Measuring the thickness of heart wall tissue during ablation

Abstract

The invention is entitled "Measuring thickness of heart wall tissue during ablation". The invention discloses a method for estimating the thickness of heart tissue undergoing ablation, the method comprising the steps of: (a) applying a sequence of ablation pulses to a region of the heart tissue so as to produce an ablation lesion, and (b) for a given ablation pulse in the sequence, estimating an incremental depth added to the ablation lesion as a result of the given ablation pulse. A cumulative depth of the ablation lesion is estimated based on the cumulative depth prior to the given pulse and based on the incremental depth. An amplitude of an electrogram signal at the region is assessed after application of the given ablation pulse, and if the amplitude exceeds a predetermined threshold, the estimate of the thickness is set to at least the cumulative depth.

Description

Technical Field

[0001] The present invention relates generally to cardiac sensing and ablation, and more specifically to estimating the characteristics of cardiac wall tissue undergoing ablation. Background Technology

[0002] Various methods can be used to estimate the thickness of the cardiac wall, such as ultrasound, fluoroscopy, and MRI. The estimated wall thickness can be further correlated with electrophysiological signals to estimate damage to the cardiac wall tissue. For example, Takeshi Sasaki et al. described a significant correlation between left ventricular wall thickness, post-infarct scar thickness, and intramural scar location seen on MRI and bipolar / unipolar voltage, duration, and offset of local endocardial electrograms on electroanatomical mapping 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). Summary of the Invention

[0003] Embodiments of the present invention provide a method for estimating the thickness of cardiac tissue undergoing ablation, the method comprising the steps of: (a) applying a sequence of ablation pulses to a region of the cardiac tissue to create an ablation focus; and (b) for a given ablation pulse in the sequence, estimating the incremental depth to the ablation focus due to the given ablation pulse. The cumulative depth of the ablation focus is estimated based on the cumulative depth prior to the given pulse and based on the incremental depth. The amplitude of an electrogram signal at the region is evaluated after the application of the given ablation pulse, and if the amplitude exceeds a predetermined threshold, the estimated thickness is set to at least the cumulative depth.

[0004] In some embodiments, the method further includes terminating the ablation pulse sequence in response to detecting that the amplitude of the electrogram signal is below a predetermined value. In 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 an interruption between ablation pulses. In another embodiment, these steps are performed automatically under processor control.

[0006] In some implementations, the ablation pulse sequence includes a planned sequence such that the expected incremental depth is less than a pre-specified incremental value.

[0007] In some implementations, the method further includes, after setting an estimate of the thickness, changing the ablation pulse sequence to alter the incremental depth caused by the next ablation step.

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

[0009] In another implementation, applying the ablation pulse includes applying radio frequency (RF) ablation.

[0010] According to another embodiment of the invention, a system for estimating the thickness of cardiac tissue undergoing ablation is also provided. The system includes an interface device and a processor. The interface device is configured to output a sequence of ablation pulses to a probe for application to a region of cardiac tissue to create an ablation focus. The processor is configured, for a given ablation pulse in the sequence, (i) to estimate the incremental depth to the ablation focus due to the given ablation pulse, (ii) to estimate the cumulative depth of the ablation focus based on the cumulative depth prior to the given pulse and based on the incremental depth, and (iii) to evaluate the amplitude of an electrogram signal at the region after the probe applies the given ablation pulse, and if the amplitude exceeds a predetermined threshold, to set the thickness estimate to at least the cumulative depth. Attached Figure Description

[0011] This disclosure will be more fully understood through the following detailed description of embodiments thereof, taken in conjunction with the accompanying drawings, wherein:

[0012] Figure 1 A schematic illustration of a catheter-based system for irreversible electroporation (IRE) treatment and electrogram sensing according to an exemplary embodiment of the present invention;

[0013] Figure 2 To illustrate, a graph showing the variation of ablation depth and electrophoresis signal amplitude at the ablation site with ablation duration according to an exemplary embodiment of the present invention; and

[0014] Figure 3 For illustrative purposes, the use of exemplary embodiments of the present invention is described. Figure 1 The flowchart describes a method for estimating the thickness of heart wall tissue undergoing ablation using a system. Detailed Implementation

[0015] Overview

[0016] Ablation foci in the cardiac wall tissue of patients undergoing ablation procedures can be created using irreversible electroporation (IRE) or other types of ablation energy such as radiofrequency (RF), both of which can be applied via a catheter. In IRE ablation, the catheter is manipulated so that electrodes positioned at the distal end of the catheter come into contact with the tissue. A high-voltage bipolar pulse is then applied between the electrodes, and the resulting strong electric field pulse in the tissue causes cell death and the creation of an ablation foci. In RF ablation, alternating RF current is applied to the tissue through one or more electrodes, thereby inducing cell death through heat.

