US20010020166A1

Movatterモバイル変換

Info

Publication number: US20010020166A1
Application number: US08/952,175
Authority: US
Inventors: Michael Daly; Pramesh Kovoor; Konstadinos Hatzianestis; David Ross
Original assignee: Individual
Current assignee: Cathrx Ltd
Priority date: 1996-04-30
Filing date: 1997-04-30
Publication date: 2001-09-06
Anticipated expiration: 2017-04-30
Also published as: US6346104B2; AUPN957296A0; WO1997040760A1

Abstract

A system (2,4), method and splitter (6) for ablating tissue (15) using radiofrequency (RF) energy is disclosed. The system (2,4) ablates tissue (15) using unipolar RF energy simultaneously delivered to multiple electrodes (22A-22D) in one or more probes (20). This is carried out by the multiple channel RF splitter (6) that can independently control the RF energy delivered through each channel (18) to a respective electrode (22A-22D) in a continuous manner. Each electrode (22A-22D) has a corresponding temperature sensor or transducer (36A-36D) that is processed independently so that the amount of RF energy delivered to each electrode (22A-22D) can be varied dependent on the temperature of the electrode (22A-22D) so that the lesion size produced by each electrode (22A-22D) can be accurately controlled. Preferably, each probe (20) has a needle-like structure with a number of electrodes (22A-22D) separated by insulative material and is adapted to puncture tissue. Each channel (18) of the splitter (6) has circuitry for interrupting current delivered to the respective channel if a predetermined temperature or current level is exceeded.

Description

BACKGROUND

The present invention relates to a system, method and apparatus for ablating tissue under temperature control of each electrode to control lesion dimensions, and in particular for ablating myocardial tissue to treat Ventricular Tachycardia (VT) or atrial fibrillation/flutter (AF).[0001]

Ventricular tachycardia is a disease of the heart which causes the heart chambers to beat excessively fast and usually degenerates to ventricular fibrillation where the heart chambers do not effectively pump blood through the body's system and hence leads to death. Ventricular tachycardia is the most common cause of cardiac arrest and sudden death. Typical features of patients with VT are (1) a history of myocardial infarction (heart attack), (2) significant left ventricular dysfunction (the main chamber effecting the pumping action), and (3) left ventricular aneurysm (dilation, thinning and stretching of the chamber). Detailed mapping studies of the electrical propagation within the myocardium during VT have shown that a re entrant pathway within and around the scarring (caused by infarction) is responsible for the arrhythmia, These studies have shown that the critical area of myocardium necessary to support reentry appears to be less than 2 to 4 cm[0002]².

Atrial fibrillation (AF) and atrial flutter are diseases of the heart which can cause the heart to beat excessively fast and frequently in an erratic manner. This usually results in distress for patients. This may also be associated with clot formation in the atria, which may become dislodged and cause strokes. AF is usually due to abnormal electrical activation of the atria. Preliminary investigations have shown that linear lesions in the atria using radiofrequency ablation can cure these arrhythmias.[0003]

A number of conventional techniques using radio frequency (RF) energy have been used to treat VT or AF. Endocardial radio frequency catheter ablation has been used in the treatment of hemodynamically stable monomorphic ventricular tachycardia secondary to coronary artery disease. The resulting lesions caused in RF ablation using catheters however have been insufficient in volume to destroy the area of tissue causing the arrhythmia.[0004]

Radiofrequency catheter ablation has been used for treatment of AF, but has been limited by the number of separate ablations required and the time required to perform the procedure.[0005]

In accordance with one conventional technique, RF energy is delivered from an RF source, incorporating phase shift networks to enable potential differences and hence current flow between multiple, separate electrode structures. Also, multiple RF power sources have been used connected to such electrodes. The independent phases of the power source lead to multiple current paths.[0006]

