Measurement of cardiac wall tissue thickness during ablation

Abstract

To provide a method for estimating a thickness of cardiac tissue undergoing ablation.SOLUTION: A method for estimating a thickness of cardiac tissue undergoing ablation includes the steps of (a) applying a sequence of ablation pulses to a region of the cardiac tissue, so as to create a lesion, and (b) estimating, for a given ablation pulse in the sequence, an incremental depth added to the lesion due to the given ablation pulse. A cumulative depth of the lesion is estimated based on the cumulative depth prior to the given pulse, and on the incremental depth. An amplitude of an electrogram signal at the region is assessed after applying the given ablation pulse, and, if the amplitude exceeds a predefined threshold, an estimate of the thickness is set to be at least the cumulative depth.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates generally to cardiac sensing and ablation, and more particularly to estimating properties of cardiac wall tissue undergoing ablation. [Background technology]

[0002] A number of methods can be used to estimate the thickness of the heart wall, such as ultrasound, fluoroscopy, and MRI imaging. The estimated wall thickness can be further correlated with electrophysical signals to estimate the damage of the heart wall tissue. For example, Takeshi Sasaki et al., in "Myocardial Structural Associations with Local Electrograms: A Study of Post-Infarct Ventricular Tachycardia Pathophysiology and Magnetic Resonance Based Non-Invasive Mapping", Circulation Arrhythmia and Electrophysiology, December, 2012, 5(6):1081-1090, describe a significant association between the left ventricular wall thickness seen on MRI, the thickness of the post-infarct scar, and the location of the scar in the wall, as well as a significant association between the local endocardial electrogram bipolar / unipolar voltage, duration, and deflection on electroanatomical mapping. Summary of the Invention [Means for solving the problem]

[0003] One embodiment of the present invention provides a method for estimating thickness of cardiac tissue undergoing ablation, comprising: (a) applying an ablation pulse sequence to a region of cardiac tissue to form a lesion; and (b) estimating, for a given ablation pulse in the sequence, an incremental depth added to the lesion by the given ablation pulse. The cumulative depth of the lesion is estimated based on the cumulative depth prior to the given pulse and the incremental depth. The amplitude of the electrogram signal in the region after application of the given ablation pulse is evaluated, and if the amplitude exceeds a predetermined threshold, the thickness estimate is set to be at least the cumulative depth.

[0004] In some embodiments, the method further includes terminating the ablation pulse sequence in response to detecting the amplitude of the electrogram signal falling below a predetermined value, hi some embodiments, the predetermined value is equal to a predetermined threshold.

[0005] In one embodiment, evaluating the amplitude of the electrogram signal includes measuring the amplitude during intervals between ablation pulses, hi another embodiment, the process is performed automatically under processor control.

[0006] In some embodiments, applying the ablation pulse sequence includes planning the sequence such that the expected incremental depth is less than a predetermined incremental value.

[0007] In some embodiments, the method further includes modifying the ablation pulse sequence after establishing the thickness estimate to modify the incremental depth caused by the next ablation step.

[0008] In one embodiment, assessing the amplitude of the electrogram signal comprises measuring the electrogram signal using the same electrodes used to apply the ablation pulse, hi another embodiment, applying the ablation pulse comprises applying an irreversible electroporation (IRE) pulse.

[0009] In yet another embodiment, applying an ablation pulse includes applying radio frequency (RF) ablation.

