RF/Microwave Delivery Systems for Therapeutic Applications - RF/Microwave Interaction with Biological Tissues - page 264

Microwave endovascular occlusion utilizes a narrow balloon catheter
similar to the one used for MBA [72-89].

6.4.3 Cardiac Ablation

In the discussion of RF/microwave catheter therapeutic modalitites, we have
chosen to concentrate on cardiac ablation. To date, RF cardiac ablation is the
“gold standard” in the treatment of a large number of supraventricular tachy-
cardias. We have chosen to detail its success and include the potential for
success in utilizing microwave cardiac ablation in new ways.
Radio-frequency ablation and to a lesser degree the microwave ablation
technique (Fig. 6.8 c ) have become the treatments of choice in many types of
cardiac arrhythmias. The actual instrumentation used as well as the catheter
based techniques are similar to those of all other ablation procedures, as men-
tioned in this topic and in the literature. From the detailed cardiac ablation
techniques described in this topic, the reader will gain a good understanding
of the ablation techniques used in the treatment of other organs.
The heart is composed of three types of cardiac tissue; atrial muscle, ven-
tricular muscle, and specialized excitatory and conduction tissues. The atrial
and ventricular muscles of the heart are normally excited in synchrony. Each
cardiac cycle begins with the generation of action potentials by the sinoatrial
(SA) or sinoauricular node located in the posterior wall of the right atrium.
These action potentials spread through the atrial muscle by means of special-
ized conduction tissues. The action potentials do not normally spread directly
from the atrial to the ventricular chambers. Instead, the action potentials con-
ducted in the atrial musculature reach the atrioventricular (AV) node and its
associated fibers, which receive and delay the impulses. Potentials from the AV
node are conducted to the His-Purkinje bundle. This structure carries the
impulses to the ventricular musculature to cause the synchronous contraction
of the ventricular muscles.
The term paroxysmal tachycardia refers to abnormal episodes involving a
sudden increase in heart rate. Such tachycardia can result from an irritable
focus in the atrium, the AV node, the bundle of His, or the ventricles. These
episodes of tachycardia either may be initiated and sustained by a reentrant
mechanism, termed a “circus” movement, or may be caused by repetitive firing
of an isolated ectopic focus. While these episodes of tachycardia are usually
amenable to treatment by medication, under certain circumstances surgical
ablation of the abnormal focus or abnormally conducting tissue may be
necessary.
The utilization of cardiac-based close chest treatment of cardiac arrhyth-
mia started in the early 1980s. Direct current is delivered to the tip of an elec-
trode with the goal of creating a localized injury to remove the cause of the
specific arrhythmia. The delivery of 300-400 J from a standard cardiac defib-
rillator through a catheter resulted in 3000 V in the vicinity of the site to be
ablated. The believed mechanisms of myocardial damage due to very high