Surgery – Diagnostic testing – Structure of body-contacting electrode or electrode inserted...
Reexamination Certificate
1997-07-03
2001-06-05
Dawson, Glenn K. (Department: 3761)
Surgery
Diagnostic testing
Structure of body-contacting electrode or electrode inserted...
C606S032000, C606S041000, C607S101000
Reexamination Certificate
active
06241666
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to ablation catheter tip assemblies. More specifically, the present invention relates to RF ablative tip catheters wherein an RF electrode is covered with a buffer layer covering the metal electrode.
DESCRIPTION OF THE RELATED ART
Supraventricular tachycardia, ventricular tachycardia, and atrial fibrillation are conditions in the heart generally known as arrhythmias. During an arrhythmia, abnormal electrical signals are generated in the endocardial tissue which cause irregular beating of the heart. One method used to treat these arrhythmias involves creating lesions within the chambers of the heart on the endocardium. These lesions are intended to stop the irregular beating of the heart by creating barriers between regions of the tissue. These barriers halt the passage through the heart of the abnormal currents generated in the endocardium.
RF energy can be used to ablate tissue, such as tissue in the heart, to form the appropriate lesion barriers to stop the flow of abnormal currents. One conventional apparatus for performing RF ablation is an RF ablation catheter with an ablative catheter tip.
An internal electrode (i.e., the ablative tip of an RF ablation catheter) is placed inside the body, adjacent to tissue which is to be ablated. An external electrode is placed on the skin surface. A power supply generates electrical power (generally radio frequency current) which is communicated between the internal electrode and the external electrode so that the RF energy ablates tissue in the vicinity of the internal electrode.
FIGS. 1 and 2
show an RF ablation catheter with an ablative tip. Note that a similar, RF ablation catheter is described in U.S. Pat. No. 5,348,554 to Imran et al., which is incorporated by reference. In the catheter shown in
FIG. 1
, an RF power supply and controller
76
supplies high frequency energy to the hollow conductive electrode tip
16
of the catheter through a lead wire
31
, in order to ablate tissue and/or cause lesions. The hollow tip conducting electrode
16
can be formed of a suitable material such as stainless steel or platinum. The lead wire
31
runs through a central lumen
28
and is physically connected to the conductive tip
16
at juncture
32
.
The catheter shown in
FIGS. 1 and 2
also includes a flexible elongate member
12
, formed of a suitable plastic, such as polyurethane. Disposed on the elongate member
12
, near its tip, is a braid
21
, which provides kink-resistance and reinforcement. The elongate member
12
may additionally include other conventional elements such as steering wires (not shown).
One problem with RF ablation catheters is clotting. Clotting has been observed to be correlated with denaturizing of the blood with high temperature and/or high current density areas in the vicinity of the catheter tip. Clots of solid organic matter are produced. These clots (or coagulum) are undesirable because they may travel in the blood stream and cause an embolic event.
Another problem in cardiac ablation is the phenomenon of “popping”. Popping refers to explosions in the body's tissue which are observed during RF ablation. Popping is generally undesirable because it causes irregularities, such as tears, in the ablated tissue, and therefore it becomes more difficult to precisely control which tissue is ablated. Also, the force of the popping explosions can cause and disperse coagulum. The exact mechanism by which popping occurs has not been fully explained.
In order to reduce the high temperatures at the catheter tip, the catheter in
FIG. 1
has a saline circulating system for cooling, which is further discussed below.
The saline circulating system is made up of pumping assembly
10
, lumens
26
,
27
,
28
and cavity
17
. The saline circulating system circulates a saline solution through lumen
26
into the cavity
17
which is inside the conductive tip electrode
16
. More specifically, the pumping assembly pumps saline solution into lumen
27
. The saline solution then flows into cavity
17
, where it cools the tip
16
. The saline solution then flows back out of the catheter through lumen
26
, and back to the pumping assembly
10
. The pressure and temperature of the saline solution can be controlled by means which are more fully discussed in U.S. Pat. No. 5,348,554.
While the conventional catheter shown in
FIG. 1
has been found to be reasonably effective in preventing hot spots on the catheter tip and resultant overheating of the tissue, popping and clotting can still occur at higher power levels. For example, there is still a high current density at the edge
16
a
of the conductive tip, which can potentially cause overheating of the tissue.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an ablative catheter assembly which exhibits reduced popping and clotting. It is now believed that popping is caused by ignition of a combination of by-products of the tissue heating and by-products of electrolysis that occurs due to transferring current between the metal tip and electrolytes in blood and tissue. This ignition occurs at areas of high current density near the metal tip where electrolysis occurs. As explained in detail below, especially in view of the foregoing theory of popping, various types of buffer layers on an ablation catheter tip can help to reduce popping by keeping the by-products of the tissue heating away from high current density areas on the metal electrode.
According to some embodiments of the present invention, the buffer layer is a layer of conductive fluid, such as saline solution, which surrounds the metal electrode. The conductive fluid will communicate RF energy from the electrode to the metal catheter tip, while providing a more uniform current density at the exterior surface of the metal catheter tip. This means that combustible products of the ablation reaction which happen to reach the outer surface of the catheter tip will not come into contact with metal at an area of high current density, and will therefore prevent ignition, combustion and popping.
In the fluid buffer layer embodiments, the catheter assembly includes an elongate catheter shaft, a hollow metal tip, a lead wire, and an electrode. The tip covers the distal end of the catheter shaft. A lumen is provided to deliver conductive fluid to the tip. The lead wire is disposed within the catheter shaft. The electrode is connected at an end of the lead wire and is disposed within the hollow tip, spaced away from the inner surface thereof. Because the electrode is spaced away from the surface of the tip (preferably by at least one millimeter) a buffer layer of conductive fluid forms around the electrode.
In some of embodiments which utilize the conductive fluid for a buffer layer, the tip can be formed from a solid material. Preferably, the conductive fluid is circulated from the proximal end of the catheter into the tip and then back out the proximal end of the catheter. In the alternative, the tip can be provided with flushing holes which allow the passage of conductive fluid through the tip and out into the blood/tissue interface. In the embodiments wherein the tip includes flushing holes, the tip can be formed from either a conductive or a nonconductive material. It should be noted that in the fluid buffer layer embodiments, the conductive fluid acts not only as a buffer layer but as a means for cooling the tip.
According to other embodiments of the present invention, a metal catheter tip is coated with a non-conductive, porous, buffer layer. The buffer layer may be made of various materials, such as cellulose, polymeric materials, adhesives, or metallic salts. The buffer layer is porous so that RF energy may be efficiently transferred from the metal catheter tip to the surrounding blood and tissue, while still preventing combustible products of the ablation reaction from reaching high current density areas on the metal tip itself, thereby preventing ignition, combustion and popping.
The embodiments which include a porous buffer layer also i
Pomeranz Mark L.
Riley Richard E.
Willis N. Parker
Cardiac Pathways Corp.
Dawson Glenn K.
Stallman & Pollock LLP
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