RF/Microwave Delivery Systems for Therapeutic Applications - RF/Microwave Interaction with Biological Tissues - page 279

the phantom model (Fig. 6.15 d ). There was a good correlation between the
phantom and animal data obtained from the 4-mm RF catheter. For the 8-mm
catheter, the phantom model underestimated lesion size when power was
increased beyond 20 W. However, the phantom model accurately predicted
that it would take approximately twice the delivered power for an 8-mm
catheter to create a lesion similar in size to a 4-mm catheter.
The flow phantom model provides a simple in vitro method for analyzing
cardiac ablation catheters. Heating of cardiac tissue to a level of cell death is
the principal mechanism of successful cardiac ablation. Therefore, under-
standing the temperature profile of a particular catheter design is important
in order to assess its effects on lesion size. The advantages of this phantom
model are (1) the ability to measure the temperature at any point along the
catheter shaft and (2) the control of chamber flow and thus its effects on
surface cooling. This flow phantom model is superior to previously described
static phantom models which do not take into account the surface-cooling
effects of cardiac blood flow.

6.5.6 Limitations

The lesion size derived from the phantom model was only an estimate of the
lesion size in vivo. The measurement in the phantom assumed that the
myocardium was flat. In addition, data points were only obtained at 2.5 and
5 mm below the surface of the phantom. More closely spaced thermometry
probes would likely have yielded more accurate information. However, closer
spacing of the thermometry probes is not feasible in this model. Furthermore,
it was assumed that a temperature of 47°C resulted in irreversible cell death.
This assumption also influenced the results. Nonetheless, this system accurately
reflects trends in temperature profiles that result from changes in catheter
design or type.
Recently, in the abstract entitled “A successful option for chronic atrial fib-
rillation” [94, 95], the authors reported that their preliminary clinical study
showed that microwave ablation represents an effective and safe option to treat
chronic AF on patients with mitral valve disease and coronary artery disease.
The authors predicted that microwave ablation would create larger and deeper
lesions and thus would be effective in patients with deeper arrhythmogenic
foci, such as those present in ventricular tachycardia. In addition, microwave
energy can be delivered through tissue coagulation, which is not the case when
the RF modality is used. Microwave heating is developed by the oscillation of
molecular dipoles in tissue by EM fields at microwave frequency. Microwave
ablation does not depend on good antenna tissue contact, whereas in the RF
ablation technique, the RF electrode-tissue interface is of prime importance.
Although the technique and general instrumentation of RF/microwave
ablation were discussed in detail in Section 6.4.3, the authors have elected to
discuss two additional areas of importance: GERD [96-103] and endometrial
ablation [104]. These two areas are consistently gaining approval and