[0010] In accordance with another embodiment of the present invention, there is further provided a system for estimating thickness of cardiac tissue undergoing ablation. The system includes an interface and a processor. The interface is configured to output an ablation pulse sequence for application by a probe to a region of cardiac tissue to form a lesion. The processor is configured to, for a given ablation pulse in the sequence, (i) estimate an incremental depth added to the lesion due to the given ablation pulse, (ii) estimate a cumulative depth of the lesion based on the cumulative depth before the given pulse and the incremental depth, and (iii) evaluate an amplitude of the electrogram signal at the region after application of the given ablation pulse and set the thickness estimate to at least the cumulative depth if the amplitude exceeds a predetermined threshold. [Brief description of the drawings]

[0011] A more complete understanding of the present disclosure will be obtained from the following detailed description of the embodiments of the present disclosure when read in conjunction with the drawings. [Figure 1] FIG. 1 is a schematic diagram of a catheter-based system for irreversible electroporation (IRE) therapy and electrogram sensing, in accordance with an exemplary embodiment of the present invention. [Diagram 2] 1 is a graph that illustrates a schematic representation of lesion depth and electrogram signal amplitude at the location of the lesion site as a function of ablation duration, in accordance with an exemplary embodiment of the present invention. [Diagram 3] 2 is a flow chart that generally illustrates a method for estimating thickness of cardiac wall tissue undergoing ablation using the system of FIG. 1, in accordance with an exemplary embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Overview Lesions in cardiac tissue of patients undergoing an ablation procedure can be created using irreversible electroporation (IRE) or other types of ablation energy, such as radio frequency (RF), that can be applied using a catheter. In IRE ablation, the catheter is manipulated so that electrodes disposed on the distal end of the catheter are in contact with the tissue. A high voltage bipolar pulse is then applied between the electrodes, and the high electric field pulses generated in the tissue cause cell death and lesion generation. In RF ablation, alternating RF currents are applied to the tissue by one or more electrodes, causing cell death by heat.

[0013] Estimation of wall thickness at tissue locations undergoing ablation can be important to avoid serious side effects of ablation, such as perforation of the heart wall. In addition, the estimation can help shorten the ablation procedure, using the estimated wall thickness as a baseline estimate at these locations, where nearby locations are ablated more rapidly. As shown in the Background, various imaging methods can be used to measure heart wall thickness. However, using such methods during an ablation procedure adds substantial complexity to the ablation system and clinical workflow.

[0014] Exemplary embodiments of the invention described below provide systems and methods for acquiring and analyzing electrograms in real time from cardiac tissue to estimate the thickness of wall tissue undergoing ablation. The disclosed system can apply an automated stepwise ablation / estimation process to achieve improved estimation that can be used to achieve optimal lesion depth (e.g., a minimal lesion depth sufficient to eliminate arrhythmias).

[0015] In some exemplary embodiments, a catheter ablation electrode, such as an IRE or RF ablation electrode, is further used to acquire the electrogram. In other exemplary embodiments, other types of ablation energy (e.g., laser, cryogenic) are applied with a catheter, and other sensing electrodes are placed on the catheter that is used to acquire the electrogram. In general, any catheter-based ablation method that creates a lesion in tissue, the depth of which can be modeled as a function of ablation duration, can be used with the disclosed ablation / estimation process.

[0016] In an exemplary embodiment, the disclosed ablation / estimation process is initiated by a physician operating a processor-controlled system to ablate tissue for a given duration to create a lesion of a predefined planned depth, with the required duration estimated using one of the lesion depth models described above associated with the ablation method. Electrodes are then used to detect electrogram signals from the ablated tissue. For IRE and RF ablation, the same electrodes can be used to perform electrogram detection.

[0017] If the measured electrogram signal has an amplitude above the predetermined threshold, the processor determines that the tissue thickness is at least as thick as the predetermined damage depth (i.e., the processor sets a lower limit on the thickness). In such a case, the processor directs the application of an additional duration to ablate the tissue to increase the damage depth by an incremental amount, and then acquire an electrogram to re-estimate the accumulated damage depth. The above steps may be repeated until the amplitude of the electrogram signal falls below the predetermined threshold.

[0018] Thus, as the disclosed ablation / estimation process progresses, a lower limit of the estimated wall thickness gradually grows (as this lower limit is equal to the accumulated damage depth in tissue that still has an electrogram signal above a predetermined threshold heart wall tissue). Eventually, the amplitude of the electrogram signal falls below the predetermined threshold. At that point in the process, the processor estimates the wall tissue thickness that is the accumulated damage depth that lowers the signal amplitude, and responsively terminates the ablation at that location.

