Patent Description:
Techniques for controlling RF ablation were previously proposed in the patent literature. For example, <CIT> describes a method and apparatus that utilize a force-time integral for real time estimation of lesion size in catheter-based ablation systems. The apparatus measures the force exerted by a contact ablation probe on a target tissue and integrates the force over an energization time of the ablation probe. 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 may also account for variations in the power delivered to the target tissue in real time to provide an improved estimation of the lesion size. In one embodiment, the force metric can be used as feedback to establish a desired power level delivered to the probe to prevent steam popping. In still other embodiments, the control system can be adapted to increase irrigation, in addition or in place of decreasing or disabling energization.

As another example, <CIT> describes tissue ablation systems and methods in which a cardiac catheter incorporates a pressure detector for sensing a mechanical force against the distal tip when engaging an ablation site. Responsively to the pressure detector, a controller computes an ablation volume according to relationships between the contact pressure against the site, the power output of an ablator, and the energy application time. The system applies a specified dosage of energy for an application time and at a power level to the tissue for ablation thereof, wherein at least one of the application time of the dosage and the power level depend on the mechanical force. A monitor may dynamically display the progress of the ablation by varying a visual indication of the computed ablation volume.

<CIT> describes a probe, including an insertion tube and an electrode mounted on a distal end of the insertion tube. A force sensor is mounted in the distal end of the insertion tube. The force sensor has a central opening and is configured to measure a force on the distal end. The probe also includes tubing, passing through the central opening, which is configured to supply irrigation fluid through apertures in the electrode.

The methods of surgery and therapy described below are not claimed but are considered as useful for understanding the invention.

To control an ablation process aimed at creating a lesion of a given size, cardiac ablation systems, such as radiofrequency (RF) ablation systems, may vary the irrigation rate, the ablation (e.g., RF) power input, and the duration of ablation, while ensuring that the temperature of the ablated tissue does not exceed a maximum value. Nevertheless, the resulting lesion size may vary due to variation in the mechanical force with which an ablation electrode is in contact with tissue during the ablation. Therefore, unless compensated for, a varying contact force may cause an uncontrolled and/or inaccurate outcome of the RF ablation.

Embodiments of the present invention that are described hereinafter provide improved systems for RF ablation. An underlying assumption in the disclosed techniques is that the only variables during RF ablation for a specific lesion size within a preset time are the instantaneous (i.e., momentary) contact force on the tissue being ablated, the irrigation flow rate, and tissue temperature. Assuming that tissue temperature is approximately constant, some embodiments keep the delivered power substantially constant, and compensate for measured changes in instantaneous contact force by adapting the irrigation flow rate.

Typically, for the ablation process to continue under elevated instantaneous contact-force conditions, a processor running an algorithm for the ablation would command an increase in the irrigation flow rate within an allowable range. If the instantaneous contact force decreases, the processor running an algorithm for the ablation would command a reduction in the irrigation flow rate so that the ablation process remains effective.

In other embodiments, changes in monitored temperature are also compensated for by adapting the irrigation flow rate. Typically, the processor monitors the temperature of the tissue and commands the irrigation flow rate to adapt (e.g., increase or decrease) responsively to the monitored temperature, and in some embodiments, responsively to a respective increase or reduction in the monitored temperature.

In some embodiments, the disclosed method comprises the steps of (a) inserting a probe, such as a catheter, into a body of a living subject, (b) putting the probe into contact with a tissue in the body, (c) presetting a power output level and time of ablation, (d) presetting a range of allowable flow rate of irrigation, (e) measuring an instantaneous contact force, (f) generating an ablation signal and providing the ablation signal to an ablation probe that is in contact with tissue (i.e., depositing into tissue the preset amount of power via one or more ablation electrodes of the probe), (g) delivering irrigation fluid to the ablation probe, for applying the irrigation fluid in a vicinity of the tissue while the ablation signal is applied to the tissue, (h) receiving from the ablation probe signals indicative of an estimated instantaneous contact force that is exerted by the ablation probe against the tissue, and (j) adapting the flow rate of the irrigation fluid responsively to the estimated instantaneous contact force.

In some embodiments, a system for body tissue ablation is provided, which includes (i) a memory, which is configured to store a value of target amount of ablation energy needed to create a specified lesion in tissue in a body of a patient, and further stores respective values of maximal power level and shortest duration of ablation (ii) an irrigation module configured to deliver irrigation fluid to the ablation probe, wherein the ablation probe is configured to: (iia) make contact with tissue, (iib) apply the irrigation fluid in a vicinity of the tissue. The ablation probe further includes means for sensing an instantaneous contact force that is exerted by the probe against the tissue.

