Patent Description:
Ablation of tissue, such as ablation performed by injecting radiofrequency (RF) power into the tissue, is a well-known procedure that is used in cardiac surgery where it is used to correct defects in the heart. Typically, in these cases the ablation is used to inactivate selected groups of cells in the myocardium, so that they no longer transfer an electropotential wave in the myocardium.

Patent application publication <CIT> describes a system for providing irrigation fluid during ablation of tissue includes a catheter, an electrode assembly, at least one thermal sensor adapted to be connected to the catheter, and a control system. The electrode assembly is adapted to be connected to an ablation generator. The thermal sensor is adapted to be operatively connected to an electronic control unit (ECU). The ECU receives as an input temperature measurement data from the thermal sensor; determines a power delivery rate value for the ablation generator responsive to the temperature measurement data; and outputs the power delivery rate value. The control system also delivers irrigation fluid to the irrigated catheter at a first flow rate in a first time period and at a second flow rate in a second time period that is temporally after the first time period. The second flow rate is at least half of the first flow rate.

An embodiment of the present invention provides apparatus, having a probe configured to be inserted into contact with a myocardium, and an electrode attached to the probe. A temperature sensor is incorporated in the probe and is configured to output a temperature signal. The apparatus also has a pump that is configured to irrigate the myocardium, via the probe, with an irrigation fluid at a controllable rate. A radiofrequency (RF) signal generator is configured to apply RF power via the electrode to the myocardium, so as to ablate the myocardium. The apparatus further includes processing circuitry that is configured to measure a temperature of the probe, based on the temperature signal, while the RF power is applied and, when the measured temperature exceeds a preset target temperature, iteratively reduce the RF power applied by the signal generator and concurrently iteratively vary a rate of irrigation of the irrigation fluid provided by the pump, until the measured temperature is reduced to the preset target temperature.

Typically, the circuitry, when the measured temperature does not exceed the preset target temperature, iteratively increases the RF power until the measured temperature is equal to the preset target temperature.

The controllable rate includes an idle irrigation rate and a high irrigation rate greater than the idle irrigation rate, and varying the rate of irrigation includes reducing the rate by pulsing the rate from the high irrigation rate to the idle irrigation rate and returning to the high irrigation rate. The disclosed embodiment may include tubing, attached to the probe, wherein pulsing the rate includes the tubing receiving a single pulse of the irrigation fluid at the idle irrigation rate and smoothing the rate of irrigation at the probe to be <NUM>% of the high irrigation rate.

The controllable rate includes an idle irrigation rate and a high irrigation rate greater than the idle irrigation rate, and varying the rate of irrigation includes increasing the rate by pulsing the rate from the idle irrigation rate to the high irrigation rate and returning to the idle irrigation rate. This embodiment may include tubing, attached to the probe, wherein pulsing the rate includes the tubing receiving a single pulse of the irrigation fluid at the high irrigation rate and smoothing the rate of irrigation at the probe so that the rate increases by between <NUM>% and <NUM>% of the idle irrigation rate.

In a yet further disclosed embodiment the circuitry, when the measured temperature is less than a low target temperature below the preset target temperature, is configured to reduce the rate of irrigation from the high irrigation rate to the idle irrigation rate.

In an alternative embodiment the circuitry, when the measured temperature is between the preset target temperature and the low target temperature, is configured to maintain the rate of irrigation at the high irrigation rate.

The present disclosure will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings, in which:.

During an ablation procedure the ablative power injected into cells needs to be well regulated, since if too little ablative energy is absorbed by the cells they may only partly inactivate, while if too much ablative energy is absorbed it may cause irreversible trauma to the heart. Another consideration for the power injected is the overall time for any given ablation procedure. Physicians typically prefer to keep the time to a minimum, so that in order to inject sufficient energy, the power injected during this time should be high. Thus, a goal for ablative power delivery is that the power level should be as high as possible, subject to not causing trauma.

To achieve this goal embodiments of the present invention provide apparatus, comprising a probe configured to be inserted into contact with a myocardium, an electrode attached to the probe, and a temperature sensor incorporated in the probe. The apparatus also comprises an irrigation module configured to irrigate the myocardium, via the probe, with an irrigation fluid at a controllable rate, and an ablation module configured to apply radiofrequency (RF) power via the electrode to the myocardium, so as to ablate the myocardium. The apparatus also has a temperature module configured, using the temperature sensor, to measure a temperature of the probe while the RF power is applied, and a processor, configured to operate the modules. When the measured temperature exceeds a preset target temperature, the processor iteratively reduces the RF power and concurrently iteratively varies a rate of irrigation of the irrigation fluid, until the measured temperature is reduced to the preset target temperature.

