments will now be described.

With continued reference toFIG. 3, the upper and lower electrode blocks161and162(e.g., as shown inFIG. 1in block form) may be embodied as a first pair of electrodes34(i.e., electrodes designated1A and1B), a second pair of electrodes36(i.e., electrodes designated2A+ and2B−) and a third pair of electrodes38(i.e., electrodes designated2A− and2B+). The first pair of electrodes34are situated in an opposing arrangement on opposing sides of the tissue site12, while the second and third pairs of electrodes36,38are situated in first and second diametrical arrangements, respectively, i.e., as taken with respect to the first pair of electrodes34or the ablation regions they create. During the first stage, only the first pair of electrodes34(i.e., electrodes1A and1B) are powered in counter-phase by the RF ablation generator14for conducting bipolar ablation.

FIG. 4shows one example of how two power signals40,42may be produced in counter-phase. As illustrated, signals40,42may be sinusoids, and further, may be offset in phase by 180 degrees (π radians). It should be appreciated that through this counter-phase configuration, electrical current will alternately flow in both directions (i.e., from electrode1A→1B, and alternately1B→1A). It should be further understood that this configuration is exemplary only, and one of ordinary skill in the art will recognize a wide variety of alternatives to accomplish bipolar ablation between a pair of electrodes34. The ablation generator14, including the RF power source22, is configured to produce RF power signals in accordance with the particular phasing approach chosen (e.g., here, a counter-phase approach). It should be further understood that still other variations are possible. For example, some unipolar component may be employed during the first stage of ablation.

For such unipolar component embodiments, the electrode configuration described herein would further include an indifferent or reference electrode, disposed either within the body or affixed to the body surface of the subject (i.e., skin patch electrode). The activation for the unipolar component (first stage) may involve, in one embodiment, sequential activation of the electrodes1A and1B of the electrode pair34(i.e., each electrode being vectored to the indifferent/reference electrode). Thus, while one electrode (e.g., electrode1A) is activated, the opposing, non-activated electrode (e.g., electrode1B) is electrically isolated from the tissue site, such as, for example, by removing the non-activated electrode from contact with the tissue site (i.e. spaced from the tissue—myocardium) or otherwise selectively electrically insulating the electrode from the tissue. In another unipolar component embodiment, both electrodes are electrically driven at the same time; however, not in counter-phase but rather with a phase offset greater than 0 degrees and less than 180 degrees (again, a reference/indifferent electrode is used). It bears emphasizing that to the extent unipolar ablation components are used, it would be used during the first stage ablation. The second stage ablation would continue to involve bipolar ablation.

FIG. 5is a cross-sectional view (profile) of the tissue site12at the end of the first stage of ablation. Conducting bipolar ablation during the first stage forms a pair of opposing first stage ablation regions441,442in the tissue at tissue site12. At the end of the first stage, for a relatively thick tissue site12, it is possible that the ablation did not progress all the way through the whole tissue depth, leaving an unablated region46in between the first stage ablation regions441,442. Further application of ablation power to the first pair of electrodes34(i.e., individual electrodes1A and1B) can result in overheating of the already-ablated tissue near these electrodes, since the impedance of the ablated tissue is often significantly higher than the impedance of the non-ablated tissue (i.e., at least within a predetermined, relatively short time after ablation, while such tissue is at an elevated temperature).

FIG. 6is a cross-sectional (profile) view of the tissue site12during/after the second stage of ablation. To address the overheating and potentially undesirable increased volume of ablated tissue, the first stage is discontinued (e.g., the electrodes1A/1B are de-energized and/or removed), and during the subsequent second stage, bipolar ablation is conducted using the second and third pairs of electrodes36,38. In the illustrated embodiment, bipolar ablation is conducted between electrodes2A+ and2B+(i.e., energized in a positive phase such as per signal40inFIG. 4) and electrodes2A− and2B− (i.e., energized in a negative phase such as per signal42inFIG. 4). The increase in the impedance of the first stage ablation regions441,442near electrodes1A/1B facilitates the ablation during the second stage, since this characteristic helps concentrate the RF current in and through the narrow, unablated channel (region46inFIG. 5) between the first stage ablation regions441,442. As a result, the tissue in and around the region46(FIG. 5) is ablated during the second stage, forming a second stage ablation region48, without affecting significant tissue volumes in undesired regions.

In the embodiment ofFIGS. 3 and 5-6, unnecessary ablation near the electrodes2A+/2A− and2B+/2B− is suppressed through a number of approaches: (1) by configuring the electrodes2A+/2A− and2B+/2B− so that they are wider than the electrodes1A/1B; (2) by arranging the electrodes2A+/2A− and2B+/2B− so that they are laterally offset from the first stage ablation regions441,442by at least a predetermined distance; and (3) by replacing each of the electrodes2A+/2A− and2B+/2B− with properly driven multiple electrodes.

