Because of its high success rate and low morbidity, radiofrequency (RF) catheter ablation has become first line treatment for many arrhythmias. In this procedure, one or more electrode catheters are advanced percutaneously through the vasculature to contact cardiac tissues. A diagnostic study is performed to define the arrhythmia mechanism, and subsequently an ablation catheter is positioned adjacent to the arrhythmogenic substrate. Radiofrequency energy of up to 50 W is delivered in the form of a continuous unmodulated sinusoidal waveform, typically for 60 seconds. Energy delivery is well tolerated by a mildly sedated patient, and results in a small (5 mm) well circumscribed lesion. Destruction of tissue critical for arrhythmogenesis (such as an accessory pathway) and its subsequent replacement with scar eliminates arrhythmia.
The small size of radiofrequency lesions has led to the greatest success in the treatment of those arrhythmias that have a focal origin or depend on a narrow isthmus for maintenance. Furthermore, since precise lesion placement is required, arrhythmias for which ablation is most effective (accessory pathways, atrioventricular nodal re-entry tachycardia (AVNRT)) have largely anatomically based or directed substrates. Accessory pathways are anomalous epicardial connections between the atria and ventricles, and are located along the mitral or tricuspid valve annulus, reducing the problem of localisation to identification of a point on a line. An electrode catheter in the coronary sinus outlines the mitral annulus fluoroscopically, and is used to guide ablation catheter position. The relative amplitude of the atrial and ventricular components of the bipolar electrogram recorded by the ablation catheter further defines tip position relative to the annulus. Earliest atrial or ventricular activation during pathway conduction identifies pathway location along the annulus. The target for catheter ablation of AVNRT (the AV nodal slow pathway) occurs even more predictably in the posteroseptum. Ablation may be guided entirely by anatomic location relative to His and coronary sinus catheter positions, which serve as fluoroscopic landmarks, or by a combined anatomic and electrogram approach. Detailed discussions of radio-frequency ablation are available elsewhere.1
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