The invention relates generally to an electrophysiological (xe2x80x9cEPxe2x80x9d) apparatus and method for providing energy to biological tissue, and more particularly, to an ablation apparatus providing for more efficient cooling of the electrodes used to apply energy to the biological tissue. 
The heart beat in a healthy human is controlled by the sinoatrial node (xe2x80x9cS-A nodexe2x80x9d) 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 (xe2x80x9cA-V nodexe2x80x9d) 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 xe2x80x9ccardiac arrhythmia.xe2x80x9d
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 (xe2x80x9cAFxe2x80x9d), a procedure published by Cox et al.  and known as the xe2x80x9cMaze procedurexe2x80x9d 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 48xc2x0 Centigrade.  Tissue heated to a temperature above 48xc2x0 C. becomes non-viable and defines the ablation volume.  The objective is to elevate the tissue temperature, which is generally at 37xc2x0 C., fairly uniformly to an ablation temperature above 48xc2x0 C., while keeping both the temperature at the tissue surface and the temperature of the electrode below 100xc2x0 C.
A basic configuration of an ablation catheter for applying RF energy includes a distal tip which is fitted with an electrode device.  The electrode device is the source of an electrical signal that causes heating of the contacting and neighboring tissue.  In the unipolar method, the electrode device may include a single electrode used for emitting RF energy.  This single electrode acts as one electrical pole.  The other electrical pole is formed by the backplate in contact with a patient""s external body part.  A RF source is applied to the electrode.  The RF source is typically in the 500 kHz region and produces a sinusoidal voltage.  When this is delivered between the distal tip of a standard electrode catheter and a backplate, it produces a localized RF heating effect and produces a well defined, deep acute lesion slightly larger than the tip electrode. 
In some procedures a lesion having a larger surface area than that produced by a single electrode in a unipolar arrangement may be required.  To this end numerous ablation catheters have been designed.  In one catheter designed to provide a larger surface ablation area, an electrode device having four peripheral electrodes which extend from a retracted mode is used.  See U.S. Pat.  No.  5,500,011 to Desai.  When extended, i. e., fanned out, the four peripheral electrodes and the central electrode form an electrode array that covers a larger surface area of the tissue than a single electrode.  When used with a conventional RF power source, and in conjunction with a backplate, the five electrodes produce five lesion spots distributed over the area spanned by the electrode array.  The lesions produced are discontinuous in relation to each other and there are areas between the electrodes that remain unablated.  This device must be manipulated so that when expanded, all electrodes are in contact with the endocardium.  An xe2x80x9cend onxe2x80x9d approach is required such that the end of the catheter, on which all five electrodes are mounted, is in intimate contact with the target tissue. 
In another catheter an electrode device having a central electrode and a number of peripheral electrodes which also fan out from a retracted mode is used.  During ablation a backplate is not used; instead the central electrode functions as the reference while the peripheral electrodes have multi-phase RF power applied to them.  For example, see U.S. Pat.  No.  5,383,917 to Desai et al.  While this technique provides a more continuous lesion covering a larger surface area of the tissue, the ablation volume is relatively shallow with a nonuniform depth of the lesion.  This arrangement also requires the same manipulation of the catheter such that an end-on contact is made by the expanded electrodes, as discussed above.  Lesions having a non-uniform ablation volume are undesirable as the depth at one part of the lesion may not be sufficient to stop the irregular signal pathways.  Arrhythmia may reoccur because the irregular signals may pass under such an ablation volume and the procedure must then be repeated to once again attempt to obtain an ablation volume having sufficient depth. 
The mechanical configuration of both of the above-described techniques comprises an expanding approach.  When used for ablation, an electrode device is typically part of a catheter system.  Accordingly, it is desirable to minimize the diameter of the electrode device during introduction to and withdrawal from the patient to lessen trauma to the patient.  Therefore, electrode devices having peripheral expandable electrodes must be configured so that the peripheral electrodes are expandable to a large size yet are retractable to as small a size as practical.  Such requirements pose design and manufacturing difficulties due to the movement of mechanical parts required for proper operation.  Further considerations are the undesirable complexity and increased manufacturing cost associated with an expandable a catheter. 
In applying power to the target tissue, it is desirable that part of the electrodes in a bipolar catheter approach are exposed to the fluids at the site.  These fluids typically provide some cooling effect to those electrodes.  If all electrodes are the same size and the cooling is the same across the entire lesion site and the impedance is the same at all electrode contacts, the same voltage can be delivered at all electrodes.  As power is the voltage squared divided by the impedance, the same power will be delivered to all of the separate electrodes.  However, this is rarely the case.  Typically, all of the above items are variables.  If the electrode is larger, it has lower resistance and can handle larger currents (thus larger power) without causing tissue clotting at the interface or coagulation.  Even if all of the electrodes are the same size, the local cooling of each electrode is typically different and thus may require different power to be delivered to each electrode. 
