Abstract:
The invention relates to an ablation system, including an ablation catheter with a plurality of ablation electrodes ( 34 ) and a neutral electrode ( 36 ), at least one energy source ( 22 ), switching elements ( 26, 28 ) for connecting the electrodes to the energy source ( 22 ) and control elements ( 16, 18, 20, 24 ) which are connected to the switching elements ( 26, 28 ). The control elements ( 16, 18, 20, 24 ) are configured in such a way that electrode pairs made up of both ablation electrodes ( 34 ) and an ablation electrode with a neutral electrode ( 36 ) can be connected to the energy source ( 22 ) in a predetermined sequence. The invention also relates to a method for controlling the electrodes of an ablation system of this type. According to said method, electrode pairs made up of both ablation electrodes ( 34 ) and an ablation electrode with a neutral electrode ( 36 ) are connected to the energy source ( 22 ) in a predetermined sequence.

Description:
FIELD OF THE INVENTION 
     The invention relates to an ablation arrangement which includes an ablation catheter with a plurality of ablation electrodes and a neutral electrode and at least one energy source and switching means for connecting the electrodes to the energy source. The invention also concerns a method of controlling the electrodes of such an ablation arrangement. 
     BACKGROUND OF THE INVENTION 
     The term ablation electrode in this respect stands for any small area energy coupling-out element and the term neutral electrode stands for any comparatively large-area energy coupling-out element. In addition the term ablation catheter is used to denote any arrangement of small area energy coupling-out elements which are spatially associated with each other by suitable connecting means. 
     The invention concerns in particular the general problem of producing line-shaped lesions of sufficient depth throughout in order, for example, to achieve an enduring curative effect in the ablation of tachyarrhythmmia phenomena. 
     To produce linear lesions of that kind it is known, for example, to use an ablation catheter with a relatively elongated electrode at the free end of the catheter. The linear lesion is produced by the electrode being slowly moved in its longitudinal direction while it is connected to an energy source. 
     Another ablation arrangement is known from international patent application WO-A97/20510. Described therein is an HF-ablation system in which HF-energy can be delivered simultaneously and in a predetermined phase association by way of a plurality of electrodes of a multipolar catheter. In that case, a number of power modules corresponding to the number of electrodes is used. That arrangement is complicated and expensive due to its structure. In addition, the various electrode systems are not decoupled from each other so that measurement or regulating circuits based thereon have a mutual influencing effect. 
     SUMMARY OF THE INVENTION 
     Therefore the object of the present invention is to provide ways and means of producing, in particular, linear ablation without involving high cost. 
     In accordance with the invention that object is attained by an ablation arrangement which includes control means which are connected to the switching means and which are of such a design that pairs of electrodes formed by two ablation electrodes and also by an ablation electrode together with the neutral electrode can be connected to the energy source in a predeterminable sequence. 
     In accordance with the invention the object is also attained by a method in which pairs of electrodes are connected to the energy source in a predetermined sequence, wherein the pairs of electrodes are formed both by two ablation electrodes and also by an ablation electrode together with the neutral electrode. 
     The invention involves the realization that different lesion patterns are formed, depending respectively on whether two ablation electrodes are connected to the energy source or whether one ablation electrode and the neutral electrode are connected to the energy source. An explanation for this phenomenon on which the invention is based is that formed between the corresponding electrodes of a pair thereof is an electrical field which in the region of the small-area ablation electrodes is strongly concentrated and has a high level of energy density while the neutral electrode is of such a large area that the energy density of the field is sufficiently low as not to cause any lesions. In accordance with the invention the result of this is that, upon actuation of a pair of electrodes comprising two ablation electrodes, the lesions occur between those ablation electrodes, while upon actuation of a pair of electrodes consisting of an ablation electrode and the neutral electrode, the lesions occur in the proximity of the one ablation electrode. The lesions can differ for example in terms of their depth: by virtue of the electrical properties of the myocardium to be treated, in comparison with the surrounding blood, the electrical field experiences concentration between two ablation electrodes connected to the energy source, and, depending on the amount of energy, the lesion is of a smaller depth, while the lesions produced by an ablation electrode in conjunction with the neutral electrode have a tendency to extend deeper into the myocardium. Actuation of a pair of electrodes comprising two ablation electrodes is referred to hereinafter as a bipolar mode of operation while actuation of a pair of electrodes consisting of an ablation electrode and the neutral electrode is referred to as a unipolar mode of operation. The core and essence of the invention is to operate an ablation catheter with a plurality of ablation electrodes both in a bipolar and in a unipolar mode in order in that way to produce in particular linear ablations. Accordingly the sequence in which the ablation electrodes are operated, as claimed, includes the unipolar and the bipolar mode of operation of the ablation electrodes. 