Tissue irrigation is necessary during ablation of the myocardium, to prevent problems such as tissue charring, or steam-pops occurring during the ablation. Legacy ablation systems typically provide irrigation at one of two rates - a low irrigation rate which, inter alia, may be used to maintain irrigation channels clear, and a high rate, which is used to prevent the problems referred to above. However, the high rate may lead to the tissue being overcooled, and in this case ablation power must be delivered for a longer-than-optimal time to correctly ablate the tissue.

An embodiment of the present invention solves the longer-than-optimal time delivery of legacy ablation systems by pulsing the irrigation rate between the low and high rates in a controlled manner. (The controlled pulsing has a similar effect to that of pulse width modulation for electronic systems. ) In some embodiments of the present invention the pulsatory irrigation rate is smoothed, by tubing used to supply the irrigation fluid, so that the irrigation rate at the tissue is substantially constant. In addition, by varying the rate at which high rate pulses are applied, the smoothed irrigation rate may be varied in a substantially continuous manner between the low rate and the high rate.

<FIG> is a schematic illustration of an invasive medical procedure using apparatus <NUM>, and <FIG> is a schematic illustration of a distal end <NUM> of a probe <NUM> used in the apparatus, according to an embodiment of the present invention. The procedure is performed by a medical professional <NUM>, and, by way of example, the procedure in the description hereinbelow is assumed to comprise ablation of a portion <NUM> of a myocardium <NUM> of the heart of a human patient <NUM>. However, it will be understood that embodiments of the present invention are not just applicable to this specific ablation procedure, and may include substantially any ablation procedure on biological tissue or on non-biological material.

In order to perform the ablation, professional <NUM> inserts a probe <NUM> into a sheath <NUM> that has been pre-positioned in a lumen of the patient. Sheath <NUM> is positioned so that distal end <NUM> of the probe may enter the heart of the patient, after exiting a distal end of the sheath, and contact tissue of the heart. Distal end <NUM> comprises a position sensor <NUM> that enables the location and orientation of the distal end to be tracked, and one or more temperature sensors <NUM> that measure the temperature at respective locations of the distal end. Distal end <NUM> also comprises an electrode <NUM> which is used to deliver radiofrequency ablation power to myocardium <NUM> in order to ablate the myocardium. (Electrode <NUM> may also be used to acquire electropotentials from the myocardium, as noted below.

Apparatus <NUM> is controlled by a system processor <NUM> which comprises real-time noise reduction circuitry <NUM>, typically configured as a field programmable gate array (FPGA), followed by an analog-to-digital (A/D) signal conversion integrated circuit <NUM>. The processor can pass the signals from A/D circuit <NUM> to modules described herein, and/or another processor and/or can be programmed to perform at least one of the algorithms disclosed herein, the algorithms comprising steps described hereinbelow. The processor uses circuitry <NUM> and circuit <NUM>, as well as features of the modules referred to above, in order to perform the algorithms. Processor <NUM> and the modules operated by the processor are herein termed processing circuitry <NUM>.

Processor <NUM> is located in an operating console <NUM> of the apparatus. Console <NUM> comprises controls <NUM> which are used by professional <NUM> to communicate with processor <NUM>, and to implement the procedure the processor communicates with modules in a module bank <NUM>. Modules in bank <NUM> are described below.

During the procedure performed by professional <NUM>, distal end <NUM> is supplied with irrigation fluid, typically normal saline solution, from a pump <NUM>, and the pump transfers the fluid to probe <NUM> via irrigation tubing <NUM>. The irrigation fluid is expelled through irrigation holes <NUM> in the distal end.

Except as stated below, pump <NUM> is assumed to be able to operate in one of two modes: an idle mode, wherein the pump pumps the irrigation fluid at a slow rate, also herein termed an idle rate, and a full flow mode, wherein the pump pumps the fluid at a fast rate, also herein termed a full flow rate. Each of the rates may be preset before the pump is used in apparatus <NUM>, and in one embodiment the idle rate may be set within a range of <NUM> - <NUM>/min, and the full rate may be set within a range of <NUM> - <NUM>/min.