However, this conventional system lacks adequate temperature control because the multiphase RF ablation cannot function satisfactorily unless certain restrictions on the dimensions of the electrode are adhered to. The ablation temperature can only be maintained at an optimum predetermined level of approximately 80° C. This is a significant shortfall of the technique.[0007]

SUMMARY

The present invention is directed to improving the efficacy of producing radio frequency lesions using multiple temperature controlled delivery by splitting high frequency current from a single generator into a number of electrodes simultaneously. Further, the system accurately measures the temperatures of these electrodes which are then used as the feedback in the system, allowing appropriate control strategies to be performed to regulate the current to each electrode.[0008]

In accordance with a first aspect of the invention, a system for ablating tissue comprises;[0009]

a device for generating RF energy;[0010]

a probe device comprising N separate electrodes, each having a corresponding device for sensing the temperature of the electrode;[0011]

a splitter device for splitting the RF energy coupled to the generating device and the probe device, the splitter device having N separate channels each being coupled to a corresponding one of the N electrodes and temperature sensing device; and[0012]

a device for controlling the splitter device, whereby the ablation of tissue at each electrode is independently controlled using closed loop feedback of the temperature of the electrode by independently regulating the amount of the RF energy delivered to each electrode.[0013]

Preferably, the system comprises a plurality of the probe devices and the splitter devices, and the controlling device separately controls each of the probe devices and the corresponding splitter device.[0014]

Preferably, the probe device has an elongated needle-like structure with one end adapted to puncture tissue and having sufficient rigidity to puncture the tissue, or a catheter which can be advanced into the heart. Bach of the electrodes may consist of a circular metal surface separated one from another by insulation.[0015]

Preferably, the RF energy has a single phase.[0016]

Preferably, the system further comprises a device for independently and continuously adjusting the RF energy delivered to each electrode in response to a control signal from the controlling device dependent on the temperature of the electrode.[0017]

Preferably, the controlling device is programmable.[0018]

Optionally, the probe device is a catheter probe device.[0019]

Preferably, each of the temperature sensing devices is a thermocouple. Preferably, the splitter device comprises one or more devices for independently interrupting current from the RF energy generating device to a respective electrode.[0020]

In accordance with a second aspect of the invention, a medical apparatus for treatment by radiofrequency ablation of tissue comprises:[0021]

an RF energy generator;[0022]

one or more probes each comprising a plurality of separate electrodes and corresponding temperature sensors for sensing the temperature of the electrodes, each temperature sensor connected to a respective one of the plurality of electrodes;[0023]

a splitter for splitting the RF energy provided by the RF energy generator, the splitter having a plurality of separate channels, wherein each of the electrodes is coupled to a respective one of the plurality of channels; and[0024]

a programmable controller coupled to the RF splitter for independently controlling the ablation of tissue at each electrode using closed loop feedback of the temperature of the electrode, whereby the amount of the RF energy delivered to each electrode is independently regulated by the programmable controller.[0025]

In accordance with a third aspect of the invention, a radio frequency energy splitter for use with one or more probes in a system for RF ablation of tissue is provided. Each probe comprises a plurality of separate electrodes and corresponding temperature sensors for sensing the temperature of the electrode. The splitter comprises:[0026]

an input device for receiving RF energy from an RF energy generator;[0027]

a plurality of channel modules for separately delivering RP energy from the input device to a respective electrode of the plurality of electrodes of the one or more probes, each channel module comprising:[0028]

a device for variably adjusting an amount of the RF energy delivered to the respective electrode in response to a control signal, the variable adjusting device being coupled between the input device and the respective electrode;[0029]

a device for interrupting the RF energy delivered to the respective electrode:[0030]

an output device coupled to the respective temperature sensor for providing a temperature signal;[0031]

a device for determining if the temperature at the respective electrode exceeds a predetermined threshold and actuating the interrupting device if the threshold is exceeded, whereby the RF energy is interrupted from delivery to the respective electrode;[0032]

wherein each channel module is capable of receiving the respective control signal from and providing the respective temperature signal to a programmable controller so that the amount of the RF energy delivered to each electrode can be independently regulated using closed loop feedback of the temperature of each electrode.[0033]