     Preferably the ablation arrangement is designed so that the ablation electrodes are arranged in succession at a determined spacing and the control means are such that the pairs of electrodes formed by two ablation electrodes are formed by mutually adjacent ablation electrodes. In a particularly preferred ablation arrangement, the control means are such that a pair of electrodes formed by ablation electrodes and a pair of electrodes formed by one of the ablation electrodes together the neutral electrode can be connected to the energy source in an alternating maimer. That means, in other words, that respectively adjacent ablation electrodes are used for the bipolar mode of operation and the unipolar and the bipolar modes preferably continuously alternate. 
     Accordingly, a preferred alternative form of the method is distinguished in that the pairs of electrodes formed by the ablation electrodes are formed by mutually adjacent ablation electrodes of a plurality of ablation electrodes which are disposed in succession at a determined spacing from each other. In a particularly preferred method, a pair of electrodes formed by two ablation electrodes together with the neutral electrode are connected to the energy source in an alternating manner. This also results in alternate unipolar and bipolar operation, in which respect preferably mutually adjacent ablation electrodes are connected to the energy source for the bipolar mode of operation. 
     A preferred method is also one in which firstly one pair of electrodes is connected to the energy source, which is formed by a first ablation electrode comprising a series of ablation electrodes which occur in succession at a spacing together with the neutral electrode. then a pair of electrodes formed by the first ablation electrode and a second ablation electrode following same, thereupon a pair of electrodes formed by the second ablation electrode and the neutral electrode, thereupon once again a pair of electrodes formed by the second ablation electrode and the next following ablation electrode, and continuing in that way in that alternate sequence, until a desired ablation length is achieved. When applied to a multipolar catheter with a tip electrode at its free end and a plurality of ring electrodes following the tip electrode, the procedure involved in the method is such that the energy of the energy source is firstly discharged in unipolar fashion between the tip electrode and the neutral electrode, then in bipolar mode between the tip electrode and the first ring electrode, then again in unipolar mode between the first ring electrode and the neutral electrode, then in bipolar mode between the first and second ring electrodes, and so forth, until the desired length of lesion is achieve. The ring electrodes are only referred to by way of example. Instead of ring electrodes, it is also possible for arrays or spirals of electrodes to be actuated in corresponding manner. 
     A further preferred alternative form of the method is distinguished in that one or more of the ablation electrodes are omitted in the sequence of the electrodes to be connected to the energy source in order to leave gaps in the ablation. This alternative form of the method does not provide a continuous linear ablation, but an ablation which is interrupted. 
     Another preferred method is one in which the power with which the energy from the energy source is discharged by way of the electrodes is different when a pair of electrodes is formed by two ablation electrodes, from the situation when a pair of electrodes is formed by an ablation electrode together with the neutral electrode. By differentiating between the power for the unipolar mode of operation and the bipolar mode of operation, it is possible for the depth of the lesions produced per ablation in the myocardium to be adapted to each other for the two modes of operation in order to achieve an optimum ablation result. 
     In general terms, but in particular also in connection with the last mentioned alternative form of the method, a preferred method is one in which the power with which the energy from the energy source is discharged for two ablation electrodes is regulated by measurement of the electrode temperature in such a way that two temperature values which are associated with the two ablation electrodes are taken into consideration for regulation purposes. Taking account of the two temperature values for the two ablation electrodes forming a pair of electrodes can be implemented, for example, by interpolation. Those temperature values may also be incorporated into the power regulation procedure in another fashion. 
     The above-mentioned ablation arrangement and the method afford on the one hand the advantage that uniform linear lesions can be produced therewith. On the other hand, the spacing of the ablation electrodes from each other on the ablation catheter used can be larger in comparison with conventional ablation catheters. That results in enhanced flexibility of the catheter and thus finer control in regard to positioning. 
     The further configuration of the ablation arrangement can correspond to that which is described in German patent application No 198 17 553.1. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The invention will now be described in greater detail by means of an embodiment with reference to the drawings in which: 
     FIG. 1 is a view showing the principle of an ablation arrangement, 
     FIG. 2 shows a pulse diagram for the switching states of switching elements which are arranged between the electrodes and the energy source, and 
     FIG. 3 shows a block circuit diagram of an alternative embodiment of the ablation arrangement. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a view in principle of an ablation arrangement  10  in which the individual components of the ablation arrangement are illustrated in the form of blocks and the connecting lines between the components are shown by broken, dash-dotted and solid lines. The broken lines in that respect identify digital signal lines, the dash-dotted lines denote analog signal lines and the solid lines denote lines for electrical powers. In that respect, the arrows on the lines indicate the direction of flow of the information or electrical currents. 
