Catheter Mediated Radiofrequency Ablation
Radiofrequency catheter ablation (RFCA) has revolutionized treatment for tachyarrhythmias and has become first-line therapy for a variety of tachycardias. Although developed in the 1980s and widely applied in the 1990s, formalized guidelines for its use in clinical practice were not developed until recently.
Catheters were first used for intracardiac recording and stimulation in the late 1960s, but surgical treatment for refractory tachyarrhythmias was the mainstay of nonpharmacologic therapy until it was superseded by catheter ablation. The initial energy source used was direct current (DC) from a standard external defibrillator. A shock was delivered between the distal catheter electrode and a cutaneous surface electrode. This high-voltage discharge was difficult to control and could cause extensive tissue damage.
Radiofrequency (RF) energy, a low-voltage high-frequency form of electrical energy familiar to physicians from its use in surgery (eg, electrocautery), quickly supplanted DC ablation. RF energy produces small, homogeneous, necrotic lesions approximately 5-7 mm in diameter and 3-5 mm in depth. The relative safety of this energy source contributed to the widespread adoption of catheter ablation as a therapeutic modality.
A survey published in 1995 indicated that approximately 15,000 RFCA procedures are performed annually in the United States. Patients found to have inducible tachycardias are candidates for catheter-mediated radiofrequency ablation that is typically offered at the time of the diagnostic EP study. Radiofrequency ablation can be either curative or ameliorative for patients depending on the mechanism of the underlying fast heart rhythm, or “tachycardia”. Tachycardia mechanisms fall broadly into one of two major categories, supraventricular and ventricular tachycardias.
Supraventricular tachycardias (SVT) are fast heart rhythms in which the top chambers of the heart (the atria) are either the source of the tachycardia or participate in the tachycardia by virtue of abnormal conduction of electrical impulses between the atria and the ventricles. The term SVT typically implies AV nodal reentry, AV reentry, or focal atrial tachycardia. Technically this term also encompasses atrial fibrillation and atrial flutter, both of which are tachycardias predominantly involving atrial tissue.
AV nodal reentrant tachycardia (AVNRT) represents a fairly common form of SVT. The AV node has multiple atrial inputs, which are broadly referred to as slow and fast pathways. These inputs provide the substrate for a reentrant tachycardia. Most commonly the posterior atrionodal input to the AV node (the slow pathway) serves as the anterograde limb of the reentry circuit and the anterior atrionodal input (the fast pathway) serves as the retrograde limb. In most instances (~85-90%), AVNRT can be cured by targeting the slow pathway region. The risk of causing heart block by ablating in this region is quite low (1-2%), and targeting the slow pathway is safer than targeting the fast pathway, which is located closer to the compact AV node.
Another common form of SVT is orthodromic reciprocating tachycardia (ORT), a reentrant rhythm that involves the AV node as the anterograde limb and an accessory AV connection (ie, the accessory pathway) as the retrograde limb. This tachycardia mechanism accounts for approximately 30% of paroxysmal SVTs. Typically, targeting the accessory pathway as it crosses the mitral or tricuspid valve annulus can cure this rhythm disturbance. Less commonly the accessory pathway can serve as the antegrade limb of the tachycardia, with retrograde conduction up the AV node. This results in a wide complex (pre-excited) tachycardia, and occurs as part of a syndrome, Wolff-Parkinson-White Syndrome. These patients are at risk for sudden death and can be cured by ablation of the accessory pathway. Unifocal atrial tachycardia is a less common form of SVT and can arise from either atrium. For those tachycardias originating from the left atrium, transseptal catheterization via a patent foramen ovale or transseptal puncture is usually required.
