CATHETER HAVING COMMON LEAD FOR ELECTRODE AND SENSOR

Abstract: An apparatus for delivering energy to a biological site includes a catheter having a plurality of electrodes arranged in a linear array, the electrodes positioned proximal the biological site. A power control system supplies power, each having a controllable duty cycle and phase angle, to each of the electrodes. A backplate is also positioned proximal the biological site so that the biological site is interposed between the electrode device and the backplate. The backplate is maintained at the reference voltage level in relation to the power. The power control system controls the phase angle of each power signal so that current flows between the electrodes and between the electrodes and the backplates results. Temperature sensors are located at the electrodes and each shares a common lead with the power circuitry. The temperature sensor signal is received by the power control system during the off-period of the duty cycle of the particular electrode.

Description: CATHETER HAVING COMMON LEAD FOR ELECTRODE AND SENSOR BACKGROUND OF THE INVENTION The invention relates generally to an electrophysiological ("EP") apparatus and method for providing energy to biological tissue, and more particularly, to a catheter having a common lead for both providing energy to an electrode and for conducting a sensor signal. The heart beat in a healthy human is controlled by the sinoatrial node ("S- A node") located in the wall of the right atrium. The S-A node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node ("A-V node") which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth of, or damage to, the conductive tissue in the heart can interfere with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as "cardiac arrhythmia." While there are different treatments for cardiac arrhythmia, including the application of anti-arrhythmia drugs, in many cases ablation of the damaged tissue can restore the correct operation of the heart. Such ablation can be performed by percutaneous ablation, a procedure in which a catheter is percutaneously introduced into the patient and directed through an artery to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. In such case, an ablation procedure is used to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities or create a conductive tissue block to restore normal heart beat or at least an improved heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels. A widely accepted treatment for arrhythmia involves the application of RF energy to the conductive tissue. In the case of atrial fibrillation ("AF"), a procedure published by Cox et al. and known as the "Maze procedure" involves continuous atrial incisions to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. While this procedure has been found to be successful, it involves an intensely invasive approach. It is more desirable to accomplish the same result as the Maze procedure by use of a less invasive approach, such as through the use of an appropriate EP catheter system. There are two general methods of applying RF energy to cardiac tissue, unipolar and bipolar. In the unipolar method a large surface area electrode; e.g., a backplate, is placed on the chest, back or other external location of the patient to serve as a return. The backplate completes an electrical circuit with one or more electrodes that are introduced into the heart, usually via a catheter, and placed in intimate contact with the aberrant conductive tissue. In the bipolar method, electrodes introduced into the heart have different potentials and complete an electrical circuit between themselves. In the bipolar method, the flux traveling between the two electrodes of the catheter enters the tissue to cause ablation. During ablation, the electrodes are placed in intimate contact with the target endocardial tissue. RF energy is applied to the electrodes to raise the temperature of the target tissue to a non-viable state. In general, the temperature boundary between viable and non-viable tissue is approximately 48 ° Centigrade. Tissue heated to a temperature above 48 ° C becomes non-viable and defines the ablation volume. The objective is to elevate the tissue temperature, which is generally at 37° C, fairly uniformly to an ablation temperature above 48 °C, while keeping both the temperature at the tissue surface and the temperature of the electrode below 100°C. During ablation, portions of the electrodes are typically in contact with the blood, so that it is possible for clotting and boiling of blood to occur if those electrodes reach an excessive temperature. Both of these conditions are undesirable. Clotting is particularly troublesome at the surface of the catheter electrode because the impedance at the electrode rises to a level where the power delivery is insufficient to effect ablation. The catheter must be removed and cleaned before the procedure can continue. Additionally, too great a rise in impedance can result in sparking and thrombus formation within the heart, both of which are also undesirable. Further, too great a temperature at the interface between the electrode and the tissue can cause the tissue to reach a high impedance which will attenuate and even block the further transmission of RF energy into the tissue thereby interfering with ablation of tissue at that location. Even though no significant amount of heat is generated in the electrodes themselves, adjacent heated endocardial tissue heats the electrodes via heat conduction through the tissue. As mentioned above, part of the active electrode will be in contact with the blood in the heart and if the electrode temperature exceeds 90-100°, it can result in blood boiling and clotting on the electrode. The application of RF energy must then be stopped. However, shutting the RF generator off due to the temperature rise may not allow sufficient time to complete the entire ablation procedure. Providing an ablation electrode capable of applying higher amounts of power for a longer period of time to ablate the damaged tissue to an acceptable depth is a goal of current ablation catheter electrode design. It has been found that higher power for longer time periods results in a higher probability of success of the ablation procedure. To avoid clotting and blood boiling, RF ablation catheters for cardiac applications typically provide temperature feedback during ablation via a temperature sensor such as a thermocouple. In its simplest form, a thermocouple consists of two dissimilar metals joined together at one end called a "bead" or junction, such as a conventional copper/constantan type "T" thermocouple. When the junction is heated a thermoelectric potential arises and can be measured across the unconnected ends. This is also known as the thermoelectric or Seebeck effect. This voltage is proportional to the temperature difference between the junction and the non-joined ends. A conventional RF ablation catheter typically has a single tip electrode and a single temperature sensor mounted along the centerline of the tip electrode where temperature readings are not affected by the rotational orientation of the catheter. Although a temperature gradient typically exists in tip electrodes, wherein the electrode is hottest at the tissue interface and coolest on the opposite side which is in contact with circulating blood, the centerline sensor provides a moderate output by which it can be determined whether the temperature of the tissue contacted by the electrode is being raised sufficiently, and whether a therapeutic lesion is being generated. In the case where a catheter has a band electrode, such as for the treatment of atrial fibrillation