WO1996010950A1

WO1996010950A1 - Combined radiofrequency and high voltage pulse catheter ablation

Info

Publication number: WO1996010950A1
Application number: PCT/US1995/013129
Authority: WO
Inventors: Wilton W. Webster, Jr.
Original assignee: Cordis Webster, Inc.
Priority date: 1994-10-07
Filing date: 1995-10-06
Publication date: 1996-04-18
Also published as: AU3955195A

Abstract

A method for treating ventricular tachycardia comprises inserting an electrode catheter into the ventricule. The ventricular wall of the heart is contacted with an ablation electrode (20) at a site where an aberrant electrical pathway is located. Radiofrequency is delivered through the ablation electrode (20) to the tissue for a time sufficient to confirm the site of the aberrant electrical pathway and to preheat the tissue. Short high voltage electrical pulses are then delivered to the tissue through the same electrode (20) to thereby form a non-conductive lesion.

Description

COMBINED RADIOFREQUENCY AND HIGH VOLTAGE

PULSE CATHETER ABLATION

Field of the Invention The present invention relates to catheter ablation of cardiac tissue and, in particular, to such treatments where radiofrequency is combined with short high voltage, low energy electrical pulses for the treatment of ventricular tachycardia.

Background of the Invention

Millions of people suffer from abnormally high heart beat rhythms, a condition referred to as "tachyrhythmia." One type of tachyrhythmia is ventricular tachycardia (VT). This is a condition originating in the ventricles and resulting in an abnormally high rate of ventricular contraction. One of the causes of VT is scarring due to cardiovascular disease. The present invention is a means for treating this form of VT by a new form of electrical ablation.

Catheter ablation (destruction of tissue in the heart by means of an electrode catheter) has been found to be an efficacious means of interrupting accessory electrical pathways between the atria and ventricles. These accessory pathways can cause tachycardia, which is referred to as supraventricular tachycardia (SVT). In such a procedure, a special electrophysiological catheter is guided through an artery or vein into the patient's heart to the site of the accessory pathway. The catheter is designed to transmit energy from an external source into the site of the accessory pathway in an amount sufficient to ablate the tissue. The ablated tissue is replaced with scar tissue which interrupts the accessory pathway. The normal activity of the heart is thereby restored.

A common form of VT is caused by coronary artery disease after it has caused scarring of the ventricle. When coronary artery disease causes blockage of blood to heart muscle, tissue may die and the result is that this dead tissue becomes scar tissue. Many times these scars contain strands of living tissue which continue to conduct and thereby become abhorrent pathways through the scar. Under certain conditions, these pathways may again trigger the surrounding muscle into a new wave of excitation much sooner than the heart is scheduled to beat again. One type of energy used to ablate cardiac tissues is high-voltage, high energy direct current delivered in one or more pulses. Such high energy electrical shocks are very effective in creating lesions of adequate size. However, severe complications, including hemodynamic deterioration, induction of ventricular fibrillation, conductive disturbances, internal embolism and perforation of the ventricular wall, have been reported with this procedure.

An alternative to direct current, high energy ablation is radiofrequency (RF) ablation. In an RF ablation procedure, RF current is delivered to the ablation site and heats the cardiac tissue sufficiently to form a lesion. RF ablation has been shown to be safe and efficacious in treating SVT caused by accessory electrical pathways between either atrium and ventricles or AV nodal reentry circuits. In SVT, the sites of the accessory electrical pathways tend to be relatively small and not too deep. A typical area of ablation has an area of about 5-6 mm diameter and a depth of 3-5 mm. Lesions of this size can usually be formed by RF ablation without the ablation electrode overheating.

RF ablation has been found to be less effective in treating VT. In VT, affected sites tend to be more diffuse and encompass a greater depth, e.g., 1 .5 cm, than the sites involved in SVT. Therefore, treatment of VT by RF ablation of the tissue requires destruction of a larger volume of tissue. In addition, the RF ablation current may have to penetrate a scarred area which is difficult because of the lack of conduction in the scar tissue. The need to destroy a larger volume of tissue and penetrate the scar to treat VT introduces a number of problems for RF catheter ablation. One problem is the inability to keep the electrode cool while causing sufficient heating to ablate the required volume of tissue. If the ablation electrode gets too hot, coagulation will occur at the tip, reducing conductivity. If this occurs, the catheter must be removed, cleaned and reinserted. Another problem is that the electrode must have sufficient size to deliver the required amount of RF current. This means that larger than desired ablation electrodes must be used. Further, it is always difficult to maintain sufficient contact pressure between the electrode and endocardium and to keep the electrode in a stabile position throughout the RF current delivery, particularly since the heart continues to beat during the procedure. Summary of the Invention

A method of treating ventricular tachycardia by a catheter from within the ventricle is described. The method comprises contacting tissue of the heart with an ablation electrode at a site of an accessory electrical pathway. Radiofrequency energy is delivered to the tissues throug the electrode for a time sufficient to confirm the site of the accessory electrical pathway and to preheat the cardiac tissue at that site. After the delivery of the radiofrequency energy, short high voltage, low energy electrical pulses are delivered to the tissue through the same electrode to thereby form a lesion and block the accessory electrical pathway.

