RF/Microwave Delivery Systems for Therapeutic Applications - RF/Microwave Interaction with Biological Tissues - page 278

power delivery. This analysis permitted a comparison of the time course of
heating among the three catheters tested. Following power delivery there
was a 3-4-s delay in temperature rise for all catheters tested. Peak tempera-
ture was achieved several seconds after power was turned off. The effect was
delayed at depths of 2.5 and 5 mm. A greater depth of heating was observed
with both the 8-mm RF and the microwave catheter. Following delivery of
power, the surface cooled more rapidly than the deeper levels studied due to
the flow of 37°C saline across the phantom surface.
In RF ablation, assuming that the current flow into the tissue is omnidi-
rectional, the average decay rate of the power deposited is Pr ~ 1/r 4 , where r is
the distance from the electrode. In the case of microwaves, the power density
decays as Pr ~ exp( - 2a r )/ r 2 , where a is the tissue attenuation constant [93].

6.5.4 Effects of Flow on Surface Temperature

The effects of cardiac blood flow on surface temperature during cardiac abla-
tion were evaluated by comparing a static and a flow phantom model. Static
measurements were performed using the previously described phantom model
but without perfusion of saline across the surface of the phantom. These values
were compared to those obtained with saline perfusion of 4 L min -1 (Fig. 6.15 c ).
Perfusion of saline caused a marked reduction in surface temperature for both
RF and microwave ablation catheters. In the static model, the peak surface
temperature measured 30 s after delivery of 30 W of power was 68°C for the
4-mm RF catheter and 61°C for the microwave catheter. Flow caused a reduc-
tion in peak surface temperature to 57°C with the RF catheter and 46°C with
the microwave catheter. In addition, the rate of the rise of surface tempera-
ture was attenuated by flow. These data suggest that static phantom models
will overestimate lesion size, since lesion size is proportional to surface tem-
perature during ablation.

6.5.5 Lesion Volume

Lesion volume was plotted as a function of power for both the RF and
microwave catheters. Lesion volumes were obtained from in vivo ablation
of canine left ventricular myocardium as well as from measurements taken
during phantom experiments. There was a direct relationship between lesion
volume and delivered power for each electrode tested. Power delivery, and
hence lesion size, was limited in the RF catheters by the development of a rise
in impedance seen at higher power. A higher power was required to optimize
lesion size with the 8-mm versus the 4-mm RF electrode (80 W versus 40 W).
The maximal lesion volume with an 8-mm RF electrode was approximately
twice that achieved with a 4-mm electrode (914 ± 362 mm versus 446 ±

150 mm 3 , p 0.01).

Lesion size was also calculated following the delivery of microwave energy.
Microwave energy in vivo was then compared to estimated lesion size in

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