by the ablation of tissue, a single temperature sensor mounted to the band may not provide the temperature of the tissue contacting the band electrode. Typically the side of the band which is in direct contact with the tissue becomes significantly hotter than the rest of the band electrode that is cooled by the blood flow. Thus, the temperature reading can be dramatically influenced by the rotational orientation of the catheter during RF ablation. If the band is oriented so that the single temperature sensor is not in contact with the tissue during the application of ablation energy, not only would there be a time lag in the sensor reaching the tissue temperature, but due to the effect of the cooling blood flow, the sensor reading may never approach the actual tissue temperature. To overcome the effect that the rotation orientation of the band electrode has on temperature sensing, two thermocouples, positioned at different locations of the band electrode, may be used. A theory is that having a sensor in contact with tissue is more likely. While attachment of multiple temperature sensors to the band electrode can result in a higher probability of sensing the actual tissue interface temperature, this also increases the number of wires occupying space within the catheter. As is well appreciated by those skilled in the art, an increase in the number of internal wires could mean an undesirable increase in catheter diameter to accommodate those wires. Conventional types of thermocouples each require a thermocouple wire pair. Two thermocouples at each band electrode would result in four wires per band electrode so that the use of multiple temperature sensors may not be practical, particularly where the catheter carries multiple band electrodes that require temperature monitoring. The larger the catheter, the more traumatic it is to the patient. Also, the more difficult it may be to negotiate the patient's vessels to position the catheter at the desired location in the heart. It is desirable to provide a catheter with as small a diameter as possible. A limiting factor in reducing the size of the catheter is the amount of devices and leads that must be carried inside the catheter. In the case of a catheter having ten band electrodes with two thermocouple temperature sensors at each electrode, a total of fifty wires would be necessary; one power wire for each electrode and two wires for each thermocouple. The size of fifty wires inside a catheter can be significant, causing an increased diameter of the catheter. Yet it is desirable to retain the electrodes and the associated temperature sensors so that more precise control over the energy applied to the biological tissue can be effected. Thus, it would be desirable to reduce the number of wires within a catheter, yet retain the same functionality. Hence, those skilled in the art have recognized a need for a minimally invasive ablation apparatus that is capable of controlling the flow of current through a biological site so that lesions with controllable surface and depth characteristics may be produced and the ablation volume thereby controlled. Additionally, a need has been recognized for reducing the size of a catheter yet retaining its ability to provide multiple electrodes and multiple temperature sensors for the control of energy applied to a biological tissue site. The invention fulfills these needs and others. SUMMARY OF THE INVENTION Briefly, and in general terms, the invention is directed to an apparatus and a method for controlling the application of energy to a biological site with a reduced number of leads in a catheter having an energy application device and a sensor device at its distal end. In a first aspect, an apparatus for delivering energy to a biological site comprises a catheter having an electrode disposed at a distal end of the catheter, the distal end adapted to be positioned so that the electrode is located at the biological site, a second device disposed at the distal end of the catheter providing a device signal, a lead connected to both the electrode and the second device, a power control system adapted to provide a power signal for the electrode on the lead and to control the duty cycle of the power signal with the duty cycle having an on-period and an off -period within a duty cycle time frame, the power control system further adapted to monitor the device signal during the off-period of the duty cycle. In a second aspect, the power control system controls the duty cycle of the power in response to the device signal received during the off-period of the duty cycle. In yet another aspect, the second device comprises a temperature sensor disposed at the distal end of the catheter providing a temperature signal, and the power control system is adapted to monitor the temperature signal produced by the temperature sensor on the common lead. In further more detailed aspects, the power control system is adapted to process the temperature signal from the temperature sensor only during the off -period in the duty cycle of the power signal, and the power control system controls the duty cycle of the power in response to the processed temperature signal. In other aspects in accordance with the invention, the catheter comprises a plurality of electrodes at its distal end and the power control system is further adapted to provide a power signal to each of the electrodes wherein the power signals are selected such that at least two electrodes have voltage levels that differ from each other so that current flows between said two electrodes. The power control system provides power signals with different phase angles to at least two of the electrodes. In a more detailed aspect, the power signals differ in phase by an amount greater than zero degrees but less than 180 degrees, and in a much more detailed aspect, the power signals differ in phase by an amount approximately equal to 132 degrees. In a further aspect, a backplate is adapted to be positioned proximal the biological site so that the biological site is interposed between the electrodes and the backplate wherein the power control system is also adapted to provide power 7 signals to the electrodes wherein the power signals are selected such that at least one electrode has a voltage level that differs from the backplate so that current flows between at least one electrode and the backplate. In another aspect, a measurement device senses at least one characteristic of the power signal applied to at least one electrode and provides a power measurement signal wherein the power control system receives the power measurement signal and determines an impedance measurement based on the power measurement signal and controls the duty cycle of a power in response to the power measurement signal. In a method for delivering energy to a biological site in accordance with the invention, the method comprising the steps of locating at the biological site a catheter having an electrode disposed at a distal end, conducting a power signal to the electrode on an electrode lead to apply energy to the biological site, and sensing temperature at the distal end of the catheter and conducting a temperature signal of the sensed temperature on the electrode lead to a power control system. In a more detailed aspect, a method further comprises the step of controlling a duty cycle of the power signal in response to the sensed temperature, the duty cycle having an on-period and an off -period within a duty cycle time frame. Furthermore, the method comprises the step of receiving the temperature signal on the lead during the off-period in the duty cycle of the power signal. In further aspects of the method, the step of locating