     The functional units of the ablation arrangement  10  are an input unit  12 . a memory unit  14 , a control unit  16 , first and second clocks  18  and  20 , an energy source  22 . a third clock  24 , first and second multiplexers  26  and  28 , a regulating unit  30 , temperature sensors  32 , ablation electrodes  34  and finally a surface electrode  36  as a neutral electrode. 
     In particular for the multiplexer  28  but also for the multiplexer  26 , it is possible to use a respective multi-channel power switch which is controlled by the control unit  16 . In addition the two multiplexers or power switches can be combined to form a unit. 
     The input unit  12  serves for parameter input and is connected to the memory unit  14 . Parameters to be input are, for example, the number of ablation electrodes  34 , the change-over switching frequencies of the multiplexers  26  and  28 , the pulse duration for an individual ablation, the ablation power for the unipolar and bipolar modes of operation or the relationship of those power levels in regard to temperature regulation, the maximum ablation temperature and the overall ablation time or the number of ablation sequences. The input parameters are stored in the memory unit  14 . 
     The control unit  16  is connected to the memory unit  14  and has access to the stored parameters by way of a digital signal line connecting the memory unit to the control unit  16 . The control unit  16  is connected at its output side by way of digital signal lines to the first, second and third clock generators  18 .  20  and  24  and to the regulating unit  30 . 
     The first clock  18  is connected on its output side by way of an analog signal line to the first multiplexer  26 . A plurality of temperature sensors  32  are connected to the first multiplexer  26 . A plurality of temperature sensors  32  are connected to the first multiplexer by way of analog signal lines. The temperature sensors  32  are, for example, thermocouple elements. In addition the first multiplexer  26  is connected at its output to the regulating unit  30  by way of an analog signal line. The first multiplexer  26 , by way of the analog signal lines, respectively transmits to the regulating unit  30  one of the temperature values ascertained by the temperature sensors  32 . In that respect, which of the temperature sensors transmits its temperature value to the regulating unit  30  depends on the switching state of the first multiplexer  26 . 
     The second multiplexer  28  is connected at its input side by way of an analog signal line to the second clock  20 . In addition, the second multiplexer  28  is connected to the regulating unit  30  by way of an electrical line. On the output side the second multiplexer  28  is connected by way of a respective electrical line to a respective one of the ablation electrodes  34  and to the surface or neutral electrode  36 . Depending on its respective switching state, the second multiplexer  28  switches electrical power received from the regulating unit  30 , for example in the form of HF-ac voltage, to one of the ablation electrodes  34 . 
     In that respect the switching state of the first multiplexer  26  and the second multiplexer  28  is determined by the first clock  18  and the second clock  20  respectively. 
     The regulating unit  30  is connected on its input side by way of an electrical line to the energy source  22 . The energy source  22  is for example an HF-generator which outputs an electrical ac voltage of a frequency of 470 kHz. The regulating unit  30  is connected at the output side by way of an electrical line to the second multiplexer  28 . The regulating unit  30  includes an amplifier and is of such a design that it regulates the power received from the energy source  22  and outputs it controlled by a clock signal received from the third clock  24  and by a control signal received from the control unit  16 . For that purpose the regulating unit  30  is connected at its input side by way of a digital signal line to the control unit  16  and by way of an analog signal line to the third clock  24 . The regulating unit  30  is further so designed that the power outputted by way of the second multiplexer  28  to two ablation electrodes  34  in the bipolar mode of operation can be different from the power outputted to an ablation electrode and the neutral electrode  36  in the unipolar mode of operation. The power which is delivered to the electrodes  34  and  36  by the regulating unit  30 , referred to hereinafter as the ablation power, can also be regulated in dependence on the temperature value received from the first multiplexer  26 . 
