Adjusting irrigation rate in radiofrequency (RF) ablation in response to contact force changes

Abstract

A method for ablating body tissue is disclosed, the method comprising generating an ablation signal and providing the ablation signal to an ablation probe in contact with tissue. An irrigation fluid is delivered to the ablation probe so as to be applied proximate to the tissue when the ablation signal is applied to the tissue. A signal is received from the ablation probe indicating an estimated instantaneous contact force applied by the ablation probe against the tissue. A flow rate of the irrigation fluid is adjusted in response to the estimated instantaneous contact force.

Description

Technical Field

[0001] The present invention relates generally to radiofrequency (RF) ablation, and particularly to cardiac RF ablation. Background Art

[0002] Techniques for controlling RF ablation have been previously proposed in the patent literature. For example, U.S. Patent Application Publication No. 2016 / 0213282 describes a method and apparatus that utilizes force-time integration to estimate lesion size in a catheter-based ablation system in real time. The apparatus measures the force applied to the target tissue by a contact ablation probe and integrates the force over the time that the ablation probe is energized. The force-time integral can be calculated and utilized to provide an estimated lesion size (depth, volume, and / or area) in real time. The force-time integral can also take into account changes in power delivered to the target tissue in real time to provide an improved estimate of lesion size. In one embodiment, the force measurement can be used as feedback to establish a desired power level delivered to the probe to prevent steam pop. In other embodiments, the control system can be adapted to increase flushing in addition to or in lieu of reducing or disabling energization.

[0003] As another example, U.S. Patent 9,962,217 describes a tissue ablation system and method in which a cardiac catheter is incorporated with a pressure detector for sensing a mechanical force against the distal tip when engaging an ablation site. A control responds to the pressure detector to calculate an ablation volume based on the relationship between the contact pressure at the site, the power output of the ablator, and the energy application time. The system applies a specified dose of energy to the tissue at a certain application time and a certain power level to ablate the tissue, wherein at least one of the application time and power level of the dose depends on the mechanical force. A monitor can dynamically display the progress of the ablation by changing a visual indication of the calculated ablation volume. Summary of the Invention

[0004] Embodiments of the present invention provide a method for ablating body tissue, the method comprising generating an ablation signal and providing the ablation signal to an ablation probe in contact with tissue. Delivering an irrigation fluid to the ablation probe so that the irrigation fluid is applied proximate to the tissue when the ablation signal is applied to the tissue. Receiving a signal from the ablation probe indicating an estimated instantaneous contact force applied by the ablation probe against the tissue. Adjusting a flow rate of the irrigation fluid in response to the estimated instantaneous contact force.

[0005] In some embodiments, the method further comprises monitoring the temperature of the tissue and adjusting the irrigation flow rate in response to the monitored temperature.

[0006] In some embodiments, adjusting the flow rate includes increasing or decreasing the flow rate while maintaining the ablation signal at a maximum power level for a minimum ablation duration.

[0007] In one embodiment, the ablation duration and maximum power level are predetermined constants.

[0008] In another embodiment, the method further includes continuously evaluating an ablation index while applying the ablation signal and stopping the ablation signal when the ablation index reaches an ablation index threshold.

[0009] In some embodiments, the ablation index depends on the power level of the ablation signal.

[0010] In some embodiments, the ablation index depends on the instantaneous contact force.

[0011] In one embodiment, adjusting the flow rate in response to the estimated instantaneous contact force includes increasing or decreasing the flow rate in response to an increase or decrease in the estimate of the instantaneous contact force.

[0012] According to an embodiment of the present invention, a system for ablation of body tissue is further provided, the system comprising a generator, an irrigation module, and a processor. The generator is configured to generate an ablation signal and provide the ablation signal to an ablation probe in contact with tissue. The irrigation module is configured to deliver an irrigation fluid to the ablation probe so that the irrigation fluid is applied near the tissue when the ablation signal is applied to the tissue. The processor is configured to receive a signal from the ablation probe indicating an estimated instantaneous contact force applied by the ablation probe against the tissue, and control the irrigation module to adjust a flow rate of the irrigation fluid in response to the estimated instantaneous contact force.