comprises the step of disposing a plurality of electrodes at the distal end of the catheter and the step of conducting further comprises the steps of conducting a power signal to each of the electrodes on an electrode lead, and selecting the power signals such that at least two electrodes have voltage levels that differ from each other so that current flows between said two electrodes. In a more detailed aspect, the method further comprises the step of conducting power signals with different phase angles to at least two of the electrodes. In yet other aspects, the method comprises the steps of locating a backplate proximal the biological site so that the biological site is interposed between the 8 electrodes and the backplate, providing power signals to the electrodes wherein the power signals are selected such that at least one electrode has a voltage level that differs from the backplate so that current flows between at least one electrode and the backplate. In further aspects, the step of conducting a power signal comprises the step of conducting respective power signals to a plurality of the electrodes on respective electrode leads, the step of sensing comprises the step of sensing temperature at a plurality of electrodes and conducting temperature signals of the sensed temperature at the respective electrode to the power control system on the respective electrode lead, and further comprising the step of receiving the temperature signals during an off-period in a duty cycle of the respective power signal. In much more detailed aspects, the method further comprises the step of controlling the duty cycle of each power in response to the respective temperature signal, sensing at least one characteristic of the power signal applied to at least one electrode and providing a power measurement signal, receiving the power measurement signal and determining an impedance measurement based on the power measurement signal, and controlling the duty cycle of a power in response to the power measurement signal. These and other aspects and advantages of the present invention will become apparent from the following more detailed description, when taken in conjunction with the accompanying drawings which illustrate, by way of example, the preferred embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematic diagram of an ablation apparatus including a power control system, electrode device and backplate; FIG. 2 is a block diagram presenting more detail of a power control system in accordance with aspects of the invention, showing phase angle control, duty cycle control, and impedance and temperature monitoring; FIG. 3 is a diagram of a multi-channel ablation apparatus in accordance with aspects of the invention wherein a single microprocessor controls the phase angle and duty cycle of each channel individually; FIG. 4 depicts a first power signal having a first phase angle and alternating instances of high peak power and very low power; FIG. 5 depicts a second power signal having a second phase angle different from the first phase angle and alternating instances of high peak power and very low power; FIG. 6 presents a time frame (TF) diagram showing a fifty-percent duty cycle; FIG. 7A depicts the phase relationship and voltage potential between the first and second power signals having first and second phase angles respectively, as a function of time; FIG. 7B depicts the phase relationship and voltage potential between the first and second power signals having second and first phase angles respectively, as a function of time; FIGS. 8A, 8B, 8C, 8D, and 8E are schematic diagrams of an embodiment of a power control system in accordance with aspects of the invention with FIG. 8A showing how FIGS. 8B, 8C, 8D and 8E are related; FIG. 9A is a three dimensional representation of an ablation apparatus having a linear array of band electrodes in contact with a biological site with a backplate at the opposite side of the biological site, in which the phase angle difference between adjacent electrodes of the linear array is zero degrees; FIGS. 9B through 9D depict, along the x, y, and z axes shown, the depth of the lesions formed by the ablation apparatus of FIG. 9A showing that the apparatus acts as a unipolar device with multiple electrodes and the resulting lesions are discontinuous; FIG. 10A is a three dimensional representation of an ablation apparatus having a linear array of band electrodes in contact with a biological site with a backplate at the opposite side of the biological site, in which the phase angle difference between adjacent electrodes is 180 degrees; FIGS. 10B through 10D depict, along the x, y, and z axes shown, the continuity and depth of a lesion formed by the ablation apparatus of FIG. 10A 10 showing that the apparatus acts as a bipolar device with no significant amount of current flowing to the backplate; FIG. 11A is a three dimensional representation of an ablation apparatus having a linear array of band electrodes in contact with a biological site with a backplate at the opposite side of the biological site, in which the phase difference between adjacent electrodes is approximately 90 degrees; and FIGS. 11B through 11D depict, along the x, y, and z axes shown, the continuity and depth of a lesion formed by the ablation apparatus of FIG. 11A showing the greater depth of lesion resulting from the phase angle difference. FIG. 12 is a diagram in partial cross section showing the junction of two dissimilar wires to form a basic thermocouple; and FIG. 13 is a diagram of a band electrode by itself showing the connection of thermocouple wires in accordance with one aspect of the invention where one of the wires also conducts ablation energy to the electrode. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Turning now to the drawings, in which like reference numerals are used to designate like or corresponding elements among the several figures, in FIG. 1 there is shown an ablation apparatus 10 in accordance with aspects of the present invention. The apparatus 10 includes a power control system 12 that provides power or drive 14 to an electrode device 16. The power control system 12 comprises a power generator 18 that may have any number of output channels through which it provides the power 14. The operation of the power generator 18 is controlled by a controller 20 which outputs control signals 21 to the power generator 18. The controller 20 monitors the power 14 provided by the power generator 18. In addition, the controller 20 also receives temperature signals 22 from the electrode device 16. Based on the power 14 and temperature signals 22 the controller 20 adjusts the operation of the power generator 18. A backplate 24 is located proximal to the biological site 26 opposite the site from the electrode device 16, and is connected by a backplate wire 28 to the power generator 18. The backplate 24 is set at the reference level to the power provided to the electrodes, as discussed in detail below. 