In some embodiments the flow rate from pump <NUM> may be continuously adjusted by using a PID (proportional integral derivative) algorithm to control the flow rate according to the radiofrequency ablation power delivered. For simplicity and clarity, the description hereinbelow assumes that pump <NUM> operates in one of the two modes (an idle mode or a full flow mode) described above, and those of ordinary skill in the art will be able to adapt the description, mutatis mutandis, if the flow from the pump can be continually adjusted.

As stated above, in order to operate apparatus <NUM>, processor <NUM> communicates with module bank <NUM>. Thus, bank <NUM> comprises a tracking module <NUM> which receives and analyzes signals from position sensor <NUM>, and which uses the signal analysis to generate a location and an orientation of distal end <NUM>. In some embodiments sensor <NUM> comprises one or more coils which provide the sensor signals in response to magnetic fields traversing the coils. In these embodiments, in addition to receiving and analyzing signals from sensor <NUM>, tracking module <NUM> also controls magnetic radiators (not shown in the figures) which radiate the magnetic fields traversing sensor <NUM>. The radiators are positioned in proximity to myocardium <NUM>, and are configured to radiate alternating magnetic fields into a region in proximity to the myocardium.

Alternatively or additionally, tracking module <NUM> may measure impedances between electrode <NUM> and electrodes (not shown in the figures) on the surface of patient <NUM>, and processor <NUM> and the tracking module may use the impedances to track the location and orientation of distal end <NUM>. The Carto® system produced by Biosense Webster, of <NUM> Technology Drive, Irvine, CA <NUM> USA, uses such a magnetic tracking system and an impedance tracking system.

As explained in more detail below, an irrigation module <NUM> controls the rate of flow of the fluid from pump <NUM>, by switching between the two modes of operation of the pump. Irrigation module <NUM> is under overall control of processor <NUM>.

Processor <NUM> uses a temperature module <NUM> to analyze signals received from one or more temperature sensors <NUM> in distal end <NUM>. From the analyzed signals, processor <NUM> determines temperatures of the distal end, and uses the temperatures in the algorithms described below.

Module bank <NUM> also comprises an ablation module <NUM>. Ablation module <NUM> comprises a radiofrequency (RF) generator <NUM>, which enables processor <NUM> to inject RF current, via electrode <NUM> of the distal end and one or more returning electrodes (not shown in the figures) on the skin of the patient, into myocardium <NUM>, in order to ablate regions of the myocardium which are in contact with the electrode. The ablation module also enables the processor to set parameters of the injected current, such as its frequency, the level of the power injected. and the duration of the injection.

In embodiments of the present invention, the level of the power injected may be provided to ablation module <NUM> by professional <NUM> as an ablation target power, which is a maximum power that may be injected into the patient's tissue by electrode <NUM>. Typically the ablation target power is set within an approximate range of <NUM> W - <NUM> W, although the ablation target power may be set outside this range.

In embodiments of the present invention apparatus <NUM> is configured to operate in one of two power modes. In a low power mode, the ablation target power is set to be less than or equal to a preset power level. In a high power mode the ablation target power is set to be greater than the preset power level. By way of example, in the description herein the preset power level is assumed to be <NUM> W. However, it will be understood that the preset power level, separating the two power modes, may be higher or lower than <NUM> W.

The software for processor <NUM> and module bank <NUM> may be downloaded to the processor in electronic form, over a network, for example. Alternatively or additionally, the software may be provided on non-transitory tangible media, such as optical, magnetic, or electronic storage media.

In order to operate apparatus <NUM>, module bank <NUM> typically comprises modules other than those described above, such as a force module which acquires signals from a force sensor in the distal end and which analyzes the signals to determine a force on the distal end. For simplicity, such other modules and their associated sensors are not illustrated in <FIG>. All modules may comprise hardware as well as software elements.

<FIG> is a first flowchart of steps of an algorithm followed by processor <NUM> when apparatus <NUM> is operating in the low power mode described above, while professional <NUM> performs the ablation procedure referred to above, and <FIG> is a second flowchart of steps of the algorithm followed by the processor, according to an embodiment of the present invention. As is described below, in the first flowchart, also referred to herein as flowchart <NUM>, the processor varies the power, and in the second flowchart, also referred to herein as flowchart <NUM>, the processor varies the irrigation rate. The processor operates both flowcharts concurrently.

