Catheter having common lead for electrode and sensor

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 backplate 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.

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.degree. Centigrade. Tissue heated to a temperature above 
48.degree. C. becomes non-viable and defines the ablation volume. The 
objective is to elevate the tissue temperature, which is generally at 
37.degree. C., fairly uniformly to an ablation temperature above 
48.degree. C., while keeping both the temperature at the tissue surface 
and the temperature of the electrode below 100.degree. 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.degree., 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 electrode, 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 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 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, byway of 
example, the preferred embodiments of the invention.

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. 
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 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 FIGS. 2-1 and 2-2, a block diagram of an ablation apparatus 
10 and method in accordance with aspects of the invention is presented. In 
FIGS. 2-1 and 2-2, 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. 2-1, a 
microprocessor 42, which is part of the controller 20 (FIG. 1), provides a 
duty cycle control signal 44 to a duty cycle generator ("DCG") 45. In this 
case, the duty cycle generator 45 receives the control signal 44 by an 
8-bit latch 46. The latch 46 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 ("FS") 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 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 ("BPA") 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 ("BPF") 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 ("IOT") 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 ("RI") 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 (FIGS. 2-1 and 2-2), a voltage and current 
monitor ("VCM") 68 is used. The monitor signal 69 is converted to digital 
form by an A-to-D converter ("ADC") 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. 2-2) 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 ("FL") 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 20 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. 
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 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.sub.e-e is defined by: 
##EQU1## 
where: .DELTA..PHI.=phase angle difference between electrodes 
V=voltage amplitude of power 
f=frequency in hertz 
t=time 
FIG. 7A shows first and second power OUT1 and OUT2 provided to first and 
second electrodes respectively having a phase angle difference 
.DELTA..PHI. 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 OUT1 by 132 degrees. 
With reference now to FIGS. 8A through 8E, schematic diagrams of an 
embodiment of the ablation apparatus 10 of FIGS. 2-1 and 2-2 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 .OMEGA. but 
transformed through the output transformer T3, it presents a load 
impedance to the FETs Q2-Q5 of approximately 0.5 to 1 .OMEGA.. In the off 
state, the FETs present an impedance of approximately 250 .OMEGA. which is 
large in comparison to the transformed load impedance of approximately 0.5 
to 1 .OMEGA.. Therefore, very little power flows when the FETs are in the 
off state. 
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 1B 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. 2-2) 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. 2-2) 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 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. 9A through 11D, 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 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.sub.e-e between adjacent band 
electrodes 32 is defined by: 
##EQU2## 
where: .DELTA..PHI.=phase angle difference between electrodes 
V=voltage amplitude of power 
Z.sub.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.sub.e-b is 
defined by: 
##EQU3## 
where: .DELTA..PHI.=phase angle difference between electrodes 
V=voltage amplitude of power 
Z.sub.e-b =impedance between electrode and backplate 
f=frequency in hertz 
t=time 
Assuming Z.sub.e-b and Z.sub.e-e are equal, the ratio of the current 
flowing between the band electrodes 32 I.sub.e-e to the current flowing 
between the band electrodes 32 and the backplate 24 I.sub.e-b is defined 
by: 
##EQU4## 
where: .DELTA..PHI.=phase angle difference between electrodes 
FIGS. 9 through 11D illustrate various current flow patterns within a 
biological site. The depths and widths of the lesions depicted in FIGS. 9A 
through 11D 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 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. 11A. 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 
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 
______________________________________ 
U1 GAL6002B Lattice 
U2 SN75372 numerous 
Q1 1RFZ34N numerous 
Q2, Q3, Q4, Q5 
1RFZ44N numerous 
Q7, Q8, Q9 MPF6601 numerous 
R3, R5 1.OMEGA. numerous 
T1, T4 CMI-4810 Corona Magnetics, Inc. 
T2 GFS97-0131-1 
GFS Manufacturing 
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 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 pending application Ser. No. 
09/072,853 entitled "Electrode Having Non-Joined Thermocouple for 
Providing Multiple Temperature-Sensitive Junctions" filed May 5, 1998. 
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.