11 The electrode device 16 is typically part of a steerable EP catheter 30 capable of being percutaneously introduced into a biological site 26, e. g., the atrium or ventricle of the heart. The electrode device 16 is shown in schematic form with the components drawn to more clearly illustrate the relationship between the components and the relationship between the components and the power control system 12. In this embodiment, the catheter 30 comprises a distal segment 34 and a handle 31 located outside the patient. A preferred embodiment of the electrode device 16 includes twelve band electrodes 32 arranged in a substantially linear array along the distal segment 34 of the catheter 30. The electrode device 16 may include a tip electrode 36. (For clarity of illustration, only four band electrodes 32 are shown in the figures although as stated, a preferred embodiment may include many more.) The band electrodes 32 are arranged so that there is space 38 between adjacent electrodes. In one configuration of the electrode device 16, the width of the band electrodes 32 is 3 mm and the space 38 between the electrodes is 4 mm. The total length of the electrode device 16, as such, is approximately 8 cm. The arrangement of the band electrodes 32 is not limited to a linear array and may take the form of other patterns. A substantially linear array is preferred for certain therapeutic procedures, such as treatment of atrial fibrillation, in which linear lesions of typically 4 to 8 cm in length are desired. A linear array is more easily carried by the catheter 30 and also lessens the size of the catheter. The band electrodes 32 are formed of a material having a significantly higher thermal conductivity than that of the biological tissue 26. Possible materials include silver, copper, gold, chromium, aluminum, molybdenum, tungsten, nickel, platinum, and platinum/10% iridium. Because of the difference in thermal conductivity between the electrodes 32 and the tissue 26, the electrodes 32 cool off more rapidly in the flowing fluids at the biological site. The power supplied to the electrodes 32 may be adjusted during ablation to allow for the cooling of the electrodes while at the same time allowing for the temperature of the tissue to build up so that ablation results. The electrodes 32 are sized so that the surface area available for contact with fluid in the heart, e. g., blood, is 12 sufficient to allow for efficient heat dissipation from the electrodes to the surrounding blood. In a preferred embodiment, the electrodes 32 are 7 French (2.3 mm in diameter) with a length of 3 mm. The thickness of the band electrodes 32 also affects the ability of the electrode to draw thermal energy away from the tissue it contacts. In the present embodiment, the electrodes 32 are kept substantially thin so that the electrodes effectively draw energy away from the tissue without having to unduly increase the outer diameter of the electrode. In a preferred embodiment of the invention, the thickness of the band electrodes is 0.05 to 0.13 mm (0.002 to 0.005 inches). Associated with the electrode device 16 are temperature sensors 40 for monitoring the temperature of the electrode device 16 at various points along its length. In one embodiment, each band electrode 32 has a temperature sensor 40 mounted to it. Each temperature sensor 40 provides a temperature signal 22 to the controller 20 which is indicative of the temperature of the respective band electrode 32 at that sensor. In another embodiment of the electrode device 16 a temperature sensor 40 is mounted on every other band electrode 32. Thus for a catheter having twelve electrodes, there are temperature sensors on six electrodes. In yet another embodiment of the electrode device 16 every other electrode has two temperature sensors 40. In FIG. 1, which shows an embodiment having one temperature sensor for each electrode, there is shown a single power lead 15 for each electrode 32 to provide power to each electrode for ablation purposes and two temperature leads 23 for each temperature sensor 40 to establish the thermocouple effect. Turning now to FIG. 2, a block diagram of an ablation apparatus 10 and method in accordance with aspects of the invention is presented. In FIG. 2, a single channel of the power control system 12 is depicted. This channel controls the application of power to a single electrode 32. As will be discussed in relation to other figures, a channel may control a plurality or group of electrodes. In FIG. 2, a microprocessor 42, which is part of the controller 20 (FIG. 1), provides a duty cycle control signal 44 to a duty cycle generator 45. In this case, the duty cycle generator 45 receives the control signal 44 by an 8-bit latch 46. The latch 46 13 provides an 8-bit signal 47 to a duty cycle comparator 48. The comparator 48 compares the 8-bit signal 47 to a count from an 8-bit duty cycle counter 50 and if the count is the same, provides a duty cycle off signal 49 to the duty cycle gate 52. The gate 52 is connected to a frequency source 54, such as an oscillator that produces 500 kHz. When the gate 52 receives the duty cycle off signal 49 from the comparator 48, it stops its output of the frequency source signal through the gate and no output exists. At a frequency of 500 kHz, an 8-bit control has a period or time frame of 0.5 msec. At a fifty-percent duty cycle, the electrode is in the off period only 0.25 msec. To allow for greater cooling of the electrode, the period or time frame 78 (FIG. 6) is lengthened by use of a prescalar 56 interposed between the frequency source 54 and the counter 50. In one embodiment, the prescalar 56 lengthens the period to 4 msec thus allowing for a 2 msec off period during a fifty-percent duty cycle. This results in a sufficient cooling time for the very thin band electrodes discussed above. Other lengths of the period may be used depending on the circumstances. It has been found that a ten percent duty cycle is particularly effective in ablating heart tissue. The combination of the application of high peak power, a ten percent duty cycle, the use of high thermal conductivity material in the band electrodes, and fluids flowing past the band electrodes which have a cooling effect on the electrodes result in a much more effective application of power to the tissue. Ablation occurs much more rapidly. A terminal count detector 58 detects the last count of the period and sends a terminal count signal 59 to the gate 52 which resets the gate for continued output of the frequency source signal. This then begins the on period of the duty cycle and the counter 50 begins its count again. In one preferred embodiment, the duty cycle is set at fifty percent and the 8-bit latch is accordingly set to 128. In another embodiment, the duty cycle is set at ten percent. A programmable logic array ("PLA") 60 receives phase control signals 61 from the microprocessor 42 and controls the phase of the frequency source 54 accordingly. In one embodiment, the PLA 60 receives the terminal count signal 14 59 from the terminal count detector 58 and only permits phase changes after receiving that terminal count signal. The output signal from the gate 52 during the on period of the duty cycle is provided to a binary power amplifier 62 that increases the signal to a higher level, in this case, 24 volts. The amplified signals are then filtered with a band pass filter 64 to convert the somewhat square wave to a sine wave. The band pass filter 64 in one embodiment is centered at 500 kHz. The filtered signal is then provided to an isolated output transformer 66 that amplifies the signal to a much higher level, for example 350 volts peak-to-peak. This signal is then sent to a relay interconnect 67 before it is provided as a power output signal OUTn 14 to an electrode 32 at the biological site to cause ablation. The power output signal 14 from the isolated output transformer 66 is monitored in one embodiment to determine the impedance at the electrode 32. In the embodiment shown in FIG. 2, a voltage and current monitor 68 is used. The monitor signal 69 is converted to digital form by an A-to-D converter 70 and provided to the microprocessor 42. As previously mentioned, some or all of the electrodes 32 may include a temperature sensor 40 (FIG. 1) that provides temperature signals 22 (FIG. 2) which are used to determine the temperature at the electrode 32. In one embodiment of the invention, the power 14, in conjunction with the temperature signals 22, are used to determine the temperature at the electrode 32. Both the temperature signals 22 and the power 14 pass through a temperature filter 73 before being sent to the microprocessor 42. In the alternative, the temperature filter 73 is contained in a printed circuit board separate from the controller 22 and contains its own processor. In either case, the filter 73 filters out any RF noise present in the power 14 so that the signal may be used for temperature monitoring purposes. In another embodiment, the microprocessor monitors the power 14 and temperature signals 22 only during the off periods of the power 14 duty cycle. Accordingly, negligible RF noise is present in the power line and filtration is not necessary. In either embodiment, the microprocessor 42 may alter the duty cycle of the power 14 in response to either or both of the impedance or temperature signals. 