In the first flowchart (<FIG>) in an initial step <NUM>, typically performed prior to the actual ablation, the professional uses controls <NUM> to assign values to parameters used by the processor in performing the algorithm.

Typical parameters set in the initial step comprise:.

Once the parameters have been set in step <NUM>, control of the algorithm proceeds to a begin ablation step <NUM>, wherein the processor ramps the power dissipated by electrode <NUM> up to the target power level set in step <NUM>. Depending whether the target power level sets the apparatus to operate in the low power mode or the high power mode, the irrigation rate is set accordingly, i.e., for the low power mode at the low irrigation rate, and for the high power mode at the high irrigation rate. Since, as stated above, the target power level is set in step <NUM> at <NUM> W, corresponding to the low power mode, then in step <NUM> the irrigation rate is set at the idle irrigation flow rate.

In a condition <NUM>, the processor uses temperature module <NUM> to check if the maximum temperature measured by any one of sensors <NUM> is lower than the target temperature set in step <NUM>. Condition <NUM> iterates at a preset rate, which in an embodiment of the present invention is every <NUM>.

If condition <NUM> returns positive, i.e., if the temperature is less than the target temperature, then in an increase power step <NUM> processor <NUM> uses the ablation module to increase the power, typically by the same value as the power reduction factor set in step <NUM>, up to the target power.

If condition <NUM> returns negative, then in a decrease power step <NUM> processor <NUM> uses the ablation module to decrease the power by the power reduction factor. Further details of the power decrease are described in flowchart <NUM> (<FIG>).

In flowchart <NUM> the initial steps of the flowchart, steps <NUM>, <NUM>, and <NUM>, are as described above with reference to flowchart <NUM> (<FIG>). If in flowchart <NUM> condition <NUM> returns positive, i.e., the maximum temperature is less than the target temperature, then in a continuing ablation step <NUM> the processor continues with the ablation, and control returns to condition <NUM>.

If condition <NUM> returns negative, i.e., the maximum temperature is equal to or greater than the target temperature, then in a power titration step <NUM> the processor uses ablation module <NUM> to titrate the power level down by the preset reduction factor set in step <NUM>. Control then continues to a second condition <NUM>.

In second condition <NUM>, the processor interrogates ablation module <NUM> to find the level of power being injected into electrode <NUM>, and the processor checks if the level has been reduced by more than the power delta set in step <NUM>. If the second condition returns negative, i.e., the power has not been reduced from the target power value by the power delta, control returns to condition <NUM>, which continues to iterate at its preset rate.

If second condition <NUM> returns positive, i.e., the power has been reduced from the target power value by more than the power delta, control of the algorithm continues to an irrigation pulse step <NUM>. In step <NUM> irrigation module <NUM> configures pump <NUM> to transfer from its idle mode, i.e., pumping at the idle rate set in step <NUM>, to its full flow mode wherein the pump pumps the irrigation fluid at its high rate set in step <NUM>. The transfer to the full flow mode continues for the irrigation pulse period set in step <NUM>, after which module <NUM> returns pump <NUM> to pumping at its idle rate.

At the conclusion of step <NUM>, control continues to a third condition <NUM>, wherein the processor checks if the power set in flowchart <NUM> (<FIG>), is equal to the target power.

If condition <NUM> returns positive, i.e., the power is equal to the target power, then in a further continuing ablation step <NUM> the processor uses the irrigation module to maintain the irrigation rate at the idle rate, and transfers control back to first condition <NUM>.

If condition <NUM> returns negative, i.e., the power has not returned to the target power, then control returns to irrigation pulse step <NUM>, so that the irrigation rate again pulses to a high rate.

Processor <NUM> continues implementing the steps of the two flowcharts <NUM>, <NUM> concurrently for the ablation time set in step <NUM>, after which the implementation ceases.

<FIG> illustrates graphically the operation of pump <NUM> while flowcharts <NUM>, <NUM> are operative, according to an embodiment of the present invention. A graph <NUM> plots irrigation flow rate vs. time, and a solid line <NUM> of the graph illustrates the output flow rate of pump <NUM>.

A section <NUM> of graph <NUM> illustrates the flow rate from pump <NUM>, as solid line <NUM>, as flowchart <NUM> proceeds to step <NUM>, and then continues via condition <NUM>, which returns positive, to step <NUM>. In this case condition <NUM> is addressed only once, so that the flow rate from the pump begins at the idle rate, pulses for one irrigation pulse period to the high rate and then returns to the idle rate.