Brief Description of the Drawings

Features, aspects and advantages of the invention will be more fully understood when considered with respect to the following detailed description, appended claims and accompanying drawings where:

FIG. 1 is an ablation catheter in accordance with a preferred embodiment of the present invention;

FIG. 2 is a mapping/ablation catheter in accordance with another preferred embodiment of the present invention; FIG. 3 is a block diagram of a preferred high voltage dc pulse generator; and

FIG. 4 is a layout of the front panel of the dc pulse generator of FIG. 3.

Detailed Description The present invention overcomes the problems presented with conventional RF ablation techniques by combining RF treatment with short duration high voltage, low energy electrical pulses.

With reference to FIG. 1 , there is shown a preferred catheter constructed in accordance with the present invention. The catheter comprises an elongated tubular catheter body 12, a deflectable tip portion 1 at the distal end of the catheter body and a control handle 16 at the proximal end of the catheter body 12. A presently preferred control handle 16 and steering mechanism is disclosed in U.S. Patent No. 4,906,134 to Webster, which is incorporated herein by reference. An ablation electrode 20 is mounted at the distal end of the tip portion 14. This could also be a ring electrode proximal to the tip electrode. An electrode lead wire (not shown) extends from the ablation electrode through the catheter body and control handle to a switch 24. Switch 24 is, in turn, connected to a source 26 of RF energy and a source 28 of short duration high voltage energy pulses. A suitable generator of RF energy is, for example, manufactured by American Cardiac Ablation and sold under the trade designation Liz 88 RF generator.

The high voltage DC pulse generator 28 shown as a block diagram in FIG. 3 consists of front panel controls and displays 30, a control board 32, an over current limit board 34 and eight power boards 36.

Front panel 30 consists of switches 54 through 62 and light emitting diode (LED) indicators 64 through 70 as illustrated in FIG. 4. Channels switch 62 allows the operator to select the number of channels to fire. This indicates to the control board 32 via line 80 the number of power boards or channels to enable to output the desired voltage level. An indication of power-on is sent to the control board 32 via line 72 upon switching the power switch 54 to the ON position. The channel status LEDs 64 on the front panel flash to indicate the channels enabled in response to pulses via lines 82 from the control board 32. The ready LED 66 lights in response to a signal from the control board 32 over line 84 once the enabled channels are fully charged. The time delay between pulses is selectable from 30 microseconds to 990 microseconds. A pair of delay switches 60 are used to select the delay in units of 10 microseconds in the range of 3 to 99 units. The delay between pulses is indicated to the control board 32 over line 78. The number of 10 microsecond pulses to fire is selected with a pair of pulse switches 58 from 1 pulse to 99 pulses in a pulse train and indicated to the control board 32 over line 76. The fire switch 56 is depressed to initiate the ablation process. The signal to fire is sent to the control board 32 via line 74.

The done LED 68 pulses on upon indication from the control board 32 via line 86 that the procedure has successfully completed within the chosen parameters. The fault LED 70 flashes to indicate that the control board 32 batteries are low upon receipt of a pulse over line 88. The fault LED 70 will continuously light to indicate that the current limit of 50 Amperes was exceeded during the ablation process as indicated by the control board 32 via line 88. Two 9 volt batteries are mounted on the rear of the front panel (not shown) to supply + 1 8 Volts and + 9 Volts to the control board 32. FIG. 4 displays a representative embodiment of a front panel layout. Other configurations may be utilized for optimal ease of operator use. The control board 32 includes a microprocessor controller and peripheral

CMOS logic. It receives operator selections and reports operational and fault status via the front panel 30 switches 54 through 62 and LED indicators 64 through 70. The control board 32 latches the number of channels to fire selected via channels switch 62 upon receipt of the power-on indication from power switch 54. The control board 32 enables the number of power boards 36, via fiber optic interface 94, corresponding to the number of channels to fire selected by the operator. It sends a pulse to channel status LEDs 64 to flash the LEDs corresponding to the channels enabled via line 82. The ready LED 66 is lighted via line 84 once the enabled power boards 36 report a fully charged status to the control board 32 via fiber optic interface 98. The control board 32 commands the firing sequence by sending the selected pulse train to the enabled power boards 36 via high speed fiber optic interface 96 upon indication via line 74 that the operator has depressed fire switch 56. If the preset current limit of 50 amperes is not exceeded during the firing sequence, the control board 32 sends a pulse to done LED 68 via line 86 indicating a successful firing sequence.

