Method for the operation of a high frequency ablation apparatus and apparatus for the high frequency tissue ablation

A method is described for the operation of a radiofrequency ablation instrument comprising a radiofrequency energy source having a plurality of regulatable outputs, with a plurality of electrodes being connectable to its outputs. A respective power value and/or current value representative of the power and/or the current transmitted by the radiofrequency energy source is detected for each output. The voltage delivered from the radiofrequency energy source is regulated in dependence on the detected power value and/or current value in such a way that the latter corresponds substantially to a predetermined power value and/or current value. A predetermined phrase relationship is in each case maintained between the currents or between the voltages at the outputs of the radiofrequency energy source. Furthermore, a corresponding apparatus is described.

The present invention relates to a method for the operation of a high
 frequency ablation instrument comprising a radiofrequency energy source
 having a plurality of regulatable outputs, with a plurality of electrodes
 being connected to its outputs. Furthermore, the invention is directed to
 an instrument for high frequency tissue ablation.
 Ablation instruments of this kind are, for example, used for the treatment
 of cardiac irregularity or disordered action of the heart. For this
 purpose an ablation catheter having one or more electrodes is connected to
 a radiofrequency energy source and is, for example, introduced via a blood
 vessel into the interior of the heart. The electrodes provided at the end
 of the catheter are positioned at the desired location at the inner or
 outer surface of the heart, whereupon the regions standing in contact with
 the electrodes are thermally obliterated by the supply of radiofrequency
 energy.
 The electrical characteristics of the treated heart tissue are so changed
 through the lesions which are produced in this manner that cardiac
 irregularities which are present are removed.
 Catheters with only one electrode are poorly suited for certain types of
 cardiac irregularities, for which larger areas of tissue have to be
 obliterated, since the sequence of a plurality of point-like lesions that
 is required with the heart beating is generally only inadequate and only
 possible with a large expenditure of time. For the obliteration of larger
 areas of tissue, catheters having a plurality of electrodes can be used.
 In this arrangement it is advantageous if the energy yield for each
 catheter electrode can be individually adjusted so that, for example, the
 ideal temperature acting in the tissue can be set for each catheter
 electrode, despite different cooling conditions in the different
 electrodes, and also despite different load resistances for each catheter
 electrode.
 The individual catheter electrodes normally cooperate with a large area
 electrode contacting the body of the patient to be treated, the so-called
 indifferent electrode, so that with an energy supply to the catheter
 electrodes, current in each case flows from the catheter electrodes
 through the body of the patient to the indifferent electrode. Since the
 current density is highest directly at the transition between the catheter
 electrodes and the tissues to be treated, as a result of the small area of
 the catheter electrodes, the temperature in this region is sufficiently
 high, with adequate energy supply, that the desired lesions are produced.
 When using catheters with a plurality of electrodes, the following problem
 arises: Since, in dependence on the requirement, different temperatures
 can be necessary at different catheter electrodes, or different energies
 can be necessary to achieve a specific temperature, and since the load
 resistances which become active at the electrodes can be different, the
 energy yield via the individual catheter electrodes must be individually
 set. This leads to different voltage values and current values being
 necessary at the different catheter electrodes. In particular, when the
 catheter outputs have low output resistances, compensation currents arise
 between electrodes to which different voltage values are applied or with
 phase shifts between the output voltages. These compensation currents are
 currents which flow, instead of to the indifferent electrode, to another
 catheter electrode at which a voltage is present which differs from the
 voltage at the electrode from which the current emerges. If a high
 potential difference exists between the two catheter electrodes, then the
 compensation current flowing into the other electrode can lead to
 undesirably high current densities at this electrode, which in turn bring
 about undesired coagulations at this electrode. This effect is
 particularly notable at electrodes at which no energy transmission or only
 a small energy transmission is desired.
 BRIEF SUMMARY OF THE INVENTION
 The object of the invention is to design an instrument and a method of the
 initially named kind so that compensation currents between the outputs or
 between electrodes connected to the outputs are largely avoided.
