Passively self-cooled electrode design for ablation catheters

Details Passively self-cooled electrode design for ablation catheters Passively self-cooled electrode design for ablation catheters

2001-03-29

2003-05-27

Gibson, Roy D. (Department: 3739)

Surgery

Instruments

Electrical application

C606S032000, C606S048000

Reexamination Certificate

active

06569162

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to ablation catheters and, more particularly, to a self-cooled electrode design for use with ablation catheters that results in more controlled heating and ablation of tissue.
BACKGROUND OF THE INVENTION
The pumping action of the heart is controlled in an orderly manner by electrical stimulation of myocardial tissue. Stimulation of this tissue in the various regions of the heart is controlled by a series of conduction pathways contained within the myocardial tissue. At the completion of ventricular stimulation, heart tissue rests to allow the cells to recover for the next stimulation. The stimulation is at the cellular level, and is a changing of the polarity of the cells from positive to negative.
Cardiac arrhythmias arise when the pattern of the heartbeat is changed by abnormal impulse initiation or conduction in the myocardial tissue. In general, there are at least two conditions under which the firing of the myocardial cells may become irregular. The first condition is an accessory pathway, which often exists at birth, and may be caused by malformation of the heart. The second condition is usually caused by scare tissue of myocardium that originated from infarction or surgical incisions. Frequently the arrhythmias are due to an arryhthmogenic site which, resulting from one of the above mentioned conditions, does not respond to treatments through the use of anti-arrhythmic drugs. These sites can dominate the normal conduction pathways in the heart resulting in abnormally rapid rhythm or tachycardia which require an interventional remedy.
Currently there are a number of medical and surgical treatments for cardiac arrhythmias. Radio frequency catheter ablation is an interventional technique in which radio frequency energy is delivered, via a catheter, to an electrode in contact with the myocardial tissue. Radio frequency catheter ablation has become a principal form of therapy for paroxysmal supraventricular tachycardia, that is unknown cause of rapid heart rate. Radio frequency catheter ablation is also being used in an increasing number of patients for treatment of ventricular tachycardia (rapid ventricular rate) associated with coronary artery disease and other forms of heart disease. Ventricular tachycardia (VT) is a type of arrhythmia with high morbidity and mortality. RF energy delivery through a standard 7 French, 4 mm distal electrode has been highly successful for ablation of arrythmogenic tissue, i.e. tissue which spontaneously repolarizes, located within a few millimeters of ablation electrodes, such as in patients with accessory atrioventricular (AV) pathways (Wolff-Parkinson-White syndrome) and AV nodal reentrant tachycardia (AVNRT). However, in approximately 1 percent of patients with accessory pathway and in approximately 30 to 50 percent of patients with VT associated with a healed myocardial infraction, the arrhythmogenic tissue, usually located at the epicardial border zone, can be difficult to destroy with a conventional ablation electrode. In order to eliminate the VT foci, deep, transmural lesions, e.g. completely through the myocardium, are necessary for ablation in these patients.
In radio frequency catheter ablation, radio frequency energy causes tissue heating, which, in turn, causes formation of a lesion. With radio frequency ablation, a catheter with a conductive inner core and a metallic tip are placed in contact with the myocardium and a circuit is completed with a patch placed on the patient's body behind the heart. The catheter is coupled to a radiofrequency generator such that application of electrical energy creates localized heating in the biological material and fluids adjacent to the distal emitting electrode. Because of the nature of radiofrequency energy, both the metallic tip and the tissue are heated simultaneously. The peak tissue temperatures during catheter delivered application of RF energy to myocardium occur close to the endocardial surface, such that the lesion size produced is approximately limited by the thermodynamics of radial heat spread from the tip. The amount of heating which occurs is dependent on the amount of contact between the electrode and the tissue and the impedance between the electrode and the exterior surface of the tissue. The higher the impedance, the lower the amount of energy transferred into the tissue.
For any given electrode size and tissue contact area, RF-induced lesion size is a function of RF power level and exposure time, i.e. duration of energy delivery. It has been shown that tissue temperature must exceed 45-50° C. for lethal tissue damage and lesion formation. However, at higher power levels, the application time is frequently limited by a rise in the electrical impedance of the electrode (reducing the application of further heating energy) which can be prevented by maintaining the electrode-tissue interface temperature to less than 100° C. During RF ablation, the tissue heating is related to the difference between the resistive heating and the heat precipitated into the surrounding tissue and blood flow. The ability to destroy tissue is a function of the thermal dose above a threshold value. The thermal dose is approximately equal to the product of temperature and time. In RF tissue ablation, the heat generated at the electrode is based on at least two factors: RF current density and the duration of the RF energy application. The RF current density is approximately equal to the RF current delivered divided by the tissue contact area. The heat dissipated in a local tissue area varies as 1/D
4
(D sup 4), where D is the distance from the ablation electrode and decreases rapidly as the distance from the ablating electrode increases. The heating is greatest at the surface interface with the electrode, and, if excessive RF power is applied, coagulation, tissue charring and/or rapid impedance rise may occur. For a conventional catheter electrode the limiting factors in creating deeper lesions are the potential risk of coagulation on the electrode, tissue charring and/or rapid impedance rise all as a result of higher electrode-tissue interface temperature when higher power is applied.
Attempts have been made to actively cool the electrode by irrigating or infusing a cooling fluid, such as saline, through the interior lumen of the catheter shaft to an electrode constructed with a degree of porosity to allow the fluid to pass through the electrode and thus cool it. While such cooling may be adequate, temperature control, which is frequently employed, is unusable due to the inhibition of temperature rise of the electrode due to the cooling fluid. As a result excessive energy is often delivered allowing the subsurface temperature to exceed 100° C. which causes steam generation and sub-surface explosions or pops within the myocardial tissue. Moreover, such irrigation procedure can add unnecessary fluid to the patient's circulation that can be potentially hazardous. Additionally, such active cooling mechanisms complicate catheter construction and limit the number of electrodes that may be used. Examples of ablation catheters with electrodes that are actively irrigated during the ablation process are disclosed in U.S. Pat. Nos. 5,545,161; 5,462,521; 5,437,662; 5,423,811; 5,348,554 and 5,334,193.
Accordingly a need exists for a simple methodology for permitting greater energy to be delivered to an ablation electrode with minimal risk of overheating the electrode-tissue interface.
A need further exists for a novel catheter electrode design which provides more profound convective electrode cooling, i.e. transferring heat to a surrounding medium, than that of conventional electrode designs.
A further need exists for a catheters employing an electrode design that will allow higher RF power application with longer application duration, resulting in deeper intra-tissue heating and deeper lesions while allowing simpler catheter construction.
A further need exists for an RF ablation catheter in which single or multi-electrode configurations permit the

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