Preferably, the variable adjusting device or circuit comprises a bridge rectifier including a fast-switching variable resistance for controlling operation of the bridge rectifier in response to the control signal.[0034]

Preferably, the RF energy interrupting device comprises a circuit for interrupting a current through the RF energy interrupting device and a circuit for limiting the current.[0035]

Preferably, the determining device compares the temperature signal with the predetermined threshold.[0036]

In accordance with a fourth aspect of the invention, a method for ablating tissue comprises the steps of:[0037]

generating RF energy;[0038]

providing a probe device comprising N separate electrodes, each having a corresponding temperature sensing device;[0039]

measuring the temperature of each electrode using the temperature sensing device of the electrode;[0040]

splitting the RF energy to the probe device into N separate channels each being coupled to a corresponding one of the N electrodes and temperature sensing device; and[0041]

controlling the splitting of the RF energy to the probe device, whereby the ablation of tissue at each electrode is independently controlled using closed loop feedback of the measured temperature of the electrode by independently regulating the amount of the RF energy delivered to each electrode.[0042]

Preferably, the method comprises the step of separately controlling the splitting of the RF energy to a plurality of the probe device.[0043]

Preferably, the probe device has an elongated needle-like structure with one end adapted to puncture tissue and having sufficient rigidity to puncture the tissue, wherein each of the electrodes consists of a metal substantially circular surface separated one from another by insulation.[0044]

Preferably, the RF energy has a single phase.[0045]

Preferably, the method further comprises the step of independently and continuously adjusting the RF energy delivered to each electrode in response to a control signal from a programmable controlling device dependent on the temperature of the electrode.[0046]

In accordance with a fifth aspect of the invention, a method for treatment by radiofrequency (RF) ablation of tissue comprises the steps of:[0047]

generating RF energy;[0048]

providing one or more probes each comprising a plurality of separate electrodes and corresponding temperature sensors, each temperature sensor connected to a respective one of the plurality of electrodes;[0049]

measuring the temperature of each electrode using the respective temperature sensor;[0050]

splitting the RF energy into a plurality of separate channels, wherein each of the electrodes is coupled to a respective one of the plurality of channels; and[0051]

programmably controlling the splitting of the RF energy so as to independently control the ablation of tissue at each electrode using closed loop feedback of the measured temperature of the electrode, whereby the amount of the RF energy delivered to each electrode is independently regulated.[0052]

Preferably, the method involves using at least two probes, and comprises the step of programmably controlling each of the probes separately.[0053]

In accordance with a sixth aspect of the invention, there is provided a method of splitting radio frequency energy delivered to one or more probes in a system for RF ablation of tissue. Each probe comprises a plurality of separate electrodes and corresponding temperature sensors for sensing the temperature of the electrode. The method comprises the steps of:[0054]

receiving RF energy from an RF energy generator;[0055]

providing a plurality of channel modules for separately delivering the RF energy to a respective electrode of the plurality of electrodes of the one or more probes, further comprising, for each channel module, the sub-steps of:[0056]

variably adjusting an amount of the RF energy delivered to the respective electrode in response to a control signal;[0057]

measuring the temperature of the respective electrode using the corresponding temperature sensor to provide a temperature signal;[0058]

determining if the temperature at the respective electrode exceeds a predetermined threshold and interrupting delivery of the RF energy to the respective electrode if the threshold is exceeded;[0059]

wherein each channel module is capable of receiving the respective control signal from and providing the respective temperature signal to a programmable controller so that the amount of the RF energy delivered to each electrode can be independently regulated using closed loop feedback of the temperature of each electrode.[0060]

Preferably, the step of variably adjusting the RF energy comprises the step of changing the resistance of a fast-switching variable resistance incorporated in a bridge rectifier in response to the control signal.[0061]

