Overview
Background
Radiofrequency (RF) catheter ablation (RFCA) has revolutionized treatment for tachyarrhythmias and has become first-line therapy for some tachycardias. Although developed in the 1980s and widely applied in the 1990s, formalized guidelines for its use in clinical practice were not developed until some years later.
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1960s, but surgical treatment for refractory tachyarrhythmias was the mainstay of nonpharmacologic therapy until it was superseded by catheter ablation.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; however, this high-voltage discharge was difficult to control and could cause extensive tissue damage.
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. The relative safety of RF energy has contributed to the widespread adoption of catheter ablation as a therapeutic modality.
RF energy produces small, homogeneous, necrotic lesions by heating tissue. Lesion size is influenced, in part, by the length of the distal ablation electrode and the type of catheter (standard vs saline-cooled). With typical power settings and good catheter contact pressure with cardiac tissue, lesions are minimally about 5-7 mm in diameter and 3-5 mm in depth.
Future directions
A curative procedure for atrial fibrillation (AF) is a major goal in clinical cardiac electrophysiology. Success has been achieved in patients with paroxysmal lone AF by eliminating conduction from the pulmonary veins to the left atrium, as many of these episodes are triggered by rapid electrical activity arising from tissue near the pulmonary vein ostia or from muscle sleeves surrounding the proximal veins. Other forms of AF may require some degree of substrate ablation (eg, linear transmural lesions in the left atrium).
Techniques are evolving to address the challenge of a catheter-based cure for all forms of AF. Three-dimensional electroanatomic maps, overlaid on magnetic resonance imaging (MRI) or computed tomography (CT) scans of the left atrium, can facilitate navigation of the catheter and mapping of the arrhythmogenic substrate. Intracardiac echocardiography may also help in avoiding collateral damage to the pulmonary veins or esophagus, ensuring adequate endocardial contact, and monitoring for complications (eg, pericardial effusion and thrombus development).
Alternative energy sources are being investigated in the ablation of AF (eg, balloon-based technologies using cryoablation,
ultrasound, and laser). In addition, robotic catheter navigation is now available to deliver RFCA.
Research is also focused on developing better methods and tools for catheter ablation of ventricular tachycardia (VT), and even ventricular fibrillation (VF), in patients with structural heart disease. Epicardial electrophysiology via subxiphoid pericardial puncture is a relatively new frontier; some tachyarrhythmia substrates (especially VT in nonischemic cardiomyopathy) cannot be reached from the endocardium.
Indications
There are three class I indications for catheter ablation. The first is symptomatic supraventricular tachycardia (SVT) due to atrioventricular (AV) nodal reentrant tachycardia (AVNRT), Wolff-Parkinson-White (WPW) syndrome, unifocal atrial tachycardia, or atrial flutter (especially common right atrial forms). For these conditions, catheter ablation is first-line therapy if that is the patient’s preference.
The second indication is AF with lifestyle-impairing symptoms and inefficacy or intolerance of at least one antiarrhythmic agent.
Both left atrial ablation for restoration of sinus rhythm and AV junction ablation for rate control are class I indications, depending on the circumstance.
The third indication is symptomatic VT.
Catheter ablation is first-line therapy in idiopathic VT if that is the patient’s preference. In structural heart disease, catheter ablation is generally performed for drug inefficacy or intolerance or as adjunctive therapy in patients with an implantable cardioverter-defibrillator (ICD) who are experiencing frequent ICD discharges.
Uncommon indications for catheter ablation include the following:
Symptomatic drug-refractory (inefficacy or intolerance) idiopathic sinus tachycardia
Lifestyle-impairing ectopic beats
Symptomatic junctional ectopic tachycardia
RFCA has been applied to most clinical tachycardias, even to polymorphic VT and VF in preliminary studies. Success rates are highest in patients with common forms of SVT, namely AVNRT and orthodromic reciprocating tachycardia (ORT).