[0017] Estimating the wall thickness at the site of ablation can be important to avoid serious side effects of ablation, such as cardiac wall perforation. Additionally, this estimation can help shorten ablation procedures, where the estimated wall thickness is used as a baseline estimate at these locations to more quickly ablate nearby sites. As noted in the background section, various imaging methods can be used to measure cardiac wall thickness. Such methods are used during ablation procedures; however, this adds considerable complexity to the ablation system and clinical workflow.

[0018] The exemplary embodiments of the invention described below provide systems and methods for real-time acquisition and analysis of electrograms from cardiac tissue to estimate the thickness of the wall tissue undergoing ablation. The disclosed systems can employ an automated, stepwise ablation / estimation process to achieve increasingly refined estimates, which can then be used to achieve optimal ablation depth (e.g., the minimum ablation depth sufficient to eliminate arrhythmias).

[0019] In some exemplary embodiments, catheter ablation electrodes, such as IRE or RF ablation electrodes, are also used to acquire electrograms. In other exemplary embodiments, other types of ablation energy (e.g., laser, cryotherapy) are applied using a catheter, with additional sensing electrodes positioned on the catheter for acquiring electrograms. Generally, any catheter-based ablation method that creates ablation foci in tissue can be used in conjunction with the disclosed ablation / estimation procedures that have depths that can be modeled based on the ablation duration.

[0020] In an exemplary embodiment, the disclosed ablation / estimation process begins with a physician operating a processor-controlled system to ablate tissue over a given duration to produce an ablation foci with a pre-specified planned depth, wherein the required duration is estimated using one of the aforementioned ablation foci depth models associated with the ablation method. Electrodes are then used to detect electrogrammatic signals from the ablated tissue. For both IRE and RF ablation, the same electrodes can be used for electrogrammatic detection.

[0021] If the measured electrogram signal has an amplitude higher than a pre-specified threshold, the processor determines that the tissue thickness is at least as thick as the pre-specified ablation depth (i.e., the processor sets a lower limit for the thickness). In this case, the processor instructs an additional duration to ablate the tissue to incrementally increase the ablation depth, and then acquires an electrogram to re-estimate the cumulative ablation depth. The above steps can be repeated until the amplitude of the electrogram signal drops below the pre-specified threshold.

[0022] Therefore, as the disclosed ablation / estimation process proceeds, the lower limit of the estimated wall thickness gradually increases (because this lower limit is equal to the cumulative ablation depth in the tissue that still has an electrogram signal above the pre-specified threshold of wall tissue). Eventually, the amplitude of the electrogram signal drops below the pre-specified threshold. At this point in the process, the processor estimates the wall tissue thickness as the cumulative ablation depth that causes the signal amplitude to decrease, and responsively terminates the ablation at that location.

[0023] Physicians control the accuracy of estimates by selecting additional durations, ensuring that the planned incremental ablation depth is less than a pre-specified increment that is considered safe.

[0024] As described above, the stepwise ablation / estimation process described herein can be executed under the control of an automated processor. Therefore, during implementation, both the physician and the patient will view the disclosed stepwise ablation / estimation process as a single continuous ablation procedure (including interruptions required for electrography acquisition).

[0025] The disclosed stepwise ablation / estimation technique allows for the estimation of thickness using a non-uniform number of ablation and estimation steps, for example, by varying the number of ablation energy pulses. Therefore, in an exemplary embodiment, a method for estimating the thickness of cardiac tissue undergoing ablation is provided, the method comprising applying a sequence of ablation pulses to a region of the cardiac tissue to create an ablation focus, and for a given ablation pulse in the sequence, (a) estimating the incremental depth from the given ablation pulse to the ablation focus, (b) estimating the cumulative depth of the ablation focus based on the cumulative depth prior to the given pulse and based on the incremental depth, and (c) evaluating the amplitude of an electrogram signal at the region after the application of the given ablation pulse, and if the amplitude exceeds a predetermined threshold, setting the thickness estimate to at least the cumulative depth.

[0026] In some exemplary embodiments, a physician or processor may terminate the ablation pulse sequence in response to detecting that the amplitude of the electrogram signal is below a predetermined value other than a predetermined threshold.

