5. Supraventricular Tachycardias Due to Accessory Atrioventricular Pathways (Preexcitation Syndromes)

Pathophysiology & Clinical Findings

Accessory pathways between the atria and the ventricle which avoid the conduction delay of the atrioventricular node predispose to reentry tachycardias, such as AVRT and atrial flutter, and to atrial fibrillation. These may be wholly or partly within the node (Mahaim fibers), yielding a short PR interval and normal QRS morphology (Lown-Ganong-Levine syndrome). More commonly, they make direct connections between the atria and ventricle through Kent bundles (Wolff-Parkinson-White syndrome). This produces a short PR interval but an early delta wave at the onset of the wide, slurred QRS complex owing to early ventricular depolarization of the region adjacent to the pathway. While the morphology and polarity of the delta wave can suggest the location of the bypass tract, mapping by intracardiac recordings is required for precise anatomic localization.

Accessory pathways occur in 0.1-0.3% of the population and facilitate reentry arrhythmias owing to the disparity in refractory periods of the atrioventricular node and accessory pathway.

Whether the tachycardia is associated with a narrow or wide QRS complex is determined by whether antegrade conduction is through the node (narrow) or the bypass tract (wide). Many patients with Wolff-Parkinson-White syndrome never conduct in an antegrade direction through the bypass tract, which is therefore “concealed.” Orthodromic tachycardia is a reentrant rhythm that conducts antegrade down the AV node and retrograde up the accessory pathway, resulting in a narrow QRS complex. Antidromic tachycardia conducts down the accessory pathway and retrograde through the AV node, resulting in a wide QRS complex. Since accessory pathways are less refractory than specialized conduction tissue, tachycardias proceeding in this direction have the potential to be more rapid. Up to 30% of patients with Wolff-Parkinson-White syndrome will develop atrial fibrillation or flutter with antegrade conduction down the accessory pathway and a rapid ventricular response.

Treatment

Some patients have a delta wave found incidentally on electrocardiography. In the absence of palpitations, lightheadedness, or syncope, these patients do not require specific therapy. They should be advised to report the onset of any of these symptoms.

A. Radiofrequency Ablation
As with AVNRT, radiofrequency ablation has become the procedure of choice in patients with accessory pathways and recurrent symptoms. Patients with preexcitation syndromes who have episodes of atrial fibrillation or flutter should be tested by induction of atrial fibrillation in the electrophysiologic laboratory, noting duration of the RR cycle; if it is less than 220 ms, a short refractory period is present. These individuals are at highest risk for sudden death, and prophylactic ablation is indicated. Success rates for ablation of accessory pathways with radiofrequency catheters exceed 90% in appropriate patients.

B. Pharmacologic Therapy
Narrow-complex reentry rhythms involving a bypass tract can be managed as discussed for AVNRT. Atrial fibrillation and flutter must be managed differently, since agents such as digoxin, calcium channel blockers, and even beta-blockers may decrease the refractoriness of the accessory pathway or increase that of the AV node, leading to sometimes faster ventricular rates. Therefore, these agents should be avoided. The class Ia antiarrhythmics, as well as the newer class Ic and class III agents, will increase the refractoriness of the bypass tract and are the drugs of choice for wide-complex tachycardias. If hemodynamic compromise is present, electrical cardioversion is warranted.

Long-term therapy often involves a combination of agents that increase refractoriness in the bypass tract (class Ia or Ic agents) and in the atrioventricular node (verapamil, digoxin, and beta-blockers), provided that atrial fibrillation or flutter with short RR cycle lengths is not present (see above). Sotalol and amiodarone are effective in refractory cases. Patients who are difficult to manage should undergo electrophysiologic evaluation.

Preferences:
_{Antz M et al: Risk of sudden death after successful accessory atrioventricular pathway ablation in resuscitated patients with Wolff-Parkinson-White syndrome. J Cardiovasc Electrophysiol 2002;13:231.}

DeLorier P et al: Should patients with asymptomatic Wolff-Parkinson-White pattern undergo a catheter ablation? Curr Cardiol Rep 2001;3:301.

Fitzsimmons PJ et al: The natural history of Wolff-Parkinson-White syndrome in 228 military aviators: a long-term follow-up of 22 years. Am Heart J 2001;142:530.