[0019] The physician controls the estimation accuracy by selecting the duration of the addition such that the planned incremental lesion depth is less than a pre-determined incremental value that is deemed safe.

[0020] As mentioned above, the above step-wise ablation / estimation process may be performed in its entirety under automatic processor control, so that in effect, both the physician and the patient experience the disclosed step-wise ablation / estimation process (including the intervals required for electrogram acquisition) as a single continuous ablation process.

[0021] The disclosed staged ablation / estimation techniques allow for the use of a non-uniform number of ablation and estimation steps, for example, by estimating thickness for each of a varying number of ablation energy pulses. Thus, in an exemplary embodiment, a method for estimating thickness of cardiac tissue undergoing ablation is provided, the method including applying an ablation pulse sequence to a region of cardiac tissue to form a lesion, and for a given ablation pulse in the sequence, (a) estimating an incremental depth added to the lesion by the given ablation pulse, (b) estimating a cumulative depth of the lesion based on the cumulative depth before the given pulse and the incremental depth, and (c) evaluating the amplitude of the electrogram signal in the region after application of the given ablation pulse, and setting the thickness estimate to at least the cumulative depth if the amplitude exceeds a predetermined threshold.

[0022] In some exemplary embodiments, the physician or processor may terminate the ablation pulse sequence in response to detecting that the amplitude of the electrogram signal falls below a preset value that is different from a predetermined threshold.

[0023] In this context, applying a pulse of ablation energy also includes applying non-electrical energy, such as cryogenic and laser energy, for a given duration.

[0024] Using already available capabilities for acquiring and analyzing electrograms to estimate the thickness of the heart wall tissue undergoing ablation may enable accurate and cost-effective ablation procedures.

[0025] System Description 1 is a schematic diagram of a catheter-based system for irreversible electroporation (IRE) therapy and electrogram sensing 12, in accordance with an exemplary embodiment of the present invention. The IRE procedure is performed by a physician 14, and by way of example, the procedure in the remainder of the description herein is assumed to include IRE of a portion of the myocardium 16 of a human patient 18's heart.

[0026] To perform the ablation, the physician 14 inserts a probe 20, illustratively a Lasso® catheter (Biosense Webster, Irvine, Calif.) into the patient's lumen so that a distal end 22 of the probe 20 enters the patient's heart. As shown in insets 25 and 70, the distal end 22 includes a plurality of electrodes 53 attached to the exterior of an arcuate portion 40 (e.g., lasso portion 40) of the distal end 22, where the electrodes 53 are in contact with a location of the myocardium. The electrodes 53 are spaced apart from one another by a distance 75, d. The distal end 22 also includes a force sensor 45, and the probe 20 also includes a proximal end 28.

[0027] The system 12 is controlled by a system processor 46 located in the device's operating console 48. The console 48 includes a control device 49 that the physician 14 uses to communicate with the processor 46. During the procedure, the processor 46 typically tracks the position and orientation of the probe's distal tip 22 using, for example, a magnetic tracking method in which a magnetic transmitter external to the patient 18 generates a signal in a coil located in the distal tip. The Carto® system manufactured by Biosense Webster uses such a tracking method. As another example, the position and orientation of the distal tip 22 may be tracked using Biosense Webster's Advanced Catheter Location (ACL) system, described in U.S. Patent No. 8,456,182, which is incorporated herein by reference. In the ACL system, the processor estimates the location of each of the multiple electrodes 53 based on the impedance measured between each of the multiple electrodes 53 and a plurality of surface electrodes 49 that are coupled to the skin of the patient 18.