15 In a manual arrangement, the temperature sensed and/or the determined impedance may be displayed to an operator. The operator in response may then manually control the duty cycle or other power parameters such as by rotating a knob mounted on a front panel of an instrument. In the case of a multiple channel instrument and catheter, as discussed below, multiple knobs may be provided in this manual arrangement for control over each channel. Referring now to FIG. 3, a multiple channel ablation apparatus is shown. Although only three complete channels are shown, the apparatus comprises many more as indicated by the successive dots. Those channels are not shown in FIG. 3 to preserve clarity of illustration. By providing different voltage levels between two electrodes 32 in an array, current flows between those electrodes in a bipolar electrode approach. By setting the backplate 24 (FIG. 1) at a voltage level different from at least one of those electrodes 32, current flows between that electrode and the backplate. By controlling the voltage levels among the three (two electrodes and backplate), the current flow through the biological site 26 can be more precisely controlled. One technique for setting different voltage levels between the electrodes 32 is to maintain a phase difference between them in an AC approach. By setting the backplate 24 at the reference level, current flows between the electrodes 32 and the backplate. The single microprocessor 42, which again is part of the controller 20 (FIG. 1), controls the duty cycle and the phase of each channel individually in this embodiment. Each channel shown comprises the same elements and each channel produces its own power output signal 14 (OUT1, OUT2, through OUTn where "n" is the total number of channels) on respective electrode leads (LEAD 1. LEAD 2, through LEAD n where "n" is the total number of leads) to the electrodes 32. This multi-channel approach permits more individual control over each electrode. For example, the duty cycle of the power applied to each electrode can be individually controlled. One electrode may have a ten percent duty cycle while another has a thirty percent duty cycle. Referring now to the first and second output signals OUT1 and OUT2 of FIG. 3, the signals, as shown in FIGS. 4, 5, and 6, have alternating instances of 16 peak power i. e., "on" periods 74, and very low power 76, i. e., "off periods. Typically, the output power 14 is a 500 kHz sine wave. In FIGS. 4 and 5, the number of cycles of the sine wave contained within one on period 74 has been substantially reduced in the drawing to emphasize the phase difference between the first and second output signals OUT1, OUT2. Preferably, the voltage of each power signal 14 during an off period 76 is substantially zero and during an on period 74 is approximately 350 volts peak-to-peak. The power OUT1 and OUT2 also have a variable duty cycle for controlling the length of the on period 74 and the off-period 76 within a time frame 78 (see FIG. 6) . The duty cycle is the ratio of the length of the on period 74 to the length of the entire time frame 78. The effective power is the peak power times the duty cycle. Thus, a signal having a peak power of 100 watts and a 50% duty cycle has an effective power of 50 watts. As shown in FIGS. 4, 5, and 6, the two power signals OUT1, OUT2 are phased differently from each other. As discussed above, the phase angle of each power signal is set and controlled by the processor 42 and PLA 60. Each power signal OUT1 and OUT2 has a respective phase angle and those phase angles differ between the two of them. The phase angle difference between the power OUT1 and OUT2 produces a voltage potential between the band electrodes 32 (FIG. 1) that receive the power. This voltage potential, in turn, induces current flow between the band electrodes 32. The phase angle relationship of the power and the voltage potential produced as a function of time is shown in FIGS. 7A and 7B. The potential between electrodes V e . e is defined by: V e _ e = 2V sinCy-) n(2πft) (Eq. 1) where: ΔΦ = phase angle difference between electrodes V = voltage amplitude of power f = frequency in hertz t = time 17 FIG. 7A shows first and second power OUT1 and OUT2 provided to first and second electrodes respectively having a phase angle difference ΔΦ with OUT1 leading OUT2 by 132 degrees. FIG. 7B shows the same power OUT1 and OUT2 but with the phase angles reversed where OUT2 is now leading OUT 1 by 132 degrees. With reference now to FIG.8, schematic diagrams of an embodiment of the ablation apparatus 10 of FIG. 2 are presented in FIGS. 8B through 8E while FIG. 8A shows how FIGS. 8B through 8E should be oriented in relation to each other. The frequency source 54 provides a signal 80, typically at 500 kHz with a phase angle controlled by the microprocessor 42 through the PLA 60, to the duty cycle generator 45. The duty cycle generator 45 modulates the frequency source signal 80 to produce the selected duty cycle in accordance with the duty cycle control signal 44 as previously described. The duty cycle generator 45 outputs two signals 82 and 84 to the binary power amplifier 62. A dual MOSFET driver U2 receives the signals, converts their 5V level to a 12V level, and sends each to a transformer T2 which transforms the signals into 24 V peak-to-peak power. The 24V power is then sent to a multi-state driver 86 which includes a configuration of FETs Q2, Q3, Q4, and Q5. During a conducting state of the driver 86, which is typically the on period 74 of the power, these FETs Q2 through Q5 conduct and forward the power to a bandpass filter 64 comprising a series LC network. During a high-impedance state of the driver 86, which is typically during the off period 76 of the power, the FETs Q2 through Q5 are nonconducting and no power is sent to the bandpass filter 64. Instead the FETs Q2 through Q5 present a high impedance load to any signals received through the electrode 32. Typically the load impedance on the FETs Q2 through Q5 presented by the circuit following the FETs , the electrode, and the tissue is approximately 150 Ω but transformed through the output transformer T3, it presents a load impedance to the FETs Q2-Q5 of approximately 0.5 to 1 Ω. In the off state, the FETs present an impedance of approximately 250 Ω which is large in comparison to the transformed load impedance of approximately 0.5 to 1 Ω. Therefore, very little power flows when the FETs are in the off state. 