[0013] The present invention will be more fully understood through the following detailed description of embodiments of the present invention in conjunction with the accompanying drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a schematic, pictorial illustration of a system for cardiac radiofrequency (RF) ablation therapy according to an embodiment of the present invention; and

[0015] Figure 2 is a schematic description of an embodiment according to the present invention Figure 1 Flowchart of the steps of the algorithm executed during the operation of the RF ablation system. DETAILED DESCRIPTION

[0016] Overview

[0017] To control the ablation process aimed at producing a lesion of a given size, cardiac ablation systems, such as radiofrequency (RF) ablation systems, can vary the irrigation rate, ablation (e.g., RF) power input, and ablation duration while ensuring that the temperature of the ablated tissue does not exceed a maximum value. However, the resulting lesion size can vary due to variations in the mechanical force with which the ablation electrode contacts the tissue during ablation. Consequently, unless compensated for, the varying contact force can lead to uncontrolled and / or inaccurate results of RF ablation.

[0018] The embodiments of the present invention described below provide improved methods and systems for RF ablation. The underlying assumption in the disclosed techniques is that the only variables used to produce a specific lesion size within a preset time during RF ablation are the instantaneous (i.e., immediate) contact force on the ablated tissue, the irrigation flow rate, and the tissue temperature. Assuming that the tissue temperature is approximately constant, some embodiments maintain the delivered power substantially constant and compensate for changes in the measured instantaneous contact force by adjusting the irrigation flow rate.

[0019] Typically, for an ablation procedure to proceed under conditions of elevated instantaneous contact force, the processor running the algorithm for ablation will command an increase in the irrigation flow rate within a tolerance range. If the instantaneous contact force decreases, the processor running the algorithm for ablation will command a decrease in the irrigation flow rate so that the ablation procedure remains effective.

[0020] In other embodiments, changes in the monitored temperature are also compensated for by adjusting the irrigation flow rate. Typically, the processor monitors the temperature of the tissue and commands an adjustment (e.g., an increase or decrease) in the irrigation flow rate in response to the monitored temperature and, in some embodiments, in response to a corresponding increase or decrease in the monitored temperature.

[0021] In some embodiments, the method disclosed herein includes the following steps: (a) inserting a probe such as a catheter into the body of a living subject, (b) positioning the probe in contact with tissue in the body, (c) presetting a power output level and an ablation time, (d) presetting a range of allowable irrigation flow rates, (e) measuring an instantaneous contact force, (f) generating an ablation signal and providing the ablation signal to the ablation probe in contact with the tissue (i.e., depositing a preset amount of power into the tissue via one or more ablation electrodes of the probe), (g) delivering an irrigation fluid to the ablation probe so that the irrigation fluid is applied near the tissue when the ablation signal is applied to the tissue, (h) receiving a signal from the ablation probe indicative of an estimated instantaneous contact force applied by the ablation probe against the tissue, and (j) adjusting the flow rate of the irrigation fluid in response to the estimated instantaneous contact force.

[0022] In some embodiments, a system for ablation of body tissue is provided, the system comprising: (i) a memory configured to store a value for a target amount of ablation energy required to produce a designated ablation lesion in tissue within a patient's body, and further storing corresponding values ​​for a maximum power level and a minimum ablation duration; (ii) an irrigation module configured to deliver an irrigation fluid to an ablation probe, wherein the ablation probe is configured to: (iia) bring it into contact with tissue; and (iib) apply the irrigation fluid near the tissue. The ablation probe further comprises means for sensing an instantaneous contact force applied by the probe against the tissue.

[0023] The provided system also includes: a generator configured to generate an ablation signal and provide the ablation signal to the ablation probe; and a processor configured to: (a) receive a signal from the ablation probe indicating an estimated instantaneous contact force applied by the ablation probe against the tissue, and (b) control the irrigation module to adjust the flow rate of the irrigation fluid in response to the estimated instantaneous contact force.

[0024] The disclosed RF ablation technique, which compensates for variations in the instantaneous contact force applied by the electrode to the tissue by responsively varying the flow rate of irrigation, may allow maximum RF power levels to be maintained for minimum ablation durations and thereby improve clinical outcomes of catheter-based RF ablation procedures.

[0025] System Description

[0026] Figure 1 FIG2 is a schematic diagram illustrating a system 12 for cardiac radiofrequency (RF) ablation therapy according to an embodiment of the present invention. Typically, the memory 45 of the system 20 stores a plurality of ablation protocols for different clinical scenarios, such as Figure 2 The scheme described in .

[0027] Physician 26 inserts catheter 28 through a blood vessel into a chamber of heart 24 of subject 22 and manipulates the catheter so that distal end 32 of the catheter contacts the area of ​​endocardium to be treated. The tip electrode 51 of catheter 28, seen in inset 25, includes one or more contact force sensors 50.