[0028] The software for the processor 46 may be downloaded to the processor in electronic form, for example, via a network. Alternatively or additionally, the software may be provided on a non-transitory tangible medium, such as an optical, magnetic, or electronic storage medium. The journey of the distal end 22 is typically displayed on a screen 62 in a three-dimensional representation 60 of the heart of the patient 18. The progress of the IRE therapy performed by the device 12 (e.g., accumulated lesion depth) is also typically displayed on the screen 62 as graphics 64 and / or alphanumeric data 66.

[0029] As inset 70 shows, lasso portion 40 accommodates the shape of the heart chamber 72 tissue of the PV 74 anatomy while pressing hard enough to bring the semi-cylindrical shaped electrode 53 into firm contact with the wall tissue of the heart chamber 72 over most or all of the area of ​​the electrode 53, i.e., with minimal, if any, electrode area exposed to blood. The heart chamber 72 wall tissue undergoing IRE ablation by electrode 53 has a local thickness D, 77 that is estimated using, for example, inter-electrode distance 75 and voltage drop between adjacent electrodes 53 in some of the disclosed exemplary embodiments of the invention.

[0030] To operate the system 12, the processor 46 communicates with a memory 50 that has several modules used by the processor to operate the device. Thus, the memory 50 includes a power control module 54 and a force module 56. The power control module 54 supplies IRE power to the electrodes 53 and also measures the instantaneous power, typically by measuring the instantaneous RMS voltage across the electrode pairs.

[0031] The force module 56 measures the instantaneous contact force by acquiring and evaluating signals from the force sensor 45 at the distal end 22. In an exemplary embodiment, based on the instantaneous voltage and contact force, the processor 46 estimates a time-dependent lesion depth 77. Systems and methods for estimating lesion depth as a function of IRE ablation duration are described in U.S. patent application Ser. No. 16 / 726,312, entitled "Calculating an Irreversible Electroporation (IRE) Index Taking IRE Field, Contact Force and Time Into Account," filed Dec. 24, 2019, and incorporated herein by reference.

[0032] In another exemplary embodiment, RF ablation energy is applied and a time-dependent lesion depth 77 is estimated by a processor using the measured instantaneous current and contact force. Systems and methods for estimating lesion depth as a function of RF ablation duration are described in U.S. Patent Application Publication No. 2017 / 0014181, which is incorporated by reference.

[0033] The memory 50 may also include other modules, such as a temperature measurement module and an irrigation module. For the sake of brevity, such other modules are not shown in this application. The modules of the memory 50 may include hardware as well as software elements.

[0034] Measurement of cardiac wall tissue thickness during ablation 2 is a graph that generally illustrates lesion depth and electrogram signal amplitude at the location of the lesion site as a function of ablation duration, according to an exemplary embodiment of the present invention. As seen over the major portion of the illustrated lesion depth vs. ablation time curve 270, the lesion depth generally increases monotonically with weak nonlinearity. At the same time, the amplitude 272 of the electrogram from the ablation location decreases as the lesion forms at greater depths. At a certain time 273 during the ablation, the lesion becomes as deep as the tissue wall thickness and the amplitude of the electrogram falls below a predetermined threshold 274.

[0035] According to an exemplary embodiment of the present invention, a physician can estimate the respective thickness 280, essentially the tissue wall thickness, at the ablated arrhythmia site. In an exemplary embodiment, the physician ablates a location of cardiac tissue for a given duration, for example, using electrode 53 of lasso catheter 20, to form a lesion in the tissue to a planned depth that is less than depth 280. Based on the method discussed in FIG. 1, a processor of the ablation system can estimate the resulting lesion depth. Then, during the interval during ablation, the processor receives an electrogram signal from the ablation location, for example, using the same electrode 53 of probe 20.

[0036] In most cases, some electrophysiologically active tissue remains in place and the amplitude of the electrogram signal is initially above a predetermined threshold. Thus, the processor estimates the tissue thickness to be the minimum lesion depth, but this initial depth is usually underestimated. The stepwise ablation / estimation process is repeated under the control of the processor or manually by the physician, and at each step of the process, the tissue is further ablated for an additional duration, such as duration 278, to add an incremental lesion depth 282, whose value the processor estimates and the accumulated lesion depth. In an exemplary embodiment, the additional duration value is selected such that the planned incremental lesion depth is less than the predetermined increment value so as not to over-ablate the tissue and / or to achieve an accurate estimation of the wall thickness.