[0027] In the context of this invention, a pulse that applies ablation energy also includes applying non-electrical energy, such as cryogenic energy and laser energy, over a given duration.

[0028] By using the already available ability to acquire and analyze electrograms to estimate the thickness of the ablated cardiac wall tissue, accurate and cost-effective ablation treatment may be possible.

[0029] System Description

[0030] Figure 1 This is a schematic illustration of a catheter-based system 12 for irreversible electroporation (IRE) procedures and electrogram sensing according to an exemplary embodiment of the present invention. The IRE procedure is performed by a physician 14, and by way of example, it is assumed that the procedure described below involves an IRE of a portion of the myocardium 16 of the heart of a human patient 18.

[0031] To perform the ablation, physician 14 will place probe 20, for example (manufactured by Biosense Webster, Irvine, California). The catheter is inserted into the patient's lumen, such that the distal end 22 of the probe 20 enters the patient's heart. As shown in illustrations 25 and 70, the distal end 22 includes a plurality of electrodes 53 mounted on the lateral side of the articular segment 40 (e.g., the lasso segment 40) of the distal end 22, which contact the myocardium. The electrodes 53 are spaced apart from each other 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.

[0032] System 12 is controlled by a system processor 46 located in an operating console 48 of the device. The console 48 includes controls 49 used by the physician 14 to communicate with the processor 46. During the procedure, the processor 46 typically uses, for example, magnetic tracking methods to track the position and orientation of the distal end 22 of the probe, where a magnetic transmitter outside the patient 18 generates a signal in a coil positioned in the distal end. Manufactured by Biosense-Webster. The system uses this type of tracking method. Alternatively, the position and orientation of the distal end 22 can be tracked using an advanced catheter positioning (ACL) system manufactured by Biosense-Webster, described in U.S. Patent 8,456,182, the disclosure of which is incorporated herein by reference. In the ACL system, the processor estimates the corresponding position of the plurality of electrodes 53 based on impedance measured between each of the electrodes 53 and a plurality of surface electrodes 49 coupled to the skin of the patient 18.

[0033] Software for processor 46 may be downloaded to the processor electronically, for example, via a network. Alternatively or otherwise, the software may be provided via a non-transitory tangible medium such as optical, magnetic, or electronic storage media. The tracking of the distal end 22 is typically displayed on a three-dimensional representation 60 of the patient 18's heart on screen 62. The progress of IRE treatment performed with device 12 (e.g., cumulative ablation depth) is also typically displayed on screen 62 in graphical form 64 and / or alphanumeric data 66.

[0034] As shown in Illustration 70, the lasso segment 40 accepts the shape of the oral 72 tissue of the PV 74 anatomy while being able to press the tissue forcefully enough to ensure that the semi-cylindrical electrode 53 makes firm contact with the oral 72 wall tissue over most or all of its area, i.e., minimal (if any) electrode area is exposed to blood. The oral 72 wall tissue subjected to IRE ablation via electrode 53 has a local thickness D, 77, which, in some exemplary embodiments disclosed herein, is estimated, for example, using the inter-electrode distance 75 and the voltage drop between adjacent electrodes 53.

[0035] For the operating system 12, the processor 46 communicates with the memory 50, which has multiple modules used by the processor to operate the device. Therefore, the memory 50 includes a power control module 54 and a force module 56. The power control module 54 delivers IRE power to the electrodes 53 and also typically measures the instantaneous power by measuring the instantaneous RMS voltage across the electrode pair.

[0036] Force module 56 measures instantaneous contact force by acquiring and estimating signals from force sensor 45 in distal end 22. In an exemplary embodiment, processor 46 estimates time-dependent ablation depth 77 based on instantaneous voltage and contact force. Systems and methods for estimating ablation depth based on IRE ablation duration are described in U.S. Patent Application 16 / 726312, filed December 24, 2019, entitled “Calculating an Irreversible Electroporation (IRE) Index Taking IRE Field, Contact Force and Time Into Account,” which is assigned to the assignee of this patent application, the contents of which are incorporated herein by reference.

[0037] In another exemplary embodiment, the processor applies RF ablation energy using measured instantaneous current and contact force, and estimates the time-dependent ablation depth 77. Systems and methods for estimating ablation depth based on RF ablation duration are described in U.S. Patent Application Publication 2017 / 0014181, the document of which is incorporated herein by reference.

[0038] The memory 50 may also include other modules, such as a temperature sensing module and a rinsing module. For the sake of simplicity, other such modules are not described in this application. The modules of the memory 50 also include hardware and software elements.

