Which patients with SVT should be referred for an intracardiac electrophysiological study (EP study)?

Which patients with SVT should be referred for an intracardiac electrophysiological study (EP study)? What are the success rates and risks of radiofrequency (RF) ablation?

The management of supraventricular tachycardia (SVT) has changed dramatically with the development of curative radiofrequency ablation (RF ablation). For most patients, the technique offers a clear alternative to long term antiarrhythmic drug therapy with its potential toxic side effects. Except for atrial fibrillation and atypical atrial flutter, most SVTs are amenable to RF ablation albeit with some variation in success rates depending on the arrhythmia mechanism.

AV nodal re-entrant tachycardia and SVTs mediated via accessory pathways are the easiest to treat with RF ablation with success rates that exceed 90%.1 Recurrence is rare occurring in less than 10%. Focal atrial tachycardias and re-entrant atrial tachycardias resulting from prior atrial surgical scars have lower success rates of about 80%. Even for the rare but troublesome atrial tachycardia that cannot be ablated, RF ablation of the AV node with permanent cardiac pacing is effective in alleviating symptoms and can reverse any tachycardia mediated cardiomyopathy. Atrial flutter of the classical variety use a single re-entrant circuit in the right atrium and typically require an isthmus of tissue between the inferior vena cava and tricuspid valve for maintenance of the arrhythmia. RF ablation to create conduction block in this isthmus is effective in preventing recurrence of atrial flutter in 80% of patients with negligible risks. Unfortunately some patients develop atrial fibrillation because both arrhythmias share common cardiac disease processes that act as substrates for the arrhythmia mechanism. Nonetheless, fibrillation is easier to manage with drugs and combination of flutter ablation and antiarrhythmic drug therapy is often successful in maintaining sinus rhythm.

In the adult patient with the symptomatic Wolff Parkinson White syndrome, it is now generally believed that RF ablation should be the treatment of choice. Recurrent arrhythmias associated with ventricular pre-excitation are difficult to treat medically and often require the use of antiarrhythmic drugs with potent pro-arrhythmic effects or organ toxicity (e.g. flecainide, amiodarone). The risk of AV block is remote (less than 1%) unless the accessory pathway is located close to the AV node in which case the risk is higher. In infants and young children, on the other hand, it is often worth deferring RF ablation if possible because there is a chance that ventricular pre-excitation may resolve over a few years.

In contrast to the above, arrhythmias such as AV nodal reentrant tachycardia often respond to acute or interval therapy with one of the more benign AV nodal blocking agents e.g. digoxin, beta blockers or calcium blocker. RF ablation should therefore be reserved for recurrent or troublesome arrhythmia. Situations that justify earlier RF ablative therapy include haemodynamic instability during episodes, intolerance of drugs, desire to avoid long term drug therapy or occupational constraints such
as in airline pilots. It is also worth bearing in mind that once a patient requires more than two drugs for prophylaxis, it becomes more cost effective to proceed to RF ablation. The risk of AV block during RF ablation for AV nodal re-entrant tachycardia is between 1 and 2%,2 and is dependent on the experience of the operator. In the younger patients, even this low rate of complication can be important considering life time commitment to cardiac pacing in the event of heart block.

The risk of RF ablation is primarily that of AV block as noted above. Other risks are those related to cardiac catheterisation and include vascular damage, cardiac tamponade, myocardial infarction, cerebrovascular or pulmonary embolism and rarely damage to the valve in left sided pathways. In experienced centres, the risk of serious complications is less than 1%.