Should the cooling by the fluids not be sufficient to maintain the tissue interface below the coagulation and boiling temperature at a particular electrode, power to the electrode must be reduced.  A method of reducing power is reducing the duty cycle of the power signal provided to an electrode.  The typical form of a power signal is one having alternating instances of peak power i. e., an xe2x80x9conxe2x80x9d period, and very low power, i. e., an xe2x80x9coffxe2x80x9d period.  The duty cycle is the ratio of the length of the on period to the total time frame (i.e., the combination of the on period and the off-period).  Ideally, during the off-period of the particular electrode, electrical current neither flows to nor from the electrode and heat from the electrode is allowed to dissipate into the surrounding tissue and fluids contacting the electrode. 
In practice, however, when the duty cycles of individual electrodes are separately adjustable, there may be instances during which the duty cycles of adjacent electrodes are different.  For example, if the duty cycle of a first electrode is fifty-percent while the duty cycle of an adjacent second electrode is twenty-five percent, there will be instances during each time frame when the second electrode is off but the first electrode is on.  Should both electrodes remain in position at the tissue and therefore have a salt bridge between them that conducts electrical energy, that second electrode may present a lower potential level to the first electrode and current may flow from the first to the second electrode.  The second electrode would therefore not actually be in an off state and would still participate in generating heat at the interface.  Thus the intended effect of turning off the second electrode to provide the electrode with an opportunity to dissipate heat is at least partially negated by the continued current flow to the electrode from those electrodes with longer duty cycles.  Yet the longer duty cycle in an adjacent electrode may be advantageous so as to continue the tissue ablation process at that electrode""s location. 
Hence, those skilled in the art have recognized a need for a structurally stable invasive ablation apparatus and method that are 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.  A need has also been recognized for an ablation apparatus and method that are capable of controlling individual electrodes in a multiple electrode array so that current flow is limited as desired.  The invention fulfills these needs and others. 
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 during ablation to thereby control the surface area, the continuity, and the depth of lesion produced during ablation. 
In a first aspect, an apparatus for delivering energy to heart tissue is provided with the apparatus comprising a catheter having a plurality of electrodes at its distal end, the distal end positionable so that the electrodes are located at the heart tissue, and a power control system adapted to provide power to the electrodes such that power to a first electrode may be turned off while power to a second electrode may be turned on wherein the power control system provides a high impedance to the first electrode when it is turned off so that substantially no current flows to the first electrode from the second electrode when the second electrode is on. 
In more detailed aspects, the power control system is adapted to vary the duty cycles of the power provided to the first and second electrodes.  The power control system provides power to the first electrode with a different phase angle from the power provided to the second electrode.  In further detail, the different phase angle is greater than zero degrees but less than 180 degrees.  In yet further detail, the phase angle differs by approximately 132 degrees. 
In another aspect, at least three electrodes arranged in a linear array with the power provided to the center electrode having a different phase angle than at least one adjacent electrode.  the power control system provides separate power to each of the plurality of electrodes with the phase angle of each being individually controllable. 
In yet another aspect, a temperature sensing device located at the electrodes is adapted to provide a temperature signal to the power control system representative of the temperature at the electrodes wherein the power control system controls the duty cycle of the power in response to a temperature signal.  In a further aspect, a measurement device senses at least one characteristic of the power applied to at least one electrode and is adapted to provide a power measurement signal wherein the power control system receives the power measurement signal, 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.  In a more detailed aspect, the power control system controls the duty cycle of the power in response to the temperature signal and in response to the power measurement signal. 
In yet another aspect, the plurality of electrodes are formed into a first electrode group and a second electrode group with at least one electrode in each group wherein all electrodes in the first group are provided with first power by the power control system and all electrodes in the second group are provided with second power by the power control system with the first power establishing a first potential at each of the electrodes in the first electrode group and the second power signal establishing a second potential at each of the electrodes in the second electrode group, with each of the first and second potentials being different from each other and from a potential at a backplate.  In further details, the first power has a different phase angle from the second power, the electrodes of the first group of electrodes are interspaced between the electrodes of the second group of electrodes such that each electrode from the first group is adjacent at least one electrode from the second group.  The electrodes are arranged into a linear array at the distal end of the catheter. 
In yet another aspect, a power interruption device is connected between the power control system and an electrode wherein the power control system is adapted to control the power interruption device to interrupt power to the selected electrode. 
In a further additional aspect, a backplate is positionable proximal the biological site so that the biological site is interposed between the electrodes and the backplate.