[0039] Measuring the thickness of the heart wall tissue during ablation

[0040] Figure 2 The diagram illustrates the variation of ablation depth and electrogram signal amplitude at the ablation site with ablation duration according to an exemplary embodiment of the present invention. As can be seen, in the main portion of the ablation depth versus ablation time curve 270, the ablation depth increases monotonically, typically exhibiting weak nonlinearity. Simultaneously, as the ablation site forms at a deeper depth, the amplitude 272 of the electrogram from the ablation site decreases. At a specific time 273 during ablation, the ablation depth is as deep as the tissue wall thickness, and the electrogram amplitude drops below a pre-specified threshold 274.

[0041] Exemplary embodiments of the present invention enable physicians to estimate the corresponding thickness 280, i.e., the tissue wall thickness at the substantially ablated arrhythmia-causing site. In an exemplary embodiment, the physician uses electrodes 53, such as those of a lasso catheter 20, to ablate the cardiac tissue site over a given duration to create an ablation focus in the tissue to a planned depth less than 280. Based on Figure 1 According to the method discussed, the processor of the ablation system can estimate the depth of the resulting ablation foci. Next, during ablation interruption, the processor uses, for example, the same electrode 53 of probe 20 to receive electrogram signals from the ablation site.

[0042] In most cases, some electrophysiologically active tissue remains at the location, and the amplitude of the electrogram signal will initially be higher than a pre-specified threshold. Therefore, the processor estimates the tissue thickness as at least the depth of the ablation focus, but this initial depth is often underestimated. The stepwise ablation / estimation process is repeated under the control of the processor or manually by the physician, wherein at each step of the process, the tissue is further ablated for an additional duration, such as duration 278, to increase the incremental ablation focus depth 282, and the processor estimates the value of this incremental ablation focus depth and the cumulative ablation focus depth. In an exemplary embodiment, the value of the additional duration is selected such that the planned incremental ablation focus depth is less than a pre-specified incremental value, so as not to over-ablate tissue and / or achieve an accurate estimation of the wall thickness.

[0043] Examples of RF ablation settings that can be used to ablate incremental ablation foci (less than 2 mm) are given in Table I:

[0044] RF low-depth parameters:

[0045] parameter scope Preset RF ablation energy 270J–540J (usually 360J) Maximum RF power level 90W Power range 0-90W Permissible temperature range 45-65℃ (usually 50℃) Permissible flushing flow rate 4ml / min-15ml / min Maximum ablation time 3-6 seconds (usually 4 seconds)

[0046] Table I

[0047] Examples of IRE ablation settings that can be used to ablate incremental ablation foci (less than 3 mm) are given in Table II:

[0048] IRE low-depth parameters:

[0049] parameter scope Preset IRE electric field 500V / cm-1000V / cm Pulse width 0.5mSec-5mSec Delay between series 1ms-1000ms Number of pulses in the series 5-20 Number of series 1-90

[0050] Table II

[0051] The ablation and subsequent wall tissue thickness estimation steps can be applied to finer increments of ablation foci depth, such as less than one millimeter, depending on the value of the additional duration selected for repeated ablation, or on other parameters such as the RF power or IRE peak voltage used in conjunction with the aforementioned ablation foci-depth model.

[0052] Similarly, immediately following the ablation step, the processor receives an electrogram signal from the ablation site and estimates the tissue wall thickness as follows. If the electrogram signal amplitude is lower than a pre-specified amplitude of 276 (e.g., lower than 0.1 mV), the processor re-estimates the tissue thickness at the ablation site as the cumulative depth of the ablation foci and terminates the ablation at that site.

[0053] On the other hand, if the electrogram signal amplitude remains higher than the pre-specified amplitude 276, the processor will estimate an updated cumulative depth of at least the ablation foci. The processor or physician can then repeat the ablation steps for an additional duration 278, estimating the incremental ablation foci depth 282 and measuring the electrogram signal. As the arrhythmogenic signal gradually decreases, the estimated tissue thickness becomes more accurate.

[0054] The disclosed technique allows tissue ablation to be stopped after a thickness of 280 mm, which would otherwise have caused damage to nearby organs and even perforation. Furthermore, further ablation has no clinical value because at time 273 mm, the tissue has effectively ceased to induce arrhythmias.

[0055] The aforementioned stepwise ablation / estimation process can be performed automatically and is experienced as a single continuous ablation during implementation, including the typically minimal interruptions required for electrogram acquisition.