Preferably, the step of interrupting the RF energy comprises the steps of interrupting a current to the respective electrode and limiting the current.[0062]

Preferably, the step of determining comprises the step of comparing the temperature signal with the predetermined threshold.[0063]

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described hereinafter with reference to the drawings, in which[0064]

FIG. 1 is a block diagram of the RF ablation system according to one embodiment;[0065]

FIG. 2. is a detailed schematic of a single channel of the system of FIG. 1;[0066]

FIG. 3 is a detailed schematic of the system of FIG. 1, wherein N=4; and[0067]

FIG. 4 is a detailed schematic of a single channel of an RF ablation system according to another embodiment.[0068]

DETAILED DESCRIPTION

First Embodiment[0069]

The RF ablating system according to a first embodiment shown in FIG. 1 comprises a programmable controller[0070]2, an N-channel RF splitter6, anRF generator8, a large conductive,dispersive plate12, and an N-electrode probe20. RF generators for RF ablation of tissue are well known in the art. It will be appreciated by a person skilled in the art that the present invention can be practiced with any of a number of RF generators without departing from the scope and spirit of the invention.

Preferably, the probe[0071]20 has a needle-like structure wherein each of theelectrodes22A to22D has a tubular or ring shape. Theelectrodes22A to22D are separated from each other by an intervening insulative portion. Such a probe structure is disclosed in International Publication No. WO 97/06727 published on Feb. 27, 1997 (International Application No. PCT/AU96/00489 by the Applicant) and incorporated herein by cross-reference. The structure of this probe20 enables theelectrodes22A to22D to be inserted into the myocardium for use in the present system. While this embodiment is described with reference to a single needle probe20, the system may be practiced with a plurality of such needle probes20 and one or more corresponding N-channel RF splitters6 that are controlled by the programmable controller2. It will be apparent to a person skilled in the art that the embodiment is not limited to the use of such needle-like probes but may be practiced with other types of ablating probes including catheters.

Further, while this embodiment is discussed with reference to ablation of reentrant pathways in relation to ventricular tachycardia, the system is not limited to this particular application, and instead can practiced in relation to a number of other applications. For example, the system may be used to ablate tissue causing atrial fibrillation or flutter, tumors, or for coagulation treatment.[0072]

The programmable controller[0073]2 may be implemented using a general purpose computer executing a control algorithm to operate theRF splitter8 in response to measured temperatures of theelectrodes22A to22D, as described below. In this embodiment, the programmable controller2 is preferably implemented using an AMLAB instrument emulator (published in International Publication No. WO92/15959 on Sep. 17, 1992; International Application No. PCT/AU92/00076), which comprises a general purpose computer having a digital signal processor subassembly that is configurable using a graphical compiler. The programmable controller2 is connected to the N-channel RF splitter6 via N output control signals14 and N temperature signals16 provided from the N-channel RF splitter6 to the programmable controller2. The N-channel RF splitter6, theRF generator8, the RMS-to-DC converter10, the probe20, and thedispersive plate12, shown asmodule4, are provided so as to meet electrical isolation barrier requirements in accordance with IEC 601 and AS3200.1 type CF standards.

The N-channel RF splitter[0074]6 provides RF energy from theRE generator8 coupled to the splitter6 via N electrical connections18 to the correspondingelectrodes22A to22D of the probe20. In addition, the N electrical connections18 are connected to corresponding thermocouples of each of theelectrodes22A to22D. While thermocouples are preferably employed, other temperature transducers or sensing circuits/devices may be practiced without departing from the scope and spirit of the invention. For example, a temperature sensing device for a respective electrode of one or more electrodes could include a thermistor or other temperature transducer. The N temperature signals16 provided to the programmable controller2 are obtained from the temperature sensing devices of theelectrodes22A to22D. TheRF generator8 is also connected to thedispersive electrode12 via the RMS-to-DC converter10.