The control board 32 processes faults and performs diagnostic functions in addition to controlling the ablation procedure. The control board 32 lights fault

LED 70 on front panel 30 continuously via line 88 when the current limit of 50 Amperes is exceeded. The over current limit can be caused by a short in the wiring that delivers power or by the selection of too many channels to fire on channel switch 62. The fault LED 70 is pulsed to flash via line 88 when low batteries are detected on the control board 32. If the ready LED 66 is not lighted by the control board 32 via line 84 once the selected number of channels have been enabled, this indicates the batteries on the power boards are too low to fully charge the power stages required. The control board 32 performs built in diagnostic tests to determine the integrity of the input/output circuitry during factory service testing when the channels switch 62 is set to 0 and the power switch 54 is switched to ON. The eight power boards 36 are enabled by the control board 32 over a fiber optic interface 94. The number of channels selected by the operator on channels switch 62 determines the number of power boards 36 that are enabled by the control board 32. Each power board 36 is comprised of two nine volt batteries, two energy storage capacitors, an inverter, a MOSFET power transistor, fiber optic detectors, a high speed power diode, amplifiers, and power sense circuitry. Input power is provided by the'two 9 volt batteries. The energy storage capacitors are charged by the inverter to -425 Volts or 27 joules. A power board 36 requires 30 to 60 seconds to charge to full power after being enabled by the control board 32. A charge status is sent to control board 32 via a fiber optic interface 98 once the energy storage capacitors are fully charged. High speed fiber optic detectors sense the firing pulse from control board 32 emitted over a high speed fiber optic interface 96. The firing pulse is amplified and used to drive the gate or input of the MOSFET power transistor that switches the 400 Volt output of each power board 36 enabled. The power boards 36 are serially connected through the high speed power diode across the output of each board to provide the high voltage output desired. The nominal voltage output is 400 Volts per power board 36. The maximum output voltage is 3200 Volts with all eight power boards 36 enabled at a maximum current of 40 Amperes. The power is output from a rear panel power output 40. The energy stored in the energy storage capacitors may be depleted if a large number of pulses are fired in one ablation. A short delay may occur during charging of the energy storage capacitors. A charge status is then sent to the control board 32 and the ready LED 66 is lighted, indicating the high voltage DC pulse generator 28 is ready to fire. The over current limit detection board 34 senses the current output of the power boards 36 over line 92 using a very low equivalent resistance. The over current circuitry trips once the current reaches approximately 50 Amperes. The over current limit status is sent to control board 32 via a high speed fiber optic interface 90 where it is processed and the fault displayed on front panel 30 fault LED 70.

Another suitable generator of high voltage, low energy pulses is described for example, in British patent application GB2225534A which is incorporated herein by reference. Switch 24 enables the physician to switch from RF energy to high voltage electrical pulses as desired.

The ablation procedure of the present invention preferably follows a conventional mapping procedure of the ventricle. In a typical mapping procedure, an electrode catheter, preferably combining a multi-electrode mapping catheter is advanced into the femoral artery to the aorta, across the aortic valve and into the left ventricle. Alteratively, the mapping catheter could also be advanced to the right ventricle. To reach this position, the catheter has been inserted into a vein (usually a femoral, subclavian, or internal jugular vein), then advanced through the inferior vena cava or superior vena cava into the right atrial cavity and then through the tricuspid valve into the right ventricle. Under fluoroscopy, one or more electrodes are positioned against the endocardium and measurements of the electrical activity are recorded and analyzed. This is done to determine the site of the aberrant electrical pathway or pathways.

Without removal of the mapping catheter, the ablation catheter of the present invention is inserted into the ventricle along the same route as the mapping catheter. Insertion of the ablation catheter may be done simultaneously with or subsequent to the insertion of the mapping catheter. (It should be noted that quite often the mapping catheter and electrode catheter are combined.) Using the mapping catheter as a guide, an ablation electrode is positioned under fluoroscopy against the ventricle wall at the site of the aberrant electrical pathway. RF current is then applied to the site. The RF current delivered to the site is preferably in the frequency range of from about 300 to about 600 KHz. Delivery time is typically 10 to 30 seconds while keeping the electrode in a stabile position.

The application of RF energy confirms the diagnostic mapping of the site by temporarily interrupting or modifying the aberrant VT circuit. The RF energy also preheats the tissue resulting in lower electrical resistance. In a subsequent ablation by means of short high voltage electrical pulses, the heated tissue is thus more susceptible to destruction for a given pulse voltage. This allows the physician to minimize the voltage required to achieve a desired lesion size. Once the site is confirmed and preheated, the energy source is then switched, via switch 24, and short high voltage, low energy electrical pulses are delivered to the site to which the RF energy was delivered. The voltage used is up to about 3,000 V to about 3500 V which results in a current of about 30-50 amps or more. If voltages lower than 400 V are used, the treatment tends to be insufficient to form a lesion necessary to result in effective ablation.