Contraindications
Few absolute contraindications to RFCA exist. Left atrial ablation and ablation for persistent atrial flutter should not be performed in the presence of known atrial thrombus. Similarly, mobile left ventricular thrombus would be a contraindication to left ventricular ablation.
Mechanical prosthetic heart valves are generally not crossed with ablation catheters. Women of reproductive age should not be exposed to fluoroscopy if any possibility exists that they are pregnant.
Outcomes
Atrial fibrillation
RFCA of the AV junction results in excellent rate control, relieves palpitations, and improves functional capacity. However, patients who undergo this procedure require permanent pacemaker implantation to manage the resulting AV block and require warfarin therapy to prevent stroke because the AF itself is not affected. AV nodal modification is less therapeutic than AV junction ablation and may result in late heart block.
Single-procedure success rates for curing AF with RFCA are as high as 80% for paroxysmal AF in the absence of structural heart disease and may be as low as 50% or less in patients with persistent AF in the presence of structural heart disease and left atrial enlargement.
Repeat procedures are typically needed in at least 25% of patients and result in an increase in these success rates.
Samuel et al investigated the long-term effectiveness of catheter ablation in patients with AF and heart failure. Patients who underwent catheter ablation had fewer rehospitalizations and all-cause mortality compared to patients who did not receive catheter ablation.
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Success rates for AF ablation have historically been based on patient symptoms and periodic electrocardiographic (ECG) monitoring. Success rates are lower if intensive ambulatory monitoring to detect asymptomatic AF recurrence is used, such as daily monitoring for a month with an auto-triggering event monitor. Some patients require the use of previously ineffective antiarrhythmic drugs to maintain success.
Supraventricular tachyarrhythmias
The common forms of SVT (eg, AVNRT, SVT associated with WPW syndrome) are usually curable with a single procedure; the success rate is typically 90-95%. Cure rates for unifocal atrial tachycardia and common right atrial flutter are somewhat lower but still approach 90%. Recurrent tachyarrhythmias typically occur in the first few months after ablation and may be amenable to cure with a second procedure.
AVNRT is usually amenable to cure with a slow pathway ablation near the inferior atrial septum, where the risk of heart block is 1-2% with RF energy. In those uncommon cases where RF ablation near the compact AV node is required (eg, fast pathway ablation for AVNRT or an accessory pathway in a para-Hisian location), the risk of heart block may approach 5% or a little higher.
In a number of centers, catheter-based cryoablation, rather than RFCA, is used near the compact AV node to minimize the risk of heart block. With cryoablation, heart block is generally reversible with prompt rewarming. However, cryoablation appears to be slightly less effective than RF as an energy source, especially for deep accessory pathways.
Ventricular tachyarrhythmias
Idiopathic VT is curable (success rate, ~80%), provided that it is readily inducible during the electrophysiologic study. The most common location for these VTs is the right ventricular outflow tract. Because these VTs are usually not reentrant in nature, a significant percentage are not inducible. Some cannot be ablated because of their deep septal location or their epicardial location near a coronary artery.
Of patients with VT associated with structural heart disease, half to two thirds can obtain palliation with catheter ablation. Extensive scarring in these ventricles may limit the efficacy of the relatively small lesions made by RFCA, and multiple VT circuits may also contribute to this moderate success rate. In practice, many of these patients have implantable cardioverter-defibrillators (ICDs), and catheter ablation is used as adjunctive therapy for frequent device activations.
For patients with structural heart disease and stable VT, the potential benefit of catheter ablation before implantation of an ICD was demonstrated in the Ventricular Tachycardia Ablation in Coronary Heart Disease (VTACH) study.
This prospective, randomized, controlled international trial in 104 patients found that time to recurrence of VT or VF was longer in the ablation group (median, 18.6 months) than in the control group (5.9 months). At 2 years, estimates for survival free from VT or VF were 47% in the ablation group and 29% in the control group.