 Furthermore, a continuous energy delivery should be possible. The
 instrument should have a high degree of efficiency and also the circuit
 complexity should be as low as possible.
 Starting from a method of the initially named kind, the part of the object
 relating to the method is satisfied in that a respective power value
 and/or current value representative of the power and/or the current
 transmitted by the radiofrequency energy source is detected for each
 output; in that the voltage transmitted from the radiofrequency energy
 source is so regulated in dependence on the detected power value and/or
 current value that the latter corresponds substantially to a predetermined
 power value and/or current value; and in that a predetermined phase
 relationship is in each case maintained between the currents or between
 the voltages at the outputs of the radiofrequency energy source.
 The part of the object relating to the apparatus is satisfied in accordance
 with the invention by an instrument of the initially named kind with at
 least one measurement element for detecting power values and/or current
 values representative for the power and/or current delivered at the
 respective outputs, and by at least one regulating element connected to
 the measurement element for the regulation of the voltage delivered by the
 radiofrequency energy source in dependence on the detected power value
 and/or current value applied to an actual value input of the regulating
 element, and on a preset power value and/or current value applied to a
 desired value input of the regulating element, with the currents or
 voltages at the outputs of the radiofrequency energy source each having a
 predetermined phase relationship to one another.
 The apparatus of the invention is thus so designed that its outputs have
 the behavior of a current or power source. In this way compensatory
 currents which would, for example, flow from an electrode with a higher
 voltage to an electrode with a lower voltage and lead to a feedback into
 this electrode are avoided. This feedback would lead to a situation in
 which, in an extreme case, a current of higher magnitude undesirably flows
 through the compensatory current to this electrode, which brings about an
 undesired coagulation at this electrode. For example, under some
 circumstances, the compensation current can be subtracted from the
 suppressed output current of the affected electrode, which can lead to a
 situation in which the direction of action of the current flowing through
 the affected electrode is reversed.
 Through the current or power regulation in accordance with the invention,
 this deviation in the desired value is directly counteracted by follow-up
 regulation, for example, by an increase of the output voltage, so that the
 desired current again flows through the electrode.
 In accordance with a preferred embodiment of the invention, the rf-energy
 source comprises at least one regulatable DC voltage source, which
 consists, for example, of a non-regulated DC voltage source and a voltage
 regulator, and at least one switching stage connected to the DC voltage
 source, with the or each switching stage being controlled by an in
 particular periodic switching signal for the generation of the rf output
 voltage, and with the voltage transmitted by the DC voltage source being
 regulated for the regulation of the voltage transmitted by the rf energy
 source. In this manner a particularly simple and cost favorable design of
 an instrument formed in accordance with the invention is possible. the
 switching signal thereby normally consists of rectangular pulses, with the
 frequency of the switching signal typically lying in the range from 300 to
 1000 kHz.
 In a further advantageous embodiment of the invention, the power
 transmitted by the DC voltage source and/or the current transmitted by the
 DC voltage source are detected, so that the power and/or current detection
 takes place at the primary side of the rf-energy source. For this purpose
 the measuring element for detecting the power and/or the current
 transmitted by the rf-energy source is arranged between the DC voltage
 source and the switching stage, or directly inside the DC voltage source.
 In the detection of the current and/or of the power at the primary side of
 the rf-energy source, the transformation factor and degree of efficiency
 of the switching stages and also of any possibly further present elements,
 such as for example a voltage regulator which is present, must be taken
 into account in the determination of the current and/or of the power.
 Through the detection of the current and/or of the power at the primary
 side of the rf-energy source, it is possible to avoid additional switching
 elements at the secondary side, which could lead to undesirable patient
 dissipation currents and to rf-leakage currents. Furthermore, the
 detection of the power and/or of the current at the primary side is
 associated with substantially less hardware cost and complexity than at
 the secondary side.
 Fundamentally, it is however, also possible for the power and/or the
 current to be detected between the switching stage and the output of the
 rf-energy source, i.e. at the secondary side of the rf-energy source.