[0037] Examples of RF ablation settings that can be used to ablate incremental lesion depths (less than 2 mm) are shown in Table I.

[0038] RF Low Depth Parameters:

[0039] [Table 1]

[0040] An example of IRE ablation settings that can be used to ablate incremental lesion depths (less than 3 mm) is given by Table II.

[0041] IRE Low Depth Parameters:

[0042] [Table 2]

[0043] The ablation and subsequent wall tissue thickness estimation process can be applied in finer incremental damage depths, e.g., sub-millimeters, depending on the additional duration value selected for the repeated ablation, or depending on other parameters such as RF power or IRE peak voltage used in conjunction with the aforementioned damage site model.

[0044] Again, immediately after the ablation step, the processor receives an electrogram signal from the ablated location and estimates the tissue wall thickness as follows: If the electrogram signal amplitude falls below a predetermined amplitude 276 (e.g., less than 0.1 mV), the processor re-estimates the tissue thickness at the ablation location to be the accumulated depth of the lesion and terminates the ablation at that location.

[0045] On the other hand, if the electrogram signal amplitude is above the predetermined amplitude 276, the processor updates the estimate to be at least the newly accumulated depth of the lesion. The processor or physician can then repeat the ablation process for an additional duration 278 and then estimate the incremental lesion depth 282 and measure the electrogram signal. As the arrhythmogenic signal gradually decreases step by step with each step, the estimated tissue thickness becomes more accurate.

[0046] The disclosed technique allows for the stopping of tissue ablation after thickness 280 has been determined, which could otherwise be dangerous in nearby organs and perforation. Moreover, at time 273, further ablation has no clinical value since the tissue has substantially ceased arrhythmogenicity.

[0047] The above stepwise ablation / estimation process may be performed automatically and may in fact be experienced as a single continuous ablation, including the typically small interval required for electrogram acquisition.

[0048] The process described in Fig. 2 is provided purely for the purpose of clarity. In practice, the actual curves of the disclosed model based on a large database of ablations are used. The given ablation duration and further ablation duration may vary depending on the location and type of arrhythmia, as well as from patient to patient.

[0049] Figure 3 is a flow chart that generally illustrates a method for estimating thickness of cardiac wall tissue undergoing ablation using the system 12 of Figure 1, in accordance with an exemplary embodiment of the present invention. The algorithm according to the presented embodiment executes a process in which, at an initial ablation step 90, the physician 14 initiates ablation of a location of cardiac tissue over a given period of time.

[0050] Processor 46 estimates the incremental and accumulated lesion depths in a lesion depth estimation step 92 using a lesion depth vs ablation duration model, such as the model described in Figure 1. In the first ablation iteration, the incremental depth is equal to the accumulated depth.

[0051] Next, in an electrogram acquisition step 94, physician 14 measures an electrogram signal at the ablation site using the same or a different catheter.

[0052] In an electrogram amplitude check step 96, processor 46 determines whether the amplitude (eg, peak or RMS value) of the electrogram signal is below a predetermined threshold.

[0053] If there is still substantial electrophysiological activity at the ablated location, i.e., the electrogram amplitude is above a threshold, processor 46 estimates the ablated cardiac wall thickness to be at least the accumulated depth of the lesion, in tissue wall thickness estimation step 98. The process then returns to step 91 to perform incremental ablation, in incremental ablation step 91.

[0054] Finally, processor 46 determines that the electrogram amplitude falls below a threshold, and processor 46 then estimates the ablated cardiac wall thickness to be the accumulated depth of the lesion in wall tissue thickness estimation step 100. In response, processor 46 terminates the ablation at that location in ablation termination step 102.