A section <NUM> of graph <NUM> illustrates the flow rate from pump <NUM>, as solid line <NUM>, as flowchart <NUM> proceeds to step <NUM>, and then continues to condition <NUM>, which returns negative, so returning to step <NUM>. In this case condition <NUM> iterates, so that while the flow rate from the pump begins at the idle rate, the flow rate from the pump continues with multiple pulses, that present as effectively one long pulse, at the high rate.

As stated above solid line <NUM> illustrates the output of pump <NUM>. However, the pulsatory output from the pump is smoothed, or averaged, by irrigation tubing <NUM>, and the smoothed output is illustrated schematically by a broken line <NUM> for section <NUM>, and a broken line <NUM> for section <NUM>. The smoothed output is the irrigation flow rate at distal end <NUM>.

For the irrigation pulse period of <NUM> of the disclosed embodiment referred to above, one pulse at a high rate of <NUM>/min, during an idle rate of <NUM>/min, typically increases the irrigation rate by between <NUM>% and <NUM>% of the idle rate, i.e., to an effective smoothed irrigation rate between <NUM>/min and <NUM>/min. A train of two or more pulses typically increases the effective irrigation rate to the high rate.

It will be understood that by varying the rate of pulsation of pump <NUM>, and due to the smoothing effect of tubing <NUM>, the irrigation flow rate at distal end <NUM> can be varied substantially continuously between the idle irrigation rate and the high irrigation rate.

<FIG> is a first flowchart of steps of an alternative algorithm followed by processor <NUM> when apparatus <NUM> is operating in the high power mode referred to above, while professional <NUM> performs the ablation procedure, and <FIG> is a second flowchart of steps of the alternative algorithm followed by the processor, according to an embodiment of the present invention. The flowchart of <FIG> is also referred to herein as flowchart <NUM>, and the flowchart of <FIG> is also referred to herein as flowchart <NUM>.

As for flowcharts <NUM> and <NUM> (<FIG> and <FIG>), in flowchart <NUM> (<FIG>) the processor varies the power, and in flowchart <NUM> (<FIG>) the processor varies the irrigation rate; the processor operates both flowcharts <NUM> and <NUM> concurrently.

An initial step <NUM> of flowchart <NUM> (<FIG>) is substantially as described above for step <NUM>, except that rather than setting one target temperature, a high target temperature and a low target temperature are set. The high target temperature is typically set to be in an approximate range of <NUM> to <NUM>, although values outside this range are possible. The low target temperature is typically set to be in an approximate range of <NUM> to <NUM>, although values outside this range are also possible. Regardless of the actual values of the high and low target temperatures, the low target temperature is set to be at least <NUM> less than the high target temperature. In a disclosed embodiment the high target temperature is set at <NUM> and the low target temperature is set at <NUM>.

A condition <NUM> is substantially similar to condition <NUM>, except that processor uses temperature module <NUM> to check if the maximum temperature measured by any one of sensors <NUM> is lower than the high target temperature.

If condition <NUM> returns positive, i.e., if the temperature is less than the high target temperature, then in an increase power step <NUM> processor <NUM> uses the ablation module to increase the power, typically by the same value as the power reduction factor set in step <NUM>, up to the target power.

If condition <NUM> returns negative, then in a decrease power step <NUM> processor <NUM> uses the ablation module to decrease the power by the power reduction factor. Further details of the power decrease are described in flowchart <NUM>.

In flowchart <NUM> (<FIG>) the initial steps of the flowchart, steps <NUM>, <NUM>, and <NUM>, are as described above with reference to flowchart <NUM>. If in flowchart <NUM> condition <NUM> returns positive, i.e., the maximum temperature is less than the high target temperature, then control transfers to a further condition <NUM>, where the processor checks if the maximum temperature is less than the low target temperature. Condition <NUM> typically iterates at the same preset rate as condition <NUM>.

If condition <NUM> returns negative, so that the maximum temperature is between the low and high target temperatures, then control transfers to a continuing ablation step <NUM>, wherein ablation is continued at the high irrigation rate set initially, and control returns to condition <NUM>.

If condition <NUM> returns positive, so that the maximum temperature is below the low target temperature, then control transfers to a reduce irrigation step <NUM>, where the processor reduces the high irrigation rate set initially to the idle irrigation rate. Ablation continues at the idle irrigation rate in a continuing ablation step <NUM> and control transfers back to iterating condition <NUM>.