The provided system further includes a generator, which is configured to generate the ablation signal and provide the ablation signal to the ablation probe, and a processor, which is configured to: (a) receive from the ablation probe signals indicative of an estimated instantaneous contact force that is exerted by the ablation probe against the tissue, (b) and control the irrigation module to adapt a flow rate of the irrigation fluid responsively to the estimated instantaneous contact force.

The disclosed RF ablation technique, which compensates for a variation in instantaneous contact force that an electrode exerts on tissue by responsively varying the flow rate of irrigation, may allow maintaining a maximal RF power level for the shortest duration of ablation, and by so may improve a clinical outcome of a catheter-based RF ablation procedure.

<FIG> is a schematic, pictorial illustration of a system <NUM> for cardiac radiofrequency (RF) ablation therapy, in accordance with an embodiment of the present invention. Typically, a memory <NUM> of system <NUM> stores numerous ablation protocols for different clinical scenarios, such as a protocol described in <FIG>.

A physician <NUM> inserts a catheter <NUM> through a blood vessel into a chamber of a heart <NUM> of a subject <NUM>, and manipulates the catheter so that a distal end <NUM> of the catheter contacts the endocardium in an area that is to be treated. A tip electrode <NUM> of catheter <NUM>, seen in inset <NUM>, comprises one or more contact force sensors <NUM>.

After positioning distal end <NUM> at an ablation site, and ensuring that the tip is in contact with the endocardium, operator <NUM> actuates an RF energy generator <NUM> in a control console <NUM> to supply RF energy via a cable <NUM> to distal end <NUM>. Meanwhile, an irrigation module <NUM>, comprising a controllable irrigation pump <NUM>, supplies a cooling fluid, such as normal saline solution, via a tube <NUM> and a lumen in catheter <NUM> to the distal end. Typically, both before and during the ablation, a display <NUM> displays values of the ablation parameters, such as those listed in Table I below, to physician <NUM>.

In order to operate system <NUM>, a processor <NUM> controls an irrigation module <NUM> as further described below. In particular, processor <NUM> runs a dedicated algorithm as disclosed herein, included in <FIG>, that enables processor <NUM> to perform the disclosed steps, as further described below.

Operation of the RF energy generator and the irrigation pump may be coordinated in order to give the appropriate volume of irrigation during ablation, so as to cool the tip of the catheter and the tissue without overloading the heart with irrigation fluid. Each contact force sensor <NUM> provides feedback to console <NUM> for use in controlling the irrigation flow rate.

In an embodiment, during ablation, one or more senor temperatures <NUM> located in tip electrode <NUM> of catheter <NUM> may sense tissue temperature and transmit a temperature-indicative signal to processor <NUM> for analysis and use.

Processor <NUM> uses software stored in a memory <NUM> to operate system <NUM>. The software may be downloaded to processor <NUM> in electronic form, over a network, for example, or it may, alternatively or additionally, be provided and/or stored on non-transitory tangible media, such as magnetic, optical, or electronic memory. In particular, processor <NUM> runs a dedicated algorithm as disclosed herein, included in <FIG>, that enables processor <NUM> to perform the disclosed steps, as further described below.

Although the pictured embodiment relates specifically to the use of a tip ablation device for ablation of heart tissue, the methods described herein may alternatively be applied in ablation devices comprising multiple ablation electrodes, in cases where the operation of irrigation with each electrode is independently controlled by processor <NUM>. In an alternative embodiment, processor <NUM> controls irrigation flow shared by all electrodes. Using feedback on the maximal instantaneous contact force among all electrodes, the processor adapts the common flow rate of irrigation.

<FIG> is a flow chart that schematically describes steps of an algorithm performed in operation of RF ablation system <NUM> of <FIG>, according to an embodiment of the present invention. The process begins in a parameter range presetting step <NUM>, during which physician <NUM> presets ablation power and time (i.e., duration). Such a step may involve generating multiple protocols for different clinical scenarios, where such protocols are saved, for example, in memory <NUM> of system <NUM>.

In an embodiment, the values/ranges are set as shown in Table I:.

Parameter range setting step <NUM> is implemented before physician <NUM> performs an ablation. In particular, ablation time (i.e., the duration of ablation) and the maximal power level are predetermined constants. However, the values/ranges of power, time, and flow rate may be set differently, for example, with a lesion depth target. Tables for low depth (less than <NUM>), medium depth (<NUM>-<NUM>. <NUM>), high depth (<NUM>-<NUM>), and extra depth (more than <NUM>) are described in a <CIT>, <CIT>, entitled "Energy-Guided Radiofrequency (RF) Ablation," which is assigned to the assignee of the present patent application.