This embodiment advantageously employs a single RF generator in which the N-channel RF splitter[0075]6 independently controls the delivery of RF energy of a single phase to one or more of theelectrodes22A to22D of the probe20. The temperature of each of theelectrodes22A to22D is independently monitored by the programmable controller2, which in turn provides the control signals14 to the N-channel RF splitter6 to simultaneously control the amount of RF energy delivered to thecorresponding electrode22A to22D.

Using closed-loop feedback and independent, simultaneous control of each electrode, the system is able to advantageously regulate temperatures to occur at each electrode at the desired temperature. This produces optimum lesion size, and avoids charring and vaporisation associated with temperatures greater than 100° C. This is in marked contrast to the prior art, since the embodiment provides a margin of at least 20° C., highlighting the lack of temperature control of all of the electrodes in the conventional system. The prior art is able to affect only the temperature of the electrode being monitored. As lesions size is proportional to the temperature of the electrodes, the system according to this embodiment is able to controllably produce larger lesion. The ability to maintain all electrodes at a desired temperature simultaneously and independently enables contiguous uniform lesions, not as dependent on the size and contact area of each electrode. Conversely, if it is desired to deliver RF energy to only one particular electrode to minimise thermal damage to “good” tissue, the system according to this embodiment is able to ensure that adjacent electrodes have minimal current. That is, the system according to this embodiment has the ability to ensure precise temperature control of each electrode individually and simultaneously.[0076]

FIG. 3 is a detailed schematic diagram of the system of FIG. 1. As shown in FIG. 3, the number of electrodes and separate channels N is preferably four (4). However, this embodiment may be practiced with a different number (e.g., N=3 or N=5) of electrodes and channels without departing from the scope and spirit of the present invention. Further, the splitter may be practiced with N channels and a number of separate probes where the total number of electrodes of the probes is less than or equal to N. A[0077]

single electrode

22A and corresponding channel of the N-channel RF splitter6 is described hereinafter with reference to FIG. 2. While asingle electrode22A and corresponding channel are described, it will be apparent to a person skilled in the art that the following description applies equally to the three remainingelectrodes22B to22D and the corresponding channels of the splitter of FIG. 3.

In FIG. 2, the[0078]

control signal

14A output by the programmable controller2 is provided to anisolation amplifier42A which in turn is connected to a fast-switching,full bridge rectifier34A. In particular, the output of theisolation amplifier42A is connected to a fast-switchingvariable resistance48A used to control operation of therectifier bridge34A. Preferably, thevariable resistance48A is implemented using a power N-channel enhancement MOSFET. The programmable controller2 receives atemperature signal16A from the output of anotherisolation amplifier40A.

One terminal of the[0079]

RF generator

8 is coupled via a decoupling capacitor9 to thedispersive electrode12. The tissue (e.g., myocardium) which the probe20 is to be applied to is generally represented by ablock15 between thedispersive plate12 and anelectrode22A of the needle probe20. In this embodiment, the needle probe is inserted into the tissue. Theelectrode22A is generally represented by a tubular or ring-like structure in accordance with the electrode structure employed in the needle probe20. However, again it will be appreciated that other electrode structures may be practised dependent on the probe type without departing from the scope and spirit of the invention. The other terminal of theRF generator8 is connected via a fail-safe relay38A and a thermal fuse,current limiter39A to therectifier bridge34A. Therelay38A consists of a fail-safe relay contact38A′ and a fail-safe relay winding38A″. These circuits act as current interrupting and current limiting devices.