[0056] Figure 2 The methods described herein are purely for clarity. In practice, actual curves from the disclosed models are used, based on a large dataset of ablation data. The given ablation duration and any additional ablation duration may vary depending on the location and type of the arrhythmia and between patients.

[0057] Figure 3 For illustrative purposes, the use of exemplary embodiments of the present invention is described. Figure 1 The flowchart illustrates a method for estimating the thickness of the ablated cardiac wall tissue using system 12. The algorithm of the presented implementation scheme is executed as follows: the process begins at the initial ablation step 90, where physician 14 ablates the cardiac tissue location within a given time period.

[0058] At step 92, the lesion depth estimation model (such as lesion depth versus ablation duration) is used. Figure 1 In the aforementioned model, processor 46 estimates the incremental ablation depth and the cumulative ablation depth. At the first ablation iteration, the incremental depth equals the cumulative depth.

[0059] Next, in electrography acquisition step 94, physician 14 uses the same or different catheters to measure the electrography signal at the ablation site.

[0060] At the electrogram amplitude check step 96, the processor 46 checks whether the amplitude (e.g., peak value or RMS value) of the electrogram signal is below a pre-specified threshold.

[0061] If significant electrophysiological activity remains at the ablation site, i.e., electrogram amplitudes exceeding a threshold, then in tissue wall thickness estimation step 98, processor 46 estimates the ablated cardiac wall thickness as at least the cumulative depth of the ablation foci. The method then returns to step 91 to perform incremental ablation at incremental ablation step 91.

[0062] Finally, processor 46 determines that the electrogram amplitude is below a threshold, and then at wall tissue thickness estimation step 100, processor 46 estimates the ablated cardiac wall thickness as the cumulative depth of the ablation foci. Correspondingly, at ablation termination step 102, processor 46 terminates the ablation at that location.

[0063] While the implementation schemes described herein primarily relate to cardiac applications, the methods and systems described herein can also be used in other medical applications, such as in neurology, for example, for monitoring neural signals.

[0064] Therefore, it should be understood that the embodiments described above are cited by way of example, and the invention is not limited to what is specifically shown and described above. Rather, the scope of the invention includes combinations and sub-combinations of the various features described above, as well as variations and modifications thereof, which will occur to those skilled in the art upon reading the above description and which are not disclosed in the prior art. Documents incorporated herein by reference are considered an integral part of this application, except that if any terminology defined in such incorporated documents conflicts with the definitions expressly or implicitly given in this specification, only the definitions in this specification shall be considered.

Claims

1. A system for estimating the thickness of cardiac tissue undergoing ablation, the system comprising: An interface device configured to output an ablation pulse sequence to a probe for application to a region of the cardiac tissue, such that the sequence is configured such that the next incremental depth is less than a pre-specified incremental value to generate an ablation focus. as well as A processor configured to, for a given ablation pulse in the sequence: Estimate the incremental depth to the ablation site due to the given ablation pulse; The cumulative depth of the ablation foci is estimated based on the cumulative depth prior to the given ablation pulse and based on the incremental depth. as well as After the given ablation pulse is applied to the probe, the amplitude of the electrogram signal at the region is evaluated, and if the amplitude exceeds a predetermined threshold, the estimated thickness is set to at least the cumulative depth. The amplitude of the electrogram signal of the arrhythmogenic signal at the region of the ablated cardiac tissue decreases after each iteration.

2. The system of claim 1, wherein the processor is further configured to terminate the ablation pulse sequence in response to detecting that the amplitude of the electrogram signal is below a predetermined value.

3. The system according to claim 2, wherein the predetermined value is equal to the predetermined threshold.

4. The system of claim 1, wherein the processor is configured to evaluate the amplitude of the electrogram signal by measuring the amplitude during an interruption between the ablation pulses.

5. The system of claim 1, wherein the processor is configured to plan the sequence such that the expected increment depth is less than a pre-specified increment value.

6. The system of claim 1, wherein the processor is configured to, after setting an estimate of the thickness, change the ablation pulse sequence to change the incremental depth caused by the next ablation step.

7. The system of claim 1, wherein the processor is further configured to evaluate the amplitude of the electrogram signal by measuring the electrogram signal using the same electrode used to apply the ablation pulse.

8. The system of claim 1, wherein the interface device is configured to output an irreversible electroporation (IRE) pulse.

9. The system of claim 1, wherein the interface device is configured to output a radio frequency (RF) ablation pulse.

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