 In accordance with a further preferred embodiment of the invention, a
 switching stage and a measurement element are associated with each output
 of the rf-energy source. In this manner it is ensured that each electrode
 which is connected to an output can be set individually and ideally.
 The switching stage preferably includes at least one transformer, through
 which the primary and secondary side of the rf-energy source are coupled
 to one another, with at least one switch, which is in particular formed as
 a transistor being provided at the primary side. If the current detection
 and/or the power detection takes place at the primary side of the RF
 energy source and thus of the transformer, then it is possible, in the
 event that the switching stage operates substantially linearly over a wide
 operating range, for this detection to take place through calculation with
 the following formulae:
EQU P.sub.HF =(P.sub.DC -P.sub.DC0).times..eta.
EQU P.sub.L =(R.sub.DC -R.sub.DC0).times.k,
 with P.sub.HF being the rf-output power at the secondary side, P.sub.DC the
 supply power at the primary side, P.sub.DC0 the supply power without RF
 output, .eta. the degree of efficiency, R.sub.L the load resistance at the
 secondary side, R.sub.DC the detectable DC resistance at the primary side,
 R.sub.DC0 the DC current resistance offset and k a circuit specific,
 resistance transformation factor.
 Fundamentally, it is however also possible for the determination of the
 secondary values to take place from the primary values via tables or a
 combination of calculations and tables. Instead of transistors any other
 desired switches, for example tubes, can be used, which can follow the
 required switching frequency of the switching signal of, for example, 300
 to 1000 kHz.
 In accordance with a further advantageous embodiment of the invention, the
 DC voltage sources deliver substantially the same maximum voltage value,
 with all outputs in particular being fed from a unitary DC voltage source.
 The use of a unitary maximum voltage value for all outputs brings about
 the following advantage in accordance with the invention. If a high
 compensation current arises as a result of a high potential difference
 between two catheter electrodes, then, as already described, the voltage
 transmitted by the associated DC voltage source is increased through the
 regulation of the impressed output current affected by the compensation
 current until the deviation from the desired value brought about by the
 compensation current has been cancelled. In the extreme case the circuit
 goes into saturation, because the maximum value of the DC voltage source
 restricts the control range. If all DC voltage sources have the same
 maximum voltage value, then in the extreme case the voltage sources
 associated with the two affected electrodes deliver the same voltage, so
 that as a result of the same potential, no compensation current can flow.
 The predetermined power values and/or current values are predetermined by a
 control, which can in particular be designed as a temperature regulation.
 For this purpose temperature sensitive sensors are provided in the region
 of the outputs of the rf-energy source, in particular in the region of the
 electrodes, and are connected to the control circuit. The desired power
 values and/or current values which are applied to the regulating element
 are determined in dependence on the temperature values measured by the
 sensors through the control circuit for each regulating element. It is
 fundamentally also possible to use other suitable measurement parameters
 for the regulation. For example, a regulation in accordance with the
 absolute impedance value and/or in accordance with changes of the
 impedance value can be used instead of or in addition to the temperature
 regulation.
 Further preferred embodiments are set forth in the subordinate claims.

The apparatus shown in FIG. 1 includes a cycle or clock generator 1, which
 produces a periodic, rectangular switching signal 2 for a plurality of
 switching stages 3. A DC voltage source 4 is associated with each
 switching stage 3 and supplies the switching stage 3 with a variable
 voltage via a respective voltage regulator 5. The DC voltage sources 4
 each deliver the same voltage value so that instead of different voltage
 sources 4 a common voltage source 4' can also be used, as is shown in
 broken lines in FIG. 1.
 The switching stages 3 each contain at least one schematically indicated
 switch 6, which is, for example, formed as a transistor or tube and is
 controlled, i.e. opened and closed by the switching signal 2', which is
 applied to the respective switching stage 3. Through the controlling of
 the switch, the DC voltage applied from the voltage regulator 5 to the
 switch 6 is chopped up, whereby the RF output voltage of the switching
 stage 3 is produced. Thus, the output voltage of the respective switching
 stage 3 is determined by the voltage delivered by the voltage regulator 5
 to the switching stage 3.