18 The bandpass filter 64 operates to shape the output signal provided by the binary amplifier 62 from a square wave to a sinusoidal wave. The filtered signal 85 then passes to the isolated output section 66 where it is step-up transformed to 350 volt peak-to-peak sinusoidal power at T3. The power is then split into two identical power signals OUT1A, OUT1B and provided to two or more respective band electrodes 32 on the output lines LEAD1A, LEAD1B. The isolated output section 66 also includes relays 88 that may be individually opened to remove the power signals OUT1A, OUT1B from the electrode leads LEAD 1A, LEAD IB when an alert condition is detected, such as high temperature or high impedance at the respective electrode 32. As previously mentioned these conditions are determined by the microprocessor 42 which receives signals indicative of the temperature and impedance at each of the band electrodes 32. The power from the isolated output section 66 is monitored and representative signals are supplied to an RF voltage and current monitor 68 where in this case, the voltage and current of each output signal are measured to determine the impedance of the particular channel. The measured signals are sent to an A-to-D converter 70 (FIG. 2) before being sent to the microprocessor 42 for impedance monitoring. If the impedance is above a threshold level indicative of blood clotting or boiling, the microprocessor 42 sends a signal to the duty cycle generator 45 to reduce or discontinue the duty cycle of the power OUT1A, OUT1B and thus lower the effective power delivered to the band electrodes 32. Similarly, the temperature at the electrodes 32 is determined by monitoring the power 14 and temperature signals 22 and measuring the voltage difference between the signals. As previously mentioned, in one embodiment of the invention, these signals pass through a filter 73 (FIG. 2) before being sent to the microprocessor 42. The voltage value is converted to a temperature and if the temperature is above a threshold level the duty cycle of the power 14 is reduced. In the case where a single lead is used to provide a signal which is used to determine the temperature as well as provide power to the electrode 32, the 19 signal from the lead is received on temperature leads 87, 89 connected at the output side of the relays 88. As shown in FIG. 3, the duty cycle of each electrode 32 may be individually controlled by the microprocessor 42. As previously mentioned, based on the temperature at an electrode 32 and the current and voltage of the output signal provided to an electrode, the duty cycle of the output signal may be adjusted. For example, one electrode 32 may have a temperature requiring a duty cycle of ten percent, while another electrode may have a temperature which allows for a fifty percent duty cycle. In an embodiment in which every other electrode 32 has a temperature sensor 40, the electrodes are grouped in pairs with each electrode in the pair having the same duty cycle. In operation, as depicted in FIGS. 9 through 11, the electrode device 16 and the backplate 24 are positioned proximal the biological site 26 undergoing ablation such that the biological site is interposed between the electrode device and the backplate. The band electrodes 32 (only one of which is indicated by a numeral 32 for clarity of illustration) of the electrode device 16 each receives power OUT1, OUT2, OUT3, OUT4 having a phase angle on LEAD 1 through LEAD 4. In one embodiment, every other electrode 32 receives the same phase angle. Therefore, the phase angle of electrode A equals the phase angle of electrode C and the phase angle of electrode B equals the phase angle of electrode D. The advantages of this arrangement are described below. In a preferred embodiment, the electrodes 32 are formed into a linear array as shown. In addition, a thermocouple temperature sensor 40 is located at each of the electrodes A, B, C, and D and uses the electrode power lead LEADS 1 through 4 as one of the sensor leads. The sensors 40 provide temperature sensor signals 22 for receipt by the power control system 12. In another embodiment, alternate electrodes 32 may be grouped together and each may receive the same power having the same phase angle and duty cycle. Another group or groups of electrodes 32 may be interspaced with the first group such that the electrodes of one group alternate with the electrodes of the other group or groups. Each electrode 32 in a particular group of electrodes has 20 the same phase angle and duty cycle. For example, electrodes A and C may be connected to the same power while interspaced electrodes B and D may be connected to a different power output signal. The use of individual power signals also provides the ability to disable any combination of electrodes 32 and thereby effectively change the length of the electrode device 16. For example, in one configuration of the present invention an electrode device 16 with twelve electrodes 32 receives twelve power signals from a twelve channel power control system 12. The electrodes 32 are 3 mm in length and are 4 mm apart. Accordingly, by disabling various electrodes, a virtual electrode of any length from 3 mm to 8 cm may be produced by the electrode device 16. In either arrangement the backplate 24 is maintained at the reference voltage level in regard to the voltage level of the power OUT1 through OUTn. As previously described, by varying the phase angles between the power OUT1, OUT2 supplied to each electrode 32, a phase angle difference is established between adjacent band electrodes. This phase angle difference may be adjusted to control the voltage potential between adjacent band electrodes 32 and thus to control the flow of current through the biological site 26. The flow of current I e . e between adjacent band electrodes 32 is defined by: ΔΦ 2Fsιn(— ) sin(2Λ/?) _. = (Eq. 2) where: ΔΦ = phase angle difference between electrodes V = voltage amplitude of power Z e . e = impedance between electrodes f = frequency in hertz t = time In addition to the current flow between the band electrodes 32 there is current flow between the band electrodes and the backplate 24. When the backplate 24 is set at the reference level, this current flow I e . b is defined by: 21 Vsm(2 ) - b = (Eq. 3) • e-b where: ΔΦ = phase angle difference between electrodes V = voltage amplitude of power Z e . b = impedance between electrode and backplate f = frequency in hertz t = time Assuming Z e . b and Z e . e are equal, the ratio of the current flowing between the band electrodes 32 I e . e to the current flowing between the band electrodes 32 and the backplate 24 I e . b is defined by: Λ-. -, • Φ (Eq. 4) —— = 2sιn(— -) 1 e-b where: ΔΦ = phase angle difference between electrodes FIGS. 9 through 11 illustrate various current flow patterns within a biological site. The depths and widths of the lesions depicted in FIGS. 9 through 11 are not necessarily to scale or in scalar proportion to each other but are provided for clarity in discerning the differences between the various power application techniques. When the phase difference between adjacent electrodes 32 is zero degrees, no current flows between the electrodes in accordance with Eq. 2 above, and the apparatus operates in a unipolar fashion with the current flowing to the backplate 24 as shown in FIGS. 9A through 9D. Substantially all current flows from the band electrodes 32 to the backplate 24 forming a series of relatively deep, acute lesions 90 along the length of the electrode device 16. As seen in the top view of FIG. 9B and the side view of FIG. 9D, the lesions are discrete. The lesions 90 are discontinuous in regard to each other. When the phase difference between adjacent electrodes 32 is 180 degrees the apparatus operates in both a unipolar and bipolar fashion and the current flow pattern is as shown in FIG. 10A. With this phase difference, approximately 22 twice as much current flows between adjacent band electrodes 32 than flows from the band