     The ablation electrodes  34  are for example the tip electrode and a plurality of ring electrodes of conventional ablation catheters. The temperature sensors  32  are integrated into such an ablation catheter and associated with a respective one of the ablation electrodes  34  of the ablation catheter. The control unit  16  controls the co-operation of the subsequent components of the ablation arrangement  10  in such a way that, to produce a linear ablation, firstly the first of the ablation electrodes (the tip electrode of a per se known ablation catheter) and the neutral electrode  36  are supplied with the high-frequency ac voltage from the energy source  22 . In that situation, an electrical field is built up in the body to be treated, between the large-area neutral electrode  36  and the first ablation electrode  34 . The ablation electrode  34  is of substantially smaller dimensions than the neutral electrode  36 , with the result that the electrical field is concentrated in the region of the ablation electrode  34 . That concentration of the electrical field results in an energy density which is sufficient to damage body tissue in the region of the ablation electrode  34 , to the desired degree. Then, the first and second of the ablation electrodes  34  are supplied in the bipolar mode of operation with high-frequency electrical energy from the energy source  22 . That results in a lesion between the first and second ablation electrodes  34 . Thereupon the second of the ablation electrodes  34  is operated in a unipolar mode, that is to say the second of the ablation electrodes  34  and the neutral electrode  36  are supplied with electrical energy from the energy source  22  so that a lesion is produced in the region of the second of the ablation electrodes  34 . Then, the second and the third of the ablation electrodes  34  are again supplied in the bipolar mode with high-frequency electrical energy from the energy source  22 . Thereupon, the third of the ablation electrodes is again operated in the unipolar mode, that is to say together with the neutral electrode  36 . until the length of the lesion produced in that way is sufficient or all ablation electrodes  34  have been operated. A sequence of that kind can be repeated so often until the strength of the lesions suffices. Between the sequential, unipolar discharge of high-frequency ac voltage to the ablation electrodes, adjacent ablation electrodes are briefly operated in a bipolar mode. The ablation catheter is therefore operated alternately in unipolar and bipolar mode. The discharge of electrical energy in the bipolar mode of operation can be reduced or increased in comparison with the unipolar mode in order thereby to control the depth of the lesions. 
     Therefore, after the discharge of ablation energy in the form of high-frequency ac voltage by way of one of the ablation electrodes  34 , before the adjacent ablation electrode is operated in a unipolar mode, high-frequency energy is discharged in a bipolar mode by way of those two adjacent ones of the ablation electrodes  34 . The ablation energy supplied can be temperature regulated in dependence on the ablation electrodes. For that purpose, as described, the temperature sensors  32  are respectively disposed in the proximity of one of the ablation electrodes  34 . Measurement of the respective ablation electrode temperature is effected by the associated one of the temperature sensors  34  in respective synchronous relationship with the discharge of energy by way of the respective one of the ablation electrodes  34 . That synchronicity is achieved by suitable actuation of the first and second multiplexers  26  and  28 . The corresponding temperature value is fed to the regulating unit  30  while it regulates the ablation power which is discharged by way of the ablation electrodes and the neutral electrode  36 . In that respect, the regulating unit  30  is designed so that differences between the measured temperatures are taken into consideration in regard to the bipolar and the unipolar modes of operation when regulating the ablation power. 
     The pulse diagram shown in FIG. 2 illustrates how the individual ablation electrodes  34  and the neutral electrode  36  are actuated for the described, strictly alternate unipolar and bipolar mode of operation. In the state identified by “1”, the respective electrode is connected to the energy source  22  while in the state identified by “0” it is separated from the energy source  22 . The pulse sequence identified by I represents in that respect the state of actuation of the first of the ablation electrodes  34 . That will generally be the tip electrode of an ablation catheter. The pulse sequence identified by II represents the actuation state of the next following second ablation electrode, and so forth, while the pulse sequence identified by “N” represents the actuation state of the neutral electrode. The change between the unipolar and bipolar modes is represented by vertical dotted lines. The cycle with which that change takes place is determined by the clocks  18 ,  20  and  24  which are actuated by the control unit  16 . The pulse sequences illustrated in FIG. 2 are plotted in relation to time which progresses from left to right. The arrows  1  through  7  shown in FIG. 2 identify the respectively co-operating electrodes, more specifically arrow  1  identifies the co-operation of the first ablation electrode with the neutral electrode in the unipolar mode, arrow  2  the co-operation of the first and second ablation electrodes in the bipolar mode, arrow  3  the co-operation of the second ablation electrode and the neutral electrode in the unipolar mode, arrow  4  the co-operation of the second and third ablation electrodes in the bipolar mode, arrow  5  the co-operation of the third ablation electrode and the neutral electrode in the unipolar mode, arrow  6  the co-operation of the third and fourth ablation electrodes in the bipolar mode and arrow  7  the co-operation of the fourth ablation electrode and the neutral electrode in the unipolar mode. 
     The sequence in which the electrodes are actuated can be repeated as often as may be desired until a desired ablation result is achieved. In addition, it is possible for some electrodes in the sequence to be omitted in order to produce lesions which are interrupted. 