In addition, to better concentrate RF current between the first stage ablation regions441,442, in one variation, the first pair of electrodes34(1A/1B) may be electrically isolated from the tissue site12so that these electrodes (1A/1B) will not partially shunt current via the first stage ablation regions441,442. Electrical isolation may be accomplished by physically removing the first pair electrodes34from electrical contact with the tissue site12or alternatively interposing a relatively high impedance material, such as an insulative sheath movable between a first conductive position and a second insulating position, between the first electrode pair34(1A/1B) and the RF power source22of the ablation generator14.

FIG. 7is a timing diagram showing further embodiments that use only four electrodes rather than three pairs (six electrodes) as used in the first embodiment while maintaining efficient ablation. In one further embodiment, during the first stage, designated as interval50on the timeline, the first pair of electrodes32(electrodes1A/1B) are used in the manner described above. However, during the second stage of ablation, designated as interval52, bipolar ablation may be conducted using only the second pair of electrodes36(i.e., electrodes2A+ and2B−). These electrodes may be positioned as inFIG. 3. A still further embodiment involves the concept of sub-dividing the second stage interval52. During a first sub-stage, designated as first interval54, the second pair of electrodes36(i.e., electrodes2A+/2B−) are situated as shown inFIG. 3and are then energized as described above to conduct bipolar ablation. Then, during interval56, the second pair36(electrodes2A+/2B−) are moved and are placed in the positions of electrodes2A−/2B+ ofFIG. 3, in essence in the same place as the now-omitted third pair of electrodes38. The electrodes2A+/2B− (now-moved) are then energized as described above to conduct bipolar ablation.

FIGS. 8-9are cross-sectional views of the tissue site12showing a still further embodiment that involves four moveable electrodes. Before the first stage of ablation, electrodes1A and2A are electrically connected and are placed relatively close to each other and disposed approximately opposite the electrodes1B and2B, which are themselves electrically connected to each other. In effect, the two pairs of electrodes, taken together, are configured in the opposing arrangement described above in connection for the electrode pair34(1A/1B) in the embodiment ofFIGS. 3 and 5-6.

As shown inFIG. 8, during the first stage, bipolar ablation is performed between the electrodes1A/2A (i.e., both driven in a positive phase, such as per signal40inFIG. 4) and the electrodes1B/2B (both driven in a negative phase, such as per signal42inFIG. 4). This first stage forms a pair of first stage ablation regions441,442as shown.

FIG. 9is a cross-sectional view showing the reconfiguration of the electrodes, as compared toFIG. 8, for purposes of conducting the second stage ablation. For the second stage, the two electrode pairs1A/1B and2A/2B are moved away from their first location (i.e., the opposing arrangement) to a second location where they are laterally spread apart, away from the first stage ablation regions441,442. Preferably, in the second location, the electrodes have at least a predetermined minimum lateral spacing between the inner edges of the electrodes and the outer edges of the first stage ablation regions441,442. This new spacing constitutes a dual diametrical arrangement, similar to that shown inFIG. 3for the first embodiment. Next, the two pair of electrodes are electrically reconfigured/reconnected in pairs1A-1B and2A-2B with these pairs being driven in the opposite phases (e.g., electrode pair1A-1B being driven in a positive phase, such as per signal40inFIG. 4, while electrode pair2A-2B is driven in a negative phase, such as per signal42, or switching positive/negative phase orientation would also work). Energizing these reconfigured pairs results in the formation of the second stage ablation region48, as shown. One advantage of this embodiment is the reduced number of electrodes.

One challenge, however, is that this embodiment necessarily involves moving the electrodes and in addition, during the second stage, the tissue site12is no longer clamped across the center near the first ablation regions (i.e., where the two electrodes pairs were previously located during the first stage). To address this challenge, in a still further embodiment, an optional mechanical clamping device60comprising clamp members601and602may be used, separable or partially separable from the RF electrodes1A/1B and2A/2B. As shown, the mechanical clamping device60takes the place vacated by the two electrode pairs after the first stage, thereby ensuring consistent compression through the tissue site12. This is particularly important where the tissue layer includes multiple, individual layers, such as may exist when conducting epicardial ablation (i.e., “pinching” or “folding” tissue, thereby lapping two individual layers).

In another embodiment, only two movable electrodes (and optionally a clamp) are used. For the first ablation stage, these electrodes are put in the central position (34inFIG. 3). For the two substages of the second ablation stage, these electrodes are repositioned into positions36and38(FIG. 3) for one and the other substages, respectively. The optional clamp could be placed in the middle of the tissue site (FIG. 9), when the two electrodes are removed herefrom.