electrodes to the backplate 24. The resulting lesion 92 is shallow but is continuous along the length of the electrode device 16. The continuity and shallow depth of the lesion 92 are illustrated in FIGS. 10B through 10D. Nevertheless, the lesion depth is still greater than that created by prior bipolar ablation methods alone. When the phase difference between adjacent electrodes 32 is set within the range of a value greater than zero to less than 180 degrees, the current flow varies from a deep, discontinuous unipolar pattern to a more continuous, shallow bipolar pattern. For example, when the phase difference between adjacent electrodes 32 is around 90 degrees, the current flows as shown in FIG. 11 A. With this phase difference, current flows between adjacent band electrodes 32 as well as between the band electrodes and the backplate 24. Accordingly, a lesion which is both deep and continuous along the length of the electrode device 16 is produced. The continuity and depth of the lesion 94 is illustrated in FIGS. 11B through 11D. In one embodiment of FIG. 11A, adjacent electrodes alternated in phase but were provided with power in groups. Electrodes A and C were provided with power at a first phase angle and electrodes B and D were provided with power at a second phase angle, different from the first. Thus, in accordance with the present invention the phase angle of the power may be adjusted in order to produce a lesion having different depth and continuity characteristics. In selecting the phase angle difference necessary to produce a continuous lesion having the greatest possible depth, other elements of the electrode device 16 are considered. For example, the width of the band electrodes 32 and the spacing between the electrodes are factors in selecting an optimum phase angle. In a preferred embodiment of the present invention, as pointed out above, the width of the band electrodes is 3 mm, the spacing between the electrodes is 4 mm and the electrodes receive power which establish a phase difference of 132 degrees between adjacent electrodes. With this configuration a long continuous lesion having a length of between approximately 3 mm and 8 cm and a depth of 5 mm or greater was produced depending on the number of 23 electrodes energized, the duty cycle employed, and the duration of power application. In another embodiment of the invention, energy is applied to the biological tissue 26 during the on period of the duty cycle in an alternating unipolar-bipolar manner. During the unipolar mode segment a voltage potential is established between the electrodes 32 and the backplate 24. Thus current flows through the tissue 26 between the electrodes 32 and the backplate 24. During the bipolar mode segment a voltage potential is established between at least two of the electrodes 32 rather than between the electrodes and the backplate 24. Thus current flows through the tissue 26 between the electrodes 32. While operating in this mode the voltage difference between the electrodes 32 may be established by providing power with different phase angles to the electrodes as previously mentioned. Alternatively, some of the electrodes 32 may be connected to a reference potential while others are maintained at a different voltage level. By adjusting the duration of the unipolar and bipolar mode segments within the on period of the duty cycle, the continuity and depth of the lesion produced may be controlled. For example, operating in the unipolar mode for one-fourth of the on period and in the bipolar mode for three-fourths of the on period produces a lesion having a continuity and depth similar to the lesion 94 illustrated in FIGS. 11B through 11D. Referring to FIGS. 8B through and 8E, the following devices are shown: Device Part No. Manufacturer Ul GAL6002B Lattice U2 SN75372 numerous Ql 1RFZ34N numerous Q2, Q3, Q4, Q5 1RFZ44N numerous Q7, Q8, Q9 MPF6601 numerous R3, R5 1Ω numerous T1, T4 CMI-4810 Corona Magnetics, Inc. T2 GFS97-0131-1 GFS Manufacturing 24 T5 CMI-4809 Corona Magnetics, Inc. The transformer denoted by "T3" is a 1:12 turns ratio, single turn primary, step up transformer wound on a TDK core PC50EER23Z. Turning now to FIG. 12, a typical thermocouple sensor 100 is shown. A first lead 102 is in physical contact forming a junction 106 with a second lead 104 of dissimilar material. A solder bead 108 assists in holding the two leads or wires 102 and 104 in contact with each other. As a result of their dissimilarity, a thermoelectric voltage is developed at the junction 106 which is proportional to the temperature. In this case, the solder bead 108 does not operate electrically as part of the thermocouple junction and performs only a mechanical function. In FIG. 13, a band electrode 32 is shown having a thermocouple 110 formed at the inside surface of the band from two leads of dissimilar metals 112 and 114. A solder bead is also shown. In this case, the two leads 112 and 114 are joined together to form a thermocouple junction and are also coupled to the band electrode 32. The first lead 112 performs a second purpose in addition to forming part of the thermocouple 110. That is, the first lead is also used to conduct power signals to the band electrode to impart ablation energy to the biological target tissue. Thus only two leads 112 and 114 are used to entirely power and sense at the band electrode 32 rather than the three leads used in prior approaches. This can result in a substantial savings in size because of the existence of one-third fewer leads to be housed by the catheter. In the case of the twelve-band catheter described above in conjunction with FIG. 1, instead of the normal thirty six leads required, only twenty four leads would be required should the invention be employed. This is a substantial decrease in the number of internal components for the catheter. Because the thermocouple voltages are typically on the order of 0.001 mV to 0.10 mV per degree C, the power signals conducted on one thermocouple lead 112 could interfere with the detection of the thermocouple signals generated by the thermocouple 110. Filtration could possibly be used to detect the DC thermocouple signals; however, such filtration approaches can be complex and costly. In accordance with a feature of the invention, the controller 20 monitors 25 the leads 112 and 114 for thermocouple signals only during the off-period 76 of the duty cycle 78, for example, as shown in FIG. 6. During this off-period, no power signals are being applied to the band electrode 32 over the first electrode lead 112 and there is less chance for interference with the thermocouple signals produced by the junction 110 and conducted on both leads 112 and 114. Thus, the temperatures may be measured briefly without electrical interference. Although the thermocouple 110 is shown in FIG. 13 as being formed by joining the two leads 112 and 114 directly together with a surrounding solder bead 116, they could also be attached to the band electrode 32 individually, thereby using the electrode as a bead filler material (analogous to using a solder or braze alloy to create the bead) . These two bead types will produce equivalent results if the wire junctions are all at the same temperature. The performance of these two bead types may differ, however, if a thermal gradient exists within the electrode 32. The inventor hereby incorporates by reference his application having docket number 40445 (12370) entitled "Electrode Having Non- Joined Thermocouple for Providing Multiple Temperature-Sensitive Junctions" filed this same day. While several particular forms of the invention have been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited, except as by the appended claims.