The output terminal of the fast-switching,[0080]

full bridge rectifier

34A is coupled via a decouplingimpedance matching capacitor44A to astainless steel conductor47A, which is connected to thestainless steel electrode22A and a terminal of thethermocouple junction36A. Thestainless steel conductor47A is also connected to alow pass filter30A, preferably composed of passive elements. Atitanium conductor46A is also coupled to thestainless steel electrode22A and the other terminal of thethermocouple junction36A embedded in theelectrode22A. Thetitanium conductor46A is further connected to the low-pass filter30A. However, other conductive materials may be used for theelectrode22A and the

conductors

46A and47A without departing from the scope and spirit of the invention. The output of thelow pass filter30A is provided to a thermocouple reference compensation amplifier andalarm32A. Theamplifier32A also provides a control signal to therelay38A. The output of theamplifier32A is provided to theisolation amplifier40A, which in turn provides thetemperature signal16A to the programmable controller2. Again, other temperature sensing devices and corresponding associated circuits to provide equivalent functionality may be practiced without departing from the scope and spirit of the invention.

The[0081]

thermocouple

36A embedded in theelectrode22A produces a temperature signal on

conductors

46A and47A in response to the heat produced by the delivery of RF energy to themyocardium tissue15. The signal produced by thethermocouple junction36A is low-pass filtered using the low-pass filter30A, the output of which is provided to the amplifier andalarm32A. The alarm andamplifier32A produces an amplified temperature signal that is provided to theisolation amplifier40A. In addition, the amplifier andalarm32A provides a control signal to operate therelay38A so as to interrupt the delivery of RF energy from the RF generator via therelay38A when the measured or sensed temperature exceeds a predetermined threshold level.

The programmable controller[0082]2 uses thetemperature signal16A to produce acontrol signal14A that is provided to thevariable resistance48A of thefull bridge rectifier34A. Thiscontrol signal14A is provided via theisolation amplifier42A. Thecontrol signal14A operates the full bridge rectifier so as to variably and continuously control the amount of RF energy delivered to thestainless steel electrode22A for ablation. Thus, this embodiment is able to precisely and independently control theelectrodes22A to22D of the needle probe20.

The heating in RF energy transfer occurs not from the[0083]

electrode

22A to22D itself but from a small volume of tissue in contact with theelectrode22A to22D, This heating source is directly proportional to the electrode surface area in contact with the tissue, contact pressure and the electrical conductivity of the tissue. Therefore, the system according to this embodiment advantageously controls the RP energy in each electrode independently of each other.

Thus, the system provides maximum control at each[0084]

electrode

22A to22D by minimising current flow betweenadjacent electrodes22A to22D. This is achieved by a single RF source (one phase)8 usingRF splitter8 to regulate current flow to eachelectrode22A to22D as a function of the temperature of each electrode.

The first embodiment illustrated in FIGS.[0085]1 to3 provides a system for simultaneous unipolar, multi-electrode ablation using simultaneous closed-loop control of temperature at eachelectrode22A to22D. This system advantageously enables multielectrode ablation for ablating ventricular tachycardia and atrial fibrillation. In contrast to conventional ablation systems which cut off current to any electrode during ablation if a temperature or impedance goes above a particular level and therefore cannot produce reliable lesions because the electrode-tissue interface surface area varies considerably during ablation, this embodiment is able to overcome this disadvantage of conventional systems. In this embodiment the control algorithm for generating the control signals and operating the system in response to the temperature of each of the electrodes is preferably implemented in software carried out using a general purpose computer.

An experimental example of the use of the system is set forth below outlining the use of another system in accordance with that of this embodiment. ablation with simultaneous closed-loop temperature control of each electrode is the optimum method for simultaneous multi-electrode ablation.[0086]

Second Embodiment[0087]

Another embodiment of the invention is illustrated in FIG. 4, in which like elements of FIGS. I to[0088]3 are indicated with the same reference numerals, For the purpose of brevity only, components of the second embodiment shared with the first embodiment are not repeated hereinafter. However, those aspects of the second embodiment will be readily understood by a person skilled in the art in view of the description with reference to FIGS.1 to3. Instead, the description hereinafter describes those aspects of the second embodiment not set forth above.