 The current flowing via the switch 6, or the corresponding power, is
 detected for each switching stage 3 by a measurement element 7 in each
 case and supplied to the actual value input 8 of a regulating element 9.
 The measurement element 7 can in this arrangement, for example, also be
 arranged in the path between the voltage regulator 5 and the switching
 stage 3, or in the path between the DC voltage source 4 and the voltage
 regulator 5. At these points the current or the power is thus measured at
 the primary side of the switching stage 3, whereby the occurrence of
 additional dissipatory currents through additional circuit elements on the
 secondary side is avoided.
 The regulating element 9 sets the current detected by the measurement
 element 7 or the power detected by the measurement element 7 in accordance
 with a desired value applied to a desired value input 10 of the regulating
 element and controls the voltage regulator 5, so that the supply voltage
 for the respective switching stage 3 is regulated in such a way that the
 detected current of the detected power is kept constant.
 At the secondary sides of the switching stages 3, there are formed
 respective outputs 20, 21 of the rf-energy source, which are each
 connected, on the one hand, to a respective catheter electrode 11 and, on
 the other hand, to a common, indifferent electrode 12.
 A respective temperature sensitive sensor 13 is arranged in the region of
 each electrode 11 and is connected to a control circuit 14 for the
 transmission of an output signal dependent on the measured temperature.
 The control circuit 14 in turn delivers the desired value for the
 regulating elements 10 in dependence on the measured temperature and also
 possibly in dependence on a preset desired temperature.
 Whereas the switching signals 2', which are applied in the previously
 described circuit to the switching stages 3, are each in phase, it is
 possible via a phase shifter 15 connected after the clock generator 1 to
 intentionally shift the phase position of the switching signals 2 relative
 to one another, whereby defined compensatory currents can be produced
 between the electrodes 11. These compensatory currents flow at the surface
 of the tissue and can serve to close lesion gaps between the catheter
 electrodes 11. Furthermore, an interrupter circuit 16 is arranged between
 the clock generator 1 and the phase shifter 15, with which the switching
 signals 2 produced by the clock generator 1 can be regularly interrupted
 in order to produce a pulsing of the output power. In this way the lesion
 profile can be advantageously controlled, with both the duration and also
 the frequency of the interruptions produced by the interruption circuit 16
 being capable of being set.
 FIG. 2 shows a design of the switching stage 3 as a single ended switching
 stage and, for the sake of easier comprehension, some of the components
 already shown in FIG. 1 and connected to the switching stage 3 are
 likewise shown in FIG. 2.
 The switch 6 of the switching stage 3 is connected in series with the
 primary winding 17 of a transformer 18, with the other end of the primary
 winding 17 being connected to the measurement element 7. The secondary
 winding 19 of the transformer is connected at its one terminal to the
 catheter electrode 11, normally via a capacitor 23, and with its other
 terminal to the indifferent electrode 12.
 The DC voltage source 4 shown in FIG. 1 and also the voltage regulator 5
 are combined in each of FIGS. 2 to 5 into a regulated voltage source 4".
 As can be seen from FIG. 2, the voltage delivered from the DC voltage
 source 4" is chopped up by the control of the switch 6 through the
 switching signal 2' and is transmitted via the transformer 18 from the
 primary winding 17 to the secondary winding 19. The current which is
 thereby induced flows via the catheter electrode 11 into the tissue to the
 indifferent electrode 12, whereby the tissue is coagulated in the region
 of the catheter electrode 11.
 Whereas the single ended switching stage shown in FIG. 2 is controlled
 solely by a single switching signal 2', the switching stages shown in
 FIGS. 3 to 5 are each controlled by two switching signals 2' of opposite
 phase. As in the single-ended switching stage of FIG. 2, the secondary
 winding 19 of the transformer 18 is connected in each of the switching
 stages in accordance with FIGS. 3 to 5 to the catheter electrode 11 and to
 the indifferent electrode 12, so that the same effect can be achieved with
 these switching stages as with the single ended switching stage in
 accordance with FIG. 2.
