RF/Microwave Delivery Systems for Therapeutic Applications - RF/Microwave Interaction with Biological Tissues - page 265

voltage are as follows: Electrolysis of water into hydrogen and oxygen gas
increases resistance, resulting in arcing. Once arcing develops, there is a
tremendous rise in temperature, which results in the creation of a shock wave.
Several investigators have modified the catheter/probe and the DC energy
source to minimize the uncontrolled size of the damaged tissue. Improvements
in the control of energy delivery increase the safety and efficiency of the tech-
niques. To date, the RF ablation technique is the standard in treating many of
the supraventricular arrhythmias. It is estimated that, internationally, tens of
millions of patients experience supraventricular tachycardia (SVT). Many
patients do not have symptoms or have very mild symptoms which are well
tolerated. On the other hand, it is worthwhile mentioning that SVT can lead
to syncope and even sudden death. The three main mechanisms responsible
for SVT are (1) atrial fibrillation (AF), (2) atrioventricular nodule reentrant
tachycardia (AVNRT), and (3) accessory-pathway-mediated tachycardia
(APMT). In AF, the electrical activity of the upper chambers of the heart
becomes disorganized, causing a rapid electrical signal to impinge upon the
AV node, which in turn creates a rapid ventricular response (Fig. 6.9 a ).
Untreated patients with SVT may suffer a minor heart attack and/or conges-
tive heart failure, even a stroke.
The most common form of SVT is AVNRT. In AVNRT there are two lon-
gitudinal disassociated pathways within the AV node. AVNRT will occur when
the electrical impulse propagates down one pathway and then travels in ret-
rograde up the other. Figure 6.9 b depicts an accessory pathway between the
atrium and ventricle. This form of SVT is know as Wolf-Parkinson-White
(WPW) syndrome. Evidence of this accessory connection can be recognized
on a surface electrocardiogram when preexcitation occurs and is defined by a
short PR interval and a “delta wave” in the up slope of the QRS interval. As
shown in Figure 6.10 a , an electrical signal propagated down the AV node and
up through the abnormal accessory pathway results in a rapid heart rate. Two
kinds of therapies are being utilized to resolve SVT: (1) acute therapy and (2)
chronic therapy. Acute therapy consists of pharmacological treatment and DC
cardioversion, while chronic therapy includes pharmacology, surgical ablation,
and catheter microablation (Fig. 6.10 b ).
The goal of any of the cardiac ablation techniques is to modify the electri-
cal conduction system of the heart of patients having arrhythmia by convert-
ing electrically active cardiac tissue to electrically inactive scar tissue, thereby
blocking the pathway or extra electrical circuit and eliminating the possibility
of arrhythmia. In the case of RF ablation resistive heating occurs, leading to
tissue coagulation and thus permanent tissue damage (Figs. 6.11 a - d ).
It is important to mention that the heat generation in the RF system occurs
within the tissues themselves, not within the catheter tip. Lesion size is a func-
tion of the power level, time of delivery, electrode tissue interface, and size of
the electrode. The last two parameters determine the tissue impedance pre-
sented to the RF generator. The concept of percutaneous cardiac tissue abla-
tion using electrode catheters was discovered in 1979 when an attempted