A single channel of the system according to the second embodiment is shown schematically in FIG. 4. The system comprises the programmable controller[0089]2 and themodule4′, which comprises the like numbered elements of FIG. 2, a voltage/current sensing module50A and thecorresponding isolation amplifier52A. Again, while theRF generator8 is illustrated within themodule4′, it will be apparent to a person skilled in the art that theRP generator8 can be equally applied to plural channels, as indicated in FIG. 3.

The[0090]

conductors

46A and47A are also coupled to the input terminals of the voltage and/orcurrent sensing module50A, which preferably detects the root-mean-square (RMS) voltage and/or current at theelectrode22A. The detected or measured voltage and/or current signal is output by thesensing module50A and provided toisolation amplifier52A. In turn the output of theisolation amplifier52A is provided to the programmable controller2.

The voltage and/or[0091]

current sensing module

50A measures the RMS voltage and current delivered to theelectrode22A Thus, the average power and impedance of eachelectrode22A can be determined independently as well. Thus, themodule50A independently senses at least one of following: the voltage, current, impedance and average power of each electrode. This is done to provide a corresponding measurement

EXAMPLE

A system in accordance with the first embodiment was implemented and tested to compare unipolar versus bipolar ablation and single electrode temperature control versus simultaneous multi-electrode temperature control during ablation.[0092]

Two types of 21 gauge needles, each with 2 cylindrical electrodes were introduced from the epicardium at thoracotomy in 3 greyhounds, The proximal electrode measured 1 mm. The distal electrode measured 1 mm in one needle and 1.5 mm in the other. The inter electrode distance was 4 mm. Seventy four intramural RF ablations were performed for 60 seconds through both the electrodes of each needle simultaneously in an unipolar (Uni) or a bipolar (Bi) fashion. During ablations the temperature of only one electrode (proximal or distal) or both the electrodes simultaneously were maintained at 80° C. by closed loop control. Lesion sizes were measured histologically.[0093]

The maximum±SD temperature (temp) measured at the proximal (P) and the distal (D) electrodes were[0094]

(electrode controlled = electrode at which temperature was Controlled)

Length of
each electrode	Uni	Electrode	Temp of P	Temp of D
in needle	or Bi	controlled	electrode	electrode	p value

P = 1 mm,	Bi	P(1	mm)	82 ± 1	82 ± 2	0.7
D = 1 mm
P = 1 mm,	Uni	P(1	mm)	83 ± 1	82 ± 2	0.01
D = 1 mm
P = 1 mm,	Bi	P(1	mm)	81 ± 1	60 ± 2	<0.001
D = 1.5 mm
P = 1 mm,	Bi	D(1.5	mm)	96 ± 2	80 ± 2	<0.001
D = 1.5 mm

P = 1 mm,	Uni	Both	82 ± 2	81 ± 1	0.24
D = 1.5 mm

Simultaneous multi-electrode ablation without closed-loop temperature control of each electrode results in higher temperature at the smaller electrode-tissue interface and lower temperature at the larger electrode-tissue interface. This results in varying lesion sizes and potentially coagulum formation and impedance rises. Unipolar RF signal which can be used by the programmable controller so that additional safety features may be implemented in the system. This preferably provides an increased level of safety by enabling predetermined cut-off levels (eg, RMS voltage, RMS current, impedance and average power) to be used to shut-down the output of each[0095]

electrode

22A. This is preferably carried out by the programmable controller2 which providescontrol signal14A dependent upon at least one of these criteria. Thus, the controller2 generates the control signal14A to independently interrupt delivery of the RF energy to the respective electrode when the meaurement signal exceeds a predetermined threshold condition. Further control structures utilising RMS voltage and/or current may also be applied to enhance the control and safety performance of the system.

Thus, the second embodiment provides, in addition to the advantages of the first embodiment, additional safety features.[0096]

While only a small number of embodiments of the invention has been described, it will be apparent to a person skilled in the art that modifications and changes thereto can be made without departing from the scope and spirit of the present invention.[0097]