ing pattern shown in FIG 1. The temperature is highest to cable 26. If axial flow is desired the layer is insulted
directly at the surface of the active electrode (distan- from the cable 26 and electrically joined to its neigh
ce=0). In operation, power is typically increased in bors.
order to increase the ablation volume until impedance The flow of heat from the electrode 20 is aided by the change in noticed due to onset of charring. Since the 5 large cross section of cable 26. The heat flow path in the tissue temperature is highest at the active electrode cable heat sink implementation, shown in FIG. 3, is surface the charring is most likely to take place there. completed by modification of the catheter tube 24 to Charring frequently necessitates the removal of the increase thermal conductivity from cable 26 to the outcatheter for cleaning. side tissue. The heat conductivity of a plastic elastomer
The objective of the thermal design of a heating cath- 10 material for tube 24 is reduced by embedding heat con
eter is to heat a controlled volume of tissue to a temper- ductive particles in the material. The region between
ature which causes ablation while at the same time the cable 26 and the tube 24 is filled with heat conduc
assuring that the peak temperature is away from the tive paste 27. The technology of improved heat conduc
electrode surface so that charring does not foul the tivity plastics, and the technology of heat conductive
active electrode surface. Graph (B) in FIG. 2 shows 15 pastes are well established in conjunction with heat sink
such a temperature profile. Graph (B) is described later techniques for solid state devices. In the cable heat sink
in conjunction with a heat sink catheter design shown in implementation above, heat dissipated in the tissue,
FIG. 3 and FIG. 4. heats active electrode 20. Active electrode 20 in turn, is
FIG. 3 shows a catheter with improved electrical and cooled by heat outflow along cable 26, through the thermal design. Axial blind hole 21, in active electrode 20 conductive paste 27 and a wall of catheter tube 24 to the 20, houses a metallic cable 26. Comparing FIG. 3 with blood and tissue surrounding tube 24. the state-of-the-art catheter in FIG. 1, cable 26 provides FIG. 4 shows an alternative heat sink design. The an electrical connection to the active electrode 20, as mounting of cable 26 and tube 24 to active electrode 28 did wire 12 in FIG. 1. Unlike the wire 12, the cross and the function of conductive skirt 23 is substantially section of cable 26 is much greater, and is typically at 25 the same as described in conjunction with FIG. 3. Acleast 20% of the cross-section of active electrode 20. tive electrode 28 in FIG. 4, preferably made from silver, Flexibility of cable 26 is maintained by stranded or which is the best heat conductor, has a different shape laminated construction from multiple metallic conduc- from active electrode 20 in FIG. 3: Active electrode 28 tors. Cable 26 provides a much greater heat conduction is longer and is shaped to seat a cylindrical film heat sink away from active electrode 20 and into catheter tube 24, 30 29. The heat sink film 29 is electrically insulating and thereby reducing a temperature rise of active electrode thermally conductive.
20 during operation. Cable 26 also provides a range of The distal end of active electrode 28 provides a bare
possibilities for movable support of active electrode 20. metal interface to tissue, generating a heating pattern
Catheter tube 24 is firmly seated on a undercut protrud- just as active electrode 20 in FIG. 3. When compared
ing proximal end 25 of active electrode 20. 35 with FIG. 3, the interface between cylindrical film heat
Active electrode 20 is tapered at its base 22 with a sink 29 and the external blood flow provides an added
tapered angle of 10 degrees. Conductive epoxy fills this cooling element. The amount of heating and cooling is
tapered region and forms a conductive skirt 23. The independently controlled by the ratio of the electrically
contours of equal heating power density, are shown in interacting bare electrode area to the heat sink area.
FIG. 3 for conductive epoxy with resistivity of 150 40 The overall effectiveness of the heat sink is deter
fl-cm. The power density percentages, are scaled the mined by the thermal conductivity of film 29 and by the
same way as in FIG. 1. It can be seen that the uniformity heat transfer coefficient. The heat transfer coefficient
of heating density at the junction of active electrode 20 associated with the thermal boundary layer in forced
and tube 24 is much improved when compared with the convection of heat between the catheter surface and the
state-of-the-art catheter in FIG. 1 due to a graduated 45 adjacent blood flow, is determined by thermal and hy
impedance, presented to the surface current flow, pro- drodynamic properties of blood. As long as the thermal
vided by the wedge-shaped cross section of conductive conductivity of film 29 is significantly smaller than the
skirt 23. heat transfer coefficient of the heat convection of the
Such a gradual transition between metallic and insu- blood flow, the heat sink is close to optimum design, lating surface properties for heating equalization can be 50 Implementation of heat sink film 29 by a 0.025 mm accomplished by alternate means to those described plastic tube meets this requirement, above. In one example, conductive skirt 23 is made of The design in FIG. 4 provides very effective forced uniform thickness but of graduated electrical properties. convective cooling by the flow of blood, while at the In another example the transition is implemented by same time, allows full control over the size of the area graduated surface capacitance, rather than graduated 55 which generates the electrical current flow. It will be surface resistance above. A skirt in the form of a tapered noted that the active electrode in FIG. 4 can also corndeposit of metal oxide on electrode 20 can accomplish prise the impedance skirt 23 which prevents the formasuch graduated capacitive implementation, e.g., tion of a hot spot at the juncture where active electrode through the formation of a tantalum oxide film, dis- 28 and electrically insulating film heat sink 29 meet. The cussed in some detail later. 60 capacitive impedance skirt implementation can be im
The impedance graduation need not be accomplished plemented using the same material as heat sink film 29.
by a surface layer but can if fact extend into the body of Cable 26, attached to active electrode 28 provides addi
the electrode: In yet another implementation, the active tional cooling of active electrode 27 by allowing the
electrode is built from axially layered regions of differ- heat flow into the catheter tube, as previously discussed
ent electrical properties. The direction of current flow 65 in conjunction with FIG. 3.
can be selectively controlled in individual layers. If An attractive heat sink/impedance skirt implementa
radial flow is desired the layer is separated from its tion involves a tantalum tube 30 (shown dashed in FIG.
neighbors by an insulator and is connected in the center 4) which is pressed onto active electrode 28 and so