High voltage, low energy electrical pulses of about 5 to 20 and preferably

10 microseconds ( /sec.) separated by intervals approximately 50 times the duration of the pulse are preferred in the practice of the present invention in order to achieve the desired tissue destruction without additional heating or arcing. Pulses which are shorter than about 5 μsec. are not preferred because of the benefits of the procedure tend not to be achieved. Pulses longer than about 20 /sec are not preferred because the titration of energy is more difficult. Intervals between the high voltage pulses shorter than about 4 times the duration of the pulse are not preferred because the heart tissue tends not to be able to differentiate between successive pulses and a single pulse and therefore results in heating. Intervals greater than 50 times the pulse duration are not believed to provide any additional benefit. As noted above, a pulse duration of about

10 / sec separated by about 500 / sec is presently preferred.

The number of pulses typically used is about 1 5 to 20 for effective ablation of the tissue required. Such treatments are sufficient to destroy the cell membrane and thereby kill the cells in contact with or in the near vicinity of the electrode. Cells can typically be destroyed to a depth of about 1 .5 cm. The killed cells are replaced with scar tissue thus preserving the integrity of the ventricle wall.

The combination of the RF and short high voltage pulses retains much of the safety advantages inherent to RF while effectively providing the larger lesion necessary to treat VT. By using the combination of RF with short, high voltage, low energy electrode pulses for treatment, the electrode does not become excessively hot during use or have to be overly large. This allows the use of a small electrode, keeping within the physical constraints of the catheter size. For effective treatment, only normal contact pressure by the electrode is required.

Excessive pressure, which is difficult to achieve and maintain is unnecessary in this procedure. Another important consideration is that the duration for which the electrode must be maintained in a stabile position against the heart wall for the increased lesion size is reduced to only a few seconds. This is extremely important considering that the treatment is being performed in a beating ventricle.

The preceding description has been presented with reference to a presently preferred embodiment of the invention shown in the drawings. It is apparent that alterations in the described structures and procedure durations can be practiced without meaningfully departing from the scope of this invention.

For example, as shown in FIG. 2, it is apparent that one could combine the mapping and ablation functions in a single multi-electrode catheter. In such an embodiment, the tip portion 14 carries a plurality of electrodes 32. All electrodes are connected by electrode leads (not shown) to switch 24 via, for example, a rotary plug connector at the proximal end of the control handle 16 as shown in

U.S. Patent No. 4,960,134. Switch 24, in turn, is connected to a conventional stimulator recorder 30. In addition, the tip or ablation electrode 20 is connected through switch 24 to an RF energy source 26 and a high voltage pulse generator 28. Mapping of the ventricle occurs first. Once the aberrant site is located, the switch is turned to supply RF current to the ablation electrode as described above. Once the site is confirmed and preheated, the switch is turned to supply high voltage electrical pulses to the site to thereby ablate the tissue. For the above reasons, the foregoing description should not be read as pertaining only to the precise structure, as described and shown in the accompanying drawings, but rather should be read consistent with and as support to the following claims which have their fullest and fair scope.

Claims

What is Claimed Is:

1 . A method for treating ventricular tachycardia in a patient comprising: inserting an ablation catheter having an ablation electrode into the ventricle of the patient's heart; contacting the ventricular wall of the heart with an ablation electrode at a site where an accessory electrical pathway is located; delivering radiofrequency to the ventricular wall through the electrode for a time sufficient to confirm the site of the accessory electrical pathway and to preheat the ventricular wall tissue; and then delivering short high voltage electrical pulses to the tissue through the same electrode to thereby form a non-conductive lesion at the site of the accessory electrical pathway.

2. A method as recited in claim 1 wherein the radiofrequency current is from about 300 KHz to about 600 KHz for a period of from about 10 to about 30 sec.

3. A method as recited in claim 1 wherein the voltage of the short high voltage electrical pulses is from about 3000 to about 3500 V.

4. A method as recited in claim 3 wherein the duration of the short high voltage electrical pulses is from about 5 to 20 / sec; and the interval between the short high voltage electrical pulses is from about 4 to about 50 times the duration of the short high voltage electrical pulses.

5. A method as recited in claim 4 wherein from about 15 to about 20 pulses are delivered.

6. A method as recited in claim 1 wherein the ablation catheter is also a mapping catheter comprising a plurality of electrodes and further comprises the step of contacting the ventricular wall of the heart with the electrodes and recording electrical impulses received by each electrode to determine sites where accessory electrical pathways are located prior to delivering radiofrequency to the ventricular wall.