[0055] Although the embodiments described herein relate primarily to cardiac applications, the methods and systems described herein may also be used in other medical applications, such as neurology, for monitoring neural signals.

[0056] It will therefore be understood that the above-described embodiments are cited by way of example, and that the present invention is not limited to what has been particularly shown and described above. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described in the above specification, as well as variations and modifications thereof that would occur to one skilled in the art upon reading the foregoing description, and that are not disclosed in the prior art. Documents incorporated by reference into this patent application are to be considered as part of this application, except that if any term is defined in these incorporated documents in a manner that is inconsistent with the definition expressly or impliedly given herein, then only the definition in this specification shall be considered.

[0057] [Embodiment] (1) A method for estimating thickness of cardiac tissue undergoing ablation, comprising: applying an ablation pulse sequence to the region of cardiac tissue to create a lesion; For a given ablation pulse in the sequence: estimating an incremental depth added to the lesion due to the given ablation pulse; estimating a cumulative depth of the damage based on a cumulative depth prior to the given pulse and the incremental depth; evaluating an amplitude of an electrogram signal at the region after application of the given ablation pulse; and setting the thickness estimate to at least the cumulative depth if the amplitude exceeds a predetermined threshold. (2) The method of claim 1, further comprising terminating the ablation pulse sequence in response to detecting that the amplitude of the electrogram signal falls below a predetermined value. (3) The method of claim 2, wherein the predetermined value is equal to the predetermined threshold value. (4) The method of embodiment 1, wherein evaluating the amplitude of an electrogram signal comprises measuring the amplitude during intervals between the ablation pulses. (5) The method of embodiment 1, wherein the steps are performed automatically under processor control.

[0058] (6) The method of embodiment 1, wherein applying the ablation pulse sequence includes planning the sequence such that an expected incremental depth is less than a predetermined incremental value. (7) The method of claim 1, further comprising, after setting the estimate of the thickness, modifying the ablation pulse sequence to modify the incremental depth caused by a subsequent ablation step. (8) The method of embodiment 1, wherein evaluating the amplitude of the electrogram signal comprises measuring the electrogram signal using the same electrodes used to apply the ablation pulses. (9) The method of embodiment 1, wherein the step of applying the ablation pulse comprises applying an irreversible electroporation (IRE) pulse. (10) The method of embodiment 1, wherein applying the ablation pulse comprises applying radio frequency (RF) ablation.

[0059] (11) A system for estimating thickness of cardiac tissue undergoing ablation, comprising: an interface configured to output an ablation pulse sequence for application by the probe to the region of cardiac tissue to create a lesion; For a given ablation pulse in the sequence: Estimating an incremental depth added to the lesion due to the given ablation pulse; estimating a cumulative depth of the damage based on a cumulative depth prior to the given pulse and the incremental depth; and a processor configured to evaluate an amplitude of an electrogram signal in the region after the probe applies the given ablation pulse, and set the thickness estimate to at least the cumulative depth if the amplitude exceeds a predetermined threshold. (12) The system of claim 11, wherein the processor is further configured to terminate the ablation pulse sequence in response to detecting that the amplitude of the electrogram signal falls below a predetermined value. (13) The system of claim 12, wherein the predetermined value is equal to the predetermined threshold value. (14) The system of claim 11, wherein the processor is configured to evaluate the amplitude of an electrogram signal by measuring the amplitude during intervals between the ablation pulses. (15) The system of embodiment 11, wherein the processor is configured to plan the sequence such that an expected increment depth is less than a predetermined increment value.

[0060] (16) The system of claim 11, wherein the processor is configured to, after setting the thickness estimate, modify the ablation pulse sequence to modify the incremental depth caused by a subsequent ablation step. (17) The system of claim 11, wherein the processor is further configured to evaluate the amplitude of the electrogram signal by measuring the electrogram signal using the same electrodes used to apply the ablation pulses. (18) The system of embodiment 11, wherein the interface is configured to output an irreversible electroporation (IRE) pulse. (19) The system of embodiment 11, wherein the interface is configured to output radio frequency (RF) ablation pulses.