In some embodiments, the duration, T, of ablation is determined during the ablation by providing a maximally allowable ablation index (AI), i.e., an ablation index threshold (AIT), that is an integral of power level times contact force over a duration, and solving the integral to extract the duration as a function of a given AIT input. The
processor is configured to continuously evaluate the AI during the ablation process, and to stop the ablation when the AI reaches the AIT.

One example for an equation of AIT is described in a <CIT>, which is assigned to the assignee of the present patent application. This example ablation index threshold (AIT), denoted AITl, has the form, <MAT>, where the integral is over duration T, of a product of the contact force, CF, raised to a first non-unity exponent, α, and the power, P, raised to a second non-unity exponent, β, k is a constant of proportionality and γ is a third non-unity exponent. The values of α, β, γ and k are determined by methods disclosed in the aforementioned <CIT>. In an alternative embodiment, the processor uses a second equation for AIT, denoted AIT2, to determine the duration of ablation, where the second AIT is given by <MAT>.

At the beginning of an ablation session, in a catheter introduction step <NUM>, physician <NUM> inserts catheter <NUM> into a desired location in heart <NUM>, using a catheter position tracking system incorporated into system <NUM>.

Next, physician <NUM> makes physical contact between electrode tip <NUM> and endocardial tissue, at an electrode-tissue contact step <NUM>. Processor <NUM> receives contact-force indicative signals from sensors on catheter <NUM> and determines the instantaneous contact force and/ or whether the contact force increased or decreased, at contact force determination step <NUM>.

At an irrigation step <NUM>, processor <NUM> controls irrigation module <NUM> to increase or decrease the irrigation flow rate responsively to a determined respective increase or reduction in instantaneous mechanical force. According to the invention, if the instantaneous contact force has increased, then processor <NUM> directs irrigation module <NUM> to momentarily increase the irrigation flow so that the rate of heat removal by irrigation will match an increase in deposited heat due to the better physical contact between electrode and tissue. On the other hand, if the instantaneous contact force has decreased, meaning less heat is deposited and tissue may be too cooled, then processor <NUM> directs irrigation module <NUM> to momentarily decrease the irrigation flow.

In RF delivery step <NUM>, physician <NUM> operates system <NUM>, with the parameter values selected in step <NUM>, in order to perform the ablation of electrode <NUM>. Display <NUM> of system <NUM> may be configured to display to physician <NUM>, by methods which are known in the art, the progress of the RF delivery to the electrode.

During the RF delivery procedure processor <NUM> monitors the instantaneous contact force using N repeated measurements, by looping back to step <NUM>. The number of repetitions N is calculated by the product of ablation time and contact-force sampling rate in Table I, wherein y example N=<NUM>. Processor <NUM> responsively commands irrigation module <NUM> to modify the irrigation flow rate according to the instantaneous contact force, by repeating step <NUM>.

At each repetition, processor <NUM> checks if the preset amount of ablative energy was delivered, at an ablation energy monitoring step <NUM>. At an ablation ending step <NUM>, processor <NUM> ends ablation after the ablation energy resulting from power and time presets in step <NUM> is achieved, or the indicated time has elapsed.

The example flow chart shown in <FIG> is chosen purely for the sake of conceptual clarity. The sampling rate of contact force is brought by way of example, where a higher rate might be used. The present embodiment also comprises additional steps of the algorithm, such as checking tissue temperature, which have been omitted from the disclosure herein purposely on order to provide a more simplified flow chart.

Although the embodiments described herein mainly address cardiac application, the methods and systems described herein can also be used in ablating other organs of the body such as kidneys (e.g., for renal denervation) and lungs.

Claim 1:
A system (<NUM>) for body tissue ablation, the system comprising:
a generator (<NUM>), configured to generate an ablation signal and to provide the ablation signal to an ablation probe (<NUM>) that is in contact with tissue;
an irrigation module (<NUM>), configured to deliver irrigation fluid to the ablation probe (<NUM>), for applying the irrigation fluid in a vicinity of the tissue while the ablation signal is applied to the tissue; and
a processor (<NUM>), configured to:
receive from the ablation probe (<NUM>) signals indicative of an estimated instantaneous contact force that is exerted by the ablation probe (<NUM>) against the tissue;
characterized in that the processor (<NUM>) is further configured to:
control the irrigation module (<NUM>) to adapt a flow rate of the irrigation fluid responsively to the estimated instantaneous contact force by commanding the irrigation module (<NUM>) to increase or decrease the flow rate responsively to a respective estimated increase or decrease in the instantaneous contact force; and
wherein if the instantaneous contact force increases, the processor (<NUM>) is configured to direct the irrigation module (<NUM>) to increase the flow rate so that a rate of heat removal by irrigation matches an increase in deposited heat due to an increase in instantaneous contact force.