Claims: 26 WHAT IS CLAIMED IS: 1. An apparatus for delivering energy to heart tissue, said apparatus comprising: a catheter having an electrode disposed at a distal end of the catheter, the distal end adapted to be positioned so that the electrode is located at the heart tissue; a second device disposed at the distal end of the catheter providing a device signal; a first lead connected to the electrode and to the second device to conduct the device signal; a power control system adapted to provide power to the electrode on the first lead and to monitor the device signal. 2. The apparatus of claim 1 wherein the power control system controls the power to the electrode to have a duty cycle with the duty cycle having an on- period and an off-period within a duty cycle time frame. 3. The apparatus of claim 2 wherein the power control system monitors the device signal during the off-period of the duty cycle. 4. The apparatus of claim 3 wherein the power control system controls the duty cycle of the power in response to the second device signal received during the off-period of the duty cycle. 5. The apparatus of claim 1 wherein the power control system controls the duty cycle of the power in response to the second device signal. 6. The apparatus of claim 5 wherein the second device comprises a temperature sensor disposed at the distal end of the catheter providing a temperature signal as the device signal; 27 wherein the power control system is adapted to control the duty cycle of the power to the electrode. 7. The apparatus of claim 6 wherein the power control system is adapted to process the temperature signal from the temperature sensor only during the off-period in the duty cycle of the power. 8. The apparatus of claim 7 wherein the power control system controls the duty cycle of the power in response to the processed temperature signal. 9. The apparatus of claim 1 wherein the second device comprises a temperature sensor disposed at the distal end of the catheter providing a temperature signal as the device signal; wherein the power control system is adapted to provide power to the electrode in response to the temperature signal. 10. The apparatus of claim 1 wherein: the catheter comprises a plurality of electrodes at its distal end; and the power control system is further adapted to provide power to each of the electrodes wherein the power is selected such that at least two electrodes have voltage levels that differ from each other so that current flows between said two electrodes. 11. The apparatus of claim 10 wherein the power control system provides power with different phase angles to at least two of the electrodes. 12. The apparatus of claim 11 wherein the power differs in phase by an amount greater than zero degrees but less than 180 degrees. 13. The apparatus of claim 12 wherein the power differs in phase by an amount approximately equal to 132 degrees. 28 14. The apparatus of claim 10 further comprising a backplate adapted to be positioned proximal the biological site so that the biological site is interposed between the electrodes and the backplate; wherein the power control system is also adapted to provide power to the electrodes wherein the power is selected such that at least one electrode has a voltage level that differs from the backplate so that current flows between at least one electrode and the backplate. 15. The apparatus of claim 1 further comprising a measurement device that senses at least one characteristic of the power applied to the electrode and provides a power measurement signal; wherein the power control system receives the power measurement signal and determines an impedance measurement based on the power measurement signal and controls the power provided to the electrode in response to the power measurement signal. 16. An apparatus for delivering energy to a biological site, said apparatus comprising: a catheter having a plurality of electrodes disposed at a distal end of the catheter, the distal end adapted to be positioned so that the electrodes are located at the biological site; a temperature sensor disposed at the distal end of the catheter providing a temperature signal; a lead connected to both an electrode and the temperature sensor adapted to conduct power to the electrode and to conduct the temperature signal from the temperature sensor; a backplate adapted to be positioned proximal the biological site so that the biological site is interposed between the electrodes and the backplate; and a power control system adapted to provide power to the electrodes wherein the power is selected such that at least two electrodes have voltage levels that differ from each other so that current flows between said two electrodes and 29 such that at least one electrode has a voltage level that differs from the backplate so that current flows between at least one electrode and the backplate, and further adapted to provide power to an electrode on the lead and to receive the temperature signal on the lead. 17. The apparatus of claim 16 wherein the power control system is further adapted to control the duty cycle of the power provided on the lead with the duty cycle having an on-period and an off-period within a duty cycle time frame, the power control system controlling the on-period of the duty cycle in response to the temperature signal. 18. The apparatus of claim 17 wherein the power control system is adapted to receive the temperature signal on the lead from the temperature sensor during the off-period in the duty cycle of the power. 19. The apparatus of claim 16 wherein the power control system provides power with different phase angles to at least two of the electrodes. 20. The apparatus of claim 17 further comprising a measurement device that senses at least one characteristic of the power applied to an electrode and provides a power measurement signal; wherein the power control system receives the power measurement signal and determines an impedance measurement based on the power measurement signal and controls the duty cycle of the power in response to the power measurement signal. 30 21. A method for delivering energy to a biological site, the method comprising the steps of: locating at the biological site a catheter having an electrode disposed at a distal end; conducting power to the electrode on an electrode lead to apply energy to the biological site; and sensing temperature at the electrode and conducting a temperature signal of the sensed temperature on the electrode lead to a power control system. 22. The method of claim 21 further comprising the step of controlling a duty cycle of the power in response to the sensed temperature, the duty cycle having an on-period and an off-period within a duty cycle time frame. 23. The method of claim 22 further comprising the step of receiving the temperature signal on the lead during the off-period in the duty cycle of the power. 24. The method of claim 21 wherein: the step of locating comprises the step of locating a plurality of electrodes disposed at the distal end of the catheter at the biological site; and the step of conducting further comprises the steps of: conducting power to each of the electrodes on a respective electrode lead; and selecting the power such that at least two electrodes have voltage levels that differ from each other so that current flows between said two electrodes. 25. The method of claim 24 further comprising the step of conducting power with different phase angles to at least two of the electrodes. 31 26. The method of claim 24 further comprising the steps of: locating a backplate proximal the biological site so that the biological site is interposed between the electrodes and the backplate; providing power to the electrodes wherein the power is selected such that at least one electrode has a voltage level that differs from the backplate so that current flows between at least one electrode and the backplate.

Inventors: SHERMAN MARSHALL L, SIMPSON JOHN A, SHERMAN, MARSHALL, L., SIMPSON, JOHN, A.
Applicants: CARDIAC PACEMAKERS INC, CARDIAC PACEMAKERS, INC.
Application year: 1999
Main classification: A61B17
IPC codes: A61B18, A61N1