FIG. 10is a side view of an apparatus62configured for use in performing the ablation method described herein. The apparatus62may include a pair of opposing substantially resilient elongate members64,66that couple the electrodes1A/1B,2A+/2A− and2B+/2B− for RF energy delivery therebetween. The elongate members may comprise clamping members64,66(as shown inFIG. 10) actuated via a pivot68in a scissors-like fashion.

FIG. 11is a side view of apparatus70also configured for use in performing the ablation method described herein. The apparatus70may also include a pair of opposing substantially resilient elongate members designated72,74configured so as to be slideably coupled so that one member (e.g., member72) is fixed and the other member (e.g., member74) is moveable in direction76toward/away from the fixed member72.

FIGS. 12-13are partial top and side views, respectively, of a still further 3-arm, scissors-type embodiment, designated apparatus78. The apparatus78includes a lower, clamping member80, a first upper clamping member82and a second upper clamping member84comprised of a pair of individual members841and842. Handle portions86,88and90are used to control the motion of members80,82and84, respectively. The apparatus78includes a main pivot92. The apparatus78provides the means to separately move each clamping arm80,82and84independent of each other. Thus, clamping arms80,82can be moved into position so that electrodes1A/1B apply a clamping force to the tissue site12during a first stage, while the clamping arm84can remain positioned so that electrodes2A+/2A− and2B+/2B− do not contact the tissue site and thus do not exert a clamping force on the tissue. During the second stage, the clamping arm84can be moved into position so that electrodes2A+/2A− and2B+/2B− contact the tissue site and thus exert a clamping force to the tissue. In the embodiments ofFIGS. 10-13, suitable elongate conductors, switches couple the electrodes to the RF generator14. Thus, generally, the apparatus78is configured such that the elongate members comprise at least two pairs of opposing members so that both pairs can engage a target tissue site in a compressive state and at least one can be relaxed in a non-compressive state thereby not engaging the target tissue site. In one form the members of the embodiments described herein are designed and fabricated of biocompatible material and of dimension providing ease of pericardial access via, for example, a sub-xiophoid incision to access one or more pulmonary veins of a subject.

The ablation process may be effectively monitored by checking the impedance between one or more different pairs of electrodes. Taking the first embodiment (i.e.,FIGS. 3 and 5-6) as exemplary, the first stage ablation may be terminated (“first stage termination conditions”) when the impedance between electrode1A and electrode1B approximately saturates (i.e., when, after increasing initially after ablation starts, levels off to an approximately constant or level value). During the second stage, the impedance between a number of electrode pairs may be monitored, for example: (1) between electrodes2A+ and2B+ (i.e., to estimate if any unwanted RF ablation occurs near these electrodes, which would reveal itself by an increase in impedance value); (2) between electrodes2A− and2B− (i.e., also to estimate if any unwanted RF ablation occurs near these electrodes); (3) between electrodes2A+ and2B−, the diametrical arrangement, to estimate if the second stage ablation succeeded (e.g., forming a satisfactory second stage ablation region48); and/or (4) between electrodes2A− and2B+, the diametrical arrangement, also to estimate if the second stage ablation succeeded (e.g., forming a satisfactory second stage ablation region48). The latter two situations may be monitored, for example, to ascertain impedance saturation as described above, and used to determine when to terminate the second stage (“second stage termination conditions”). It should be understood, as alluded to above, that the impedance monitoring may be performed manually (i.e., as observed by the user), or, alternatively, may be implemented in automated monitoring/automatic start/stop of the ablation stages. In the automated embodiment, the main controller18of the ablation generator14may be configured in software to implement the above steps, consistent with the description herein.

It should be understood that the system10, particularly the RF ablation generator14, as described above may include conventional processing apparatus known in the art, capable of executing pre-programmed instructions stored in an associated memory, all performing in accordance with the functionality described herein. Certain aspects of the methods described herein may be programmed (as indicated), with the resulting software being stored in an associated memory and where so described, may also constitute the means for performing such methods. Implementation in software, in view of the foregoing enabling description, would require no more than routine application of programming skills by one of ordinary skill in the art. Such a system may further be of the type having both ROM, RAM, a combination of non-volatile and volatile (modifiable) memory so that the software can be stored and yet allow storage and processing of dynamically produced data and/or signals.

In addition, the ablation electrodes described herein may be coupled to only an RF generator or one or more electrodes can also couple to a cardiac pacing circuit and/or a cardiac sensing circuit or impedance measuring circuit (as described above) according to various forms of the invention. The pacing and/or sensing circuitry can be used to confirm transmurality of a particular lesion set (e.g., a continuous transmural lesion around a plurality of pulmonary veins) whether or not the set was produced by a system according to the foregoing or in part by another apparatus. That is, pacing or sensing “above” the lesion set (above natural sinus rhythm) should not conduct or capture the myocardium thus indicating continuity of the lesion set. Likewise, sensing above the lesion set should not detect cardiac activity (or at least not at typical amplitude(s)).