Claims

1. 1. A method, operating a processor, for estimating thickness of cardiac tissue undergoing ablation, comprising: for a given ablation pulse in an ablation pulse sequence applied by a probe to a region of cardiac tissue to form a lesion, the processor estimating an incremental depth added to the lesion due to the given ablation pulse; the processor estimating a cumulative depth of the lesion based on a cumulative depth prior to the given ablation pulse and the incremental depth; a step of: the processor evaluating an amplitude of an electrogram signal in the region after application of the given ablation pulse; and the processor setting the thickness estimate to at least the cumulative depth if the amplitude exceeds a predetermined threshold.

2. A method of operating a processor as described in claim 1, comprising the step of the processor terminating the ablation pulse sequence in response to the processor detecting that the amplitude of the electrogram signal falls below a predetermined value.

3. 3. The method of claim 2, wherein the predetermined value is equal to the predetermined threshold value.

4. A method of operating a processor as described in claim 1, wherein the step of the processor evaluating the amplitude of the electrogram signal includes a step of the processor measuring the amplitude during intervals between the ablation pulses.

5. A method of operating a processor as described in claim 1, wherein the step of the processor applying the ablation pulse sequence includes a step of the processor planning the ablation pulse sequence so that an expected incremental depth is less than a predetermined incremental value.

6. A method of operating a processor as described in claim 1, comprising the step of, after the processor has set the estimated value of the thickness, the processor modifying the ablation pulse sequence to change the incremental depth caused by a next ablation step.

7. A method of operating a processor as described in claim 1, wherein the step of the processor evaluating the amplitude of the electrogram signal includes a step of the processor measuring the electrogram signal using the same electrodes used to apply the ablation pulse.

8. A method of operating a processor as described in claim 1, wherein the step of the processor applying the ablation pulse includes the step of the processor applying an irreversible electroporation (IRE) pulse.

9. A method of operating a processor as described in claim 1, wherein the step of the processor applying the ablation pulse includes the step of the processor applying radio frequency (RF) ablation.

10. 1. A system for estimating thickness of cardiac tissue undergoing ablation, comprising: an interface configured to output an ablation pulse sequence for application by the probe to the region of cardiac tissue to create a lesion; For a given ablation pulse in the ablation pulse sequence: Estimating an incremental depth added to the lesion due to the given ablation pulse; estimating a cumulative depth of the lesion based on a cumulative depth prior to the given ablation pulse and the incremental depth; and a processor configured to evaluate an amplitude of an electrogram signal in the region after the probe applies the given ablation pulse, and set the thickness estimate to at least the cumulative depth if the amplitude exceeds a predetermined threshold.

11. 11. The system of claim 10, wherein the processor is further configured to terminate the ablation pulse sequence in response to detecting the amplitude of the electrogram signal falling below a predetermined value.

12. The system of claim 11 , wherein the predetermined value is equal to the predetermined threshold value.

13. The system of claim 10 , wherein the processor is configured to evaluate the amplitude of an electrogram signal by measuring the amplitude during intervals between the ablation pulses.

14. The system of claim 10 , wherein the processor is configured to plan the ablation pulse sequence such that an expected incremental depth is less than a predetermined incremental value.

15. 11. The system of claim 10, wherein the processor is configured to, after setting the thickness estimate, modify the ablation pulse sequence to modify the incremental depth caused by a subsequent ablation step.

16. 11. The system of claim 10, wherein the processor is further configured to evaluate the amplitude of the electrogram signal by measuring the electrogram signal using the same electrodes used to apply the ablation pulses.

17. The system of claim 10 , wherein the interface is configured to output an irreversible electroporation (IRE) pulse.

18. The system of claim 10 , wherein the interface is configured to output radio frequency (RF) ablation pulses.

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