[0028] After positioning the distal tip 32 at the ablation site and ensuring that the tip is in contact with the endocardium, the operator 26 activates the RF energy generator 44 in the console 42 to supply RF energy to the distal tip 32 via the cable 38. Simultaneously, an irrigation module 55, including a controllable irrigation pump 48, supplies a cooling fluid, such as saline solution, to the distal tip via the lumen in the tube 40 and the catheter 28. Typically, before and during ablation, the display 46 displays the values ​​of the ablation parameters, such as those listed in Table 1 below, to the physician 26.

[0029] To operate the system 12, the processor 41 controls the flush module 55 as further described below. Specifically, the processor 41 executes the instructions disclosed herein including Figure 2 , which enables the processor 41 to perform the disclosed steps, as further described below.

[0030] Operation of the RF energy generator and irrigation pump can be adjusted to provide an appropriate volume of irrigation to cool the catheter tip and tissue during ablation without adding excessive irrigation fluid to the heart. Each contact force sensor 50 provides feedback to the console 42 for controlling the irrigation flow rate.

[0031] In one embodiment, during ablation, one or more temperature sensors 52 located in the tip electrode 51 of the catheter 28 can sense tissue temperature and transmit temperature-indicative signals to the processor 41 for analysis and use.

[0032] The processor 41 operates the system 20 using software stored in the memory 45. The software may be downloaded to the processor 41 in electronic form over a network, for example, or alternatively or additionally, the software may be provided and / or stored on a non-transitory tangible medium such as magnetic memory, optical memory, or electronic memory. Specifically, the processor 41 executes the software disclosed herein including Figure 2 , which enables the processor 41 to perform the disclosed steps, as further described below.

[0033] Although the illustrated embodiment specifically relates to ablating cardiac tissue using a tip ablation device, the methods described herein may alternatively be applied to an ablation device comprising multiple ablation electrodes, in which case the irrigation operation of each electrode is independently controlled by the processor 41. In an alternative embodiment, the processor 41 controls the irrigation fluid common to all electrodes. Using feedback on the maximum instantaneous contact force on all electrodes, the processor adjusts the common flow rate of the irrigation.

[0034] Adjusting irrigation rate in radiofrequency (RF) ablation in response to contact force changes

[0035] Figure 2 For a schematic description of an embodiment according to the present invention Figure 1 FIG2 is a flow chart illustrating the steps of an algorithm executed in the operation of the RF ablation system 20. The process begins with a parameter range preset step 70, during which the physician 26 presets the ablation power and time (i.e., duration). This step may involve storing multiple protocols for different clinical scenarios, wherein such protocols are saved, for example, in the memory 45 of the system 20.

[0036] In one embodiment, these values / ranges are set as shown in Table 1:

[0037] parameter Value / Range Preset maximum power level 50W90-W Preset ablation time 2s-20s Contact force sampling rate 5Hz-70Hz Flushing flow rate 2-30ml / min

[0038] Table I

[0039] The parameter range setting step 70 is implemented before the physician 26 performs the ablation. Specifically, the ablation time (i.e., the ablation duration) and the maximum power level are predetermined constants. However, the values / ranges of power, time, and flow rate can be set differently, for example, depending on the ablation lesion depth target. Tables for low depth (less than 2 mm), medium depth (2 mm-3.5 mm), high depth (3.5 mm-5.0 mm), and ultra-high depth (greater than 5.0 mm) are described in U.S. patent application 16 / 288,838, entitled "Energy-Guided Radiofrequency (RF) Ablation," filed on February 28, 2019, which has been assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference.

[0040] In some embodiments, during ablation, the duration T of ablation is determined by providing a maximum allowable ablation index (AI), i.e., an ablation index threshold (AIT), which is the power level multiplied by the integral of the contact force over the duration, and solving the integral to extract a duration that varies with a given AIT input. The processor is configured to continuously evaluate the AI ​​during the ablation process and stop ablation when the AI ​​reaches the AIT.

[0041] One example of a formula for AIT is described in U.S. Patent Publication No. 2017 / 0014181, which is assigned to the assignee of the present patent application and the disclosure of which is incorporated herein by reference. This exemplary ablation index threshold (AIT) (denoted as AIT1) has the form Wherein the integral is the product of the first non-invariant exponent α of the contact force CF and the second non-invariant exponent β of the power P over the duration T, k is a proportionality constant, and γ is a third non-invariant exponent. The values ​​of α, β, γ, and k are determined by the method disclosed in the above-mentioned U.S. Patent Application Publication No. 2017 / 0014181. In an alternative embodiment, the processor uses a second formula of AIT (denoted as AIT2) to determine the duration of the ablation, where the second AIT is given by given.

[0042] At the start of ablation therapy, in a catheter introduction step 72 , physician 26 inserts catheter 28 into a desired location in heart 24 using a catheter position tracking system incorporated into system 20 .