The path of condition <NUM>, step <NUM>, and step <NUM> illustrates that while the maximum temperature is below the low target temperature, the processor maintains the irrigation at its low idle rate.

Returning to condition <NUM>, if the condition returns negative, i.e., the maximum temperature is equal to or greater than the high target temperature, then in a power titration step <NUM> the processor titrates the power down, substantially as described in power titration step <NUM>. Control then continues to a power reduction condition <NUM>.

Condition <NUM> is substantially as described for condition <NUM>, i.e., the processor interrogates ablation module <NUM> to check if the power level has been reduced by more than the power delta set in step <NUM>. If condition <NUM> returns negative, i.e., the power has not been reduced from the target power value by the power delta, control returns to condition <NUM>, which continues to iterate at its preset rate.

If condition <NUM> returns positive, i.e., the power has been reduced from the target power value by more than the power delta, control of the algorithm continues to an irrigation pulse step <NUM>. In step <NUM> irrigation module <NUM> configures pump <NUM> to transfer from its full flow mode, i.e., pumping at the high rate set in step <NUM>, to its idle mode wherein the pump pumps the irrigation fluid at its low rate set in step <NUM>. The transfer to the idle mode continues for the irrigation pulse period set in step <NUM>, after which module <NUM> returns pump <NUM> to pumping at its full rate.

At the conclusion of step <NUM>, control continues to a power check condition <NUM>, wherein the processor checks if the power set in flowchart <NUM> (<FIG>), is equal to the target power.

If condition <NUM> returns positive, i.e., the power is equal to the target power, then control continues at continuing ablation step <NUM>, where the irrigation module maintains the irrigation rate at the full rate, and transfers control back to condition <NUM>.

If condition <NUM> returns negative, i.e., the power has not returned to the target power, then control returns to irrigation pulse step <NUM>, so that the irrigation rate again pulses to a low rate.

A section <NUM> of graph <NUM> illustrates the flow rate from pump <NUM>, as solid line <NUM>, as flowchart <NUM> proceeds to step <NUM>, and then continues via condition <NUM>, which returns positive, to step <NUM>. In this case condition <NUM> is addressed only once, so that the flow rate from the pump begins at the full rate, pulses for one irrigation pulse period to the low rate and then returns to the idle rate.

A section <NUM> of graph <NUM> illustrates the flow rate from pump <NUM>, as solid line <NUM>, as flowchart <NUM> proceeds to step <NUM>, and then continues to condition <NUM>, which returns negative, so returning to step <NUM>. In this case condition <NUM> iterates, so that while the flow rate from the pump begins at the high rate, the flow rate from the pump continues with multiple pulses, that present as effectively one long pulse, at the low rate.

The smoothing is generally similar to that described above with respect to <FIG>. Thus, for an irrigation pulse period of <NUM>, a single pulse at an idle rate of <NUM>/min, during a high rate of <NUM>/min, typically reduces the irrigation rate by approximately <NUM>% of the high rate, i.e., to approximately <NUM>/min. A train of two or more pulses typically reduces the effective irrigation rate to the idle rate.

Claim 1:
Apparatus (<NUM>), comprising:
a probe (<NUM>) configured to be inserted into contact with a myocardium;
an electrode (<NUM>) attached to the probe;
a temperature sensor incorporated in the probe and configured to output a temperature signal;
a pump (<NUM>) configured to irrigate the myocardium, via the probe, with an irrigation fluid at a controllable rate;
a radiofrequency (RF) signal generator (<NUM>) configured to apply RF power via the electrode to the myocardium, so as to ablate the myocardium; and
processing circuitry (<NUM>) configured to measure a temperature of the probe, based on the temperature signal, while the RF power is applied and, when the measured temperature exceeds a preset target temperature, iteratively reduce the RF power applied by the signal generator and concurrently iteratively vary a rate of irrigation of the irrigation fluid provided by the pump, until the measured temperature is reduced to the preset target temperature, wherein the controllable rate comprises an idle irrigation rate and a high irrigation rate greater than the idle irrigation rate, and wherein varying the rate of irrigation comprises
a) reducing the rate by pulsing the rate from the high irrigation rate to the idle irrigation rate; and
b) increasing the rate by pulsing the rate from the idle irrigation rate to the high irrigation rate.