 The manner of operation of an apparatus formed in accordance with the
 invention will be described again in more detail in the following with
 reference to the Figures.
 A switching signal 2 is generated by the clock generator 1, which can be
 interrupted timewise by the interrupter circuit 16 and can be converted by
 the phase shifter 15 into switching signals 2' with different phase
 positions.
 The switching signals 2' respectively control the switch or switches 6 at
 the switching stage 3, so that the DC voltage delivered by the DC voltage
 source 4, as set via the voltage regulator 5 and present at the switching
 stage 3, is converted into an AC voltage. The current flowing at the
 primary side is measured by the measurement element 7 and passed on to the
 regulating element 9, which so controls the voltage regulator 5 on the
 basis of a desired value for power or current applied to the control
 circuit 14 that the current or power detected remain constant.
 At the secondary side the switching stage 3 is connected via the outputs
 20, 21 of the RF energy source to the catheter electrode 11 and also to
 the indifferent electrode 12, so that the tissue contacting the catheter
 electrode 11 is heated and denatured as a result of the high current
 density arising at this location.
 The temperature prevailing in the region of the catheter electrode 11 is
 measured by the sensor 13 and transmitted to the control circuit 14,
 through which a corresponding desired value is applied to the regulating
 element 9.
 If a potential drop exists between two adjacently disposed catheter
 electrodes 11, then it is possible for a compensation current to flow from
 the electrode 11 with the higher voltage to the electrode 11 with the
 lower voltage. Since this compensation current is superimposed on the
 impressed output current of the electrode 11 with the lower voltage, the
 regulating element 9 controls the voltage regulator 5, after detection of
 the resulting current value by the measurement element 7, in such a way
 that the voltage applied to the switching stage 3 is subjected to
 follow-up regulation and the desired value of the current or the power is
 set again.
 In the extreme case the voltage applied to the switching stage 3 will be
 regulated up to the saturation limit, with it being ensured, through the
 use of a unitary voltage source 4', that in this extreme case the same
 voltage is applied both to the electrode 11 delivery the compensation
 current and also to the electrode 11 receiving the compensation current,
 so that no compensation current can flow between these two electrodes 11.
 While it has previously been described that a temperature sensor is
 associated with each catheter electrode, it is basically also possible to
 provide a plurality of sensors for each electrode or a common reference
 sensor for a plurality of electrodes or no sensor for some of the
 electrodes or all of the electrodes.
 The use of the described switching stages is of advantage because these
 have a high degree of efficiency, since the power losses via the switches
 are low.
 In order to further reduce the power feedback, it can be possible to switch
 all the switches into the open position if the corresponding switching
 stage is controlled by a supply voltage of 0 Volt.
 Undesired compensation currents can also be prevented in an advantageous
 further development, in which no circuit elements are used, which would
 permit a significant passive current impression, such as is the case, for
 example, with relief networks. Furthermore, preferred switching elements
 are provided which reduce the harmonic content of the output voltage. The
 operation of apparatus being operated in the vicinity can be disturbed by
 harmonics, so that the amplitudes of the harmonic waves can be reduced by
 corresponding circuit elements, for example, capacitors 22 (FIG. 3)
 connected in parallel to the primary winding of the output transformer or
 a capacitor-resistor combination 22, 22' (FIG. 3).
 It is fundamentally also possible, instead of using a common, indifferent
 electrode, to provide further electrodes, for example, on the same
 catheter or on another catheter, which serve as the counterpole to the
 described catheter electrodes. In addition, the circuit can be of modular
 design, so that, for example, the control circuit of each individual
 electrode can be formed by a separate module.
 It can also be sensible, for all medical applications in particular, to
 provide the rf-energy source with a voltage control characteristic. In
 this respect it can be sensible to provide regulating elements both for
 the current regulation and also for the voltage regulation, with a
 determination advantageously being made through the load conditions, as to
 which of the two regulations is active. The measurement of the voltage
 value required for the voltage regulation can in turn selectively take
 place either at the primary side or at the secondary side of the switching
 stages.