[0043] Next, physician 26 achieves physical contact between electrode tip 51 and endocardial tissue at an electrode-tissue contact step 74. At a contact force determination step 76, processor 41 receives a contact force indication signal from a sensor on catheter 28 and determines the instantaneous contact force and / or determines whether the contact force is increasing or decreasing.

[0044] At the flushing step 78, the processor 41 controls the flushing module 55 to increase or decrease the flushing flow rate in response to the determined corresponding increase or decrease in the instantaneous mechanical force. For example, if the instantaneous contact force has increased, the processor 41 directs the flushing module 55 to immediately increase the flushing flow rate so that the heat removal rate achieved by flushing will match the increase in deposited heat resulting from better physical contact between the electrode and the tissue. On the other hand, if the instantaneous contact force has decreased, meaning that less heat is being deposited and the tissue may be too cold, the processor 41 directs the flushing module 55 to immediately decrease the flushing flow rate.

[0045] In RF delivery step 80, physician 26 operates system 20 using the parameter values ​​selected in step 70 to perform ablation of electrode 51. Display 46 of system 20 may be configured to display the progress of RF delivery to the electrode to physician 26 by methods known in the art.

[0046] During the RF delivery process, the processor 41 monitors the instantaneous contact force using N repeated measurements by looping back to step 78. The number of repetitions N is calculated by multiplying the ablation time and the contact force sampling rate in Table I, where, for example, N = 20. The processor 41 responsively commands the irrigation module 55 to modify the irrigation flow rate by repeating step 78 based on the instantaneous contact force.

[0047] At the ablation energy monitoring step 82, the processor 41 checks whether the preset amount of ablation energy is delivered at each repetition. At the ablation end step 84, the processor 84 ends the ablation after the ablation energy generated based on the power and time preset in step 70 has been achieved or the indicated time has elapsed.

[0048] Figure 2 The exemplary flow chart shown is chosen solely for conceptual clarity. The contact force sampling rate is introduced by way of example, where higher rates may be used. This embodiment also includes additional steps of the algorithm, such as checking tissue temperature, which have been intentionally omitted from the disclosure herein in order to provide a more simplified flow chart.

[0049] Although the embodiments described herein are primarily directed to cardiac applications, the methods and systems described herein can also be used to ablate other organs of the body, such as the kidneys (eg, for renal denervation) and lungs.

[0050] It should therefore be understood that the embodiments described above are cited by way of example, and that the present invention is not limited to what is specifically shown and described above. On the contrary, the scope of the present invention includes combinations and subcombinations 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. The documents incorporated by reference into this patent application are considered an integral part of this application, except that if any term defined in these incorporated documents conflicts with a definition explicitly or implicitly given in this specification, only the definition in this specification shall be considered.

Claims

1. A system for ablation of body tissue, the system comprising: a generator configured to generate an ablation signal and provide the ablation signal to an ablation probe in contact with tissue; an irrigation module configured to deliver an irrigation fluid to the ablation probe so as to apply the irrigation fluid proximate the tissue when the ablation signal is applied to the tissue; and a processor configured to: receiving a signal from the ablation probe indicative of an estimated instantaneous contact force applied by the ablation probe against the tissue; and controlling the irrigation module to adjust the flow rate of the irrigation fluid in response to the estimated transient contact force by commanding the irrigation module to increase or decrease the flow rate of the irrigation fluid accordingly in response to an increase or decrease in the estimate of the transient contact force; and wherein if the transient contact force has increased, the processor is configured to direct the flushing module to increase the flow rate such that the rate of heat removal achieved by flushing matches the increase in deposited heat resulting from the increase in the transient contact force; The processor is configured to check at each repetition whether a preset amount of ablation energy has been delivered and to end ablation after the preset amount of ablation energy has been delivered. 2 . The system of claim 1 , wherein the processor is further configured to monitor a temperature of the tissue and control the irrigation module to adjust the flow rate in response to the monitored temperature. 3 . The system of claim 1 , wherein the processor is configured to control the irrigation module to adjust the flow rate and maintain the ablation signal at a maximum power level for a minimum ablation duration.

4. The system of claim 3, and comprising a memory configured to save the values ​​of the maximum power level and the shortest ablation duration as predetermined constants. 5 . The system of claim 1 , wherein the processor is configured to continuously evaluate an ablation index and to stop the ablation signal when the ablation index reaches an ablation index threshold. The system of claim 5 , wherein the ablation index depends on a power level of the ablation signal. The system of claim 5 , wherein the ablation index is dependent on the instantaneous contact force.

Images