     FIG. 3 shows a block circuit diagram of an embodiment of the ablation arrangement, showing alternatives in parts thereof. The ablation arrangement  100  shown in FIG. 3 comprises the components ablation device  103 , stimulation device  105 , clinical measuring station  107 , switching device  109 , multipolar ablation catheter  111  and surface electrode  113  which co-operate in known manner. In that respect, in regard to describing the invention, the ablation device  103 , the HF-generator  115  and the output terminals are of particular interest. There are two serial data terminals  103   a ,  103   b , a power supply terminal  103   c , an HF-power output  103   d ,  103   e  and a reference terminal  103   f  for the surface electrode  113 . 
     The ablation catheter  111  is also of per se known type and, on an insulating catheter body  111   a  made from a biocompatible plastic material, besides a tip or point electrode  111   b , has five ring electrodes  111   c  through  111   g  which are arranged equidistantly relative to each other and in relation to the tip electrode and which—like the tip electrode  111   a —are preferably made from platinum or a Pt—Ir-alloy and together with the tip electrode form the ablation electrodes. The catheter body  111   a  accommodates feed lines (not individually shown) for the individual electrodes and the constantan or copper wires (also not individually shown) for thermocouple elements  117   a  through  117   f  spatially associated with the electrodes. HF-lines  119 . 1  and the temperature signal lines  119 . 2  are guided in a connecting line  119  from the switching device  109  to the ablation catheter  111 . 
     The switching device is connected on its input side (by lines which once again are not shown specifically) to the output terminals  115   a  to  115   e  of the ablation device. As the most important components for ablation control it includes a microcontroller  109 . 1 , six electronic switches  109 . 3  connected to the microcontroller by way of a bus line  109 . 2  for the catheter electrodes  111   b  through  111   g  and a safety cut-out switch  109 . f  and finally a watchdog timer  109 . 5 . Associated with the HF-output lines  119 . 1  are current monitoring sensors  109 . 6  which are connected to the microcontroller  109 . 1  by way of a second bus line  109 . 7 . 
     The microcontroller is connected by way of data inputs and outputs  109 . 1   a ,  109 . 1   b  to the data inputs and outputs  103   a ,  103   b  of the ablation device and by way of a current supply input  109 . 1   c  to the current supply output  103 . c  thereof, while a separate control output  109 . 1   d  is provided for the watchdog timer  109 . 5 . 
     The Cu-lines of the thermocouple elements  117   a  through  117   f  are connected by way of a multiplexer  109 . 8  and the constantan line directly to a single measuring amplifier  109 . 9 , and the output thereof is connected to a first T-signal input  109 . 1   d  of the microcontroller. A cold junction  109 . 10  serving as a reference with a thermistor (not shown) for T-detection is connected directly to a second T-signal input  109 . 1   e  of the microcontroller. 
     The primary control functions for an ablation treatment (choice of electrode and sequence of electrode actuation, duration and spacings of the HF-pulses and so forth) are implemented after evaluation of the results, obtained by means of the measuring station  107 , of the clinical investigation, on the basis of a suitable programming, by the doctor, in the ablation device  103  (which in practice is also referred to as an ablator or ablation system). The ablation device  103  is of such a design configuration that the ablation electrodes are operated both in unipolar and in bipolar mode within a sequence and that the ablation power for the unipolar mode is different than from the bipolar mode. Control commands for the ablation device are fed by way of the data outputs  103   a ,  103   b  to the microcontroller  109 . 1  which inter alia converts the control commands into control signals for the switches  109 . 3  and the multiplexer  109 . 9  which monitors switching functions by means of the current sensors  109 . 6  and calculates the temperature at the individual electrodes  117   a  through  117   f.    
     The illustrated switch arrangement makes it possible to discharge HF-energy with any desired combination of ablation electrodes with each other or with the surface or neutral electrode  113  and the application of stimulation pulses and the detection of an intracardial electrogram IEGM at the electrodes which are respectively switched on. At the same time, by virtue of synchronous control of the multiplexer  109 . 8 , it is possible to monitor the temperature at the electrodes, but if necessary the multiplexer can also be controlled in an asynchronous relationship with the switches. The results of T-monitoring are transmitted to the ablation device  103  to be displayed for the doctor and for any modification to the ablation program, but they can also be used internally in the microcontroller  109 . 1  for triggering an electrode switch-off procedure in the event of unacceptably high T-values. 
     The switches  109 . 3  and  109 . 4  are preferably in the form of MOSFET-switches but—if the limitation in change-over switching speed is acceptable—they can also be formed by relays. The bidirectional transmission of data between the ablator  103  and the switching device  109  or suitable programming of the ablator permits short-term blanking out of the HF at each electrode change-over switching operation in order to reduce the switch loading, which is advantageous in particular when using the relays.