Atrial fibrillation and accessory pathway

ECG 11A: Characteristic pattern of atrial fibrillation due to accessory pathway; Irregular tachycardia with very short RR cycles (cycles measured at 200 ms) with cycle-to-cycle variations in QRS pattern including narrow QRS and QRS with various degree of pre-excitation;

ECG 11B:

ECG 11B: This patient underwent electrical shock under brief sedation; sinus rhythm with left lateral pre-excitation pattern (short PR, delta wave);

Comments: Atrial fibrillation episodes are documented in approximately one third of patients with Wolff-Parkinson-White syndrome. The incidence of atrial flutter and tachycardia on the other hand does not appear to be increase. Various mechanisms involved in the genesis of these episodes have been suggested: 1) certain reciprocating (reentry) tachycardias degenerate spontaneously into atrial fibrillation. Clinically, the patient initially feels a regular and well-supported tachycardia which becomes irregular and often ill-supported. During an episode of antidromic or orthodromic tachycardia, the atria are activated in a rapid and retrograde manner, which increases the likelihood of occurrence of an atrial fibrillation. The incidence of atrial fibrillation decreases considerably after effective ablation of the accessory pathway. This mechanism may not be the only mechanism involved in the genesis of arrhythmias as some patients present episodes of fibrillation without ever having previously presented reciprocating tachycardias; 2) it would appear that the presence of an accessory pathway induces changes in atrial architecture and an increase in atrial vulnerability. Electrophysiological studies have revealed an atrial substrate propitious to arrhythmia with evidence of fractionated atrial endocardial electrograms of prolonged duration, of altered atrial refractory periods and of increased inter-atrial conduction time.

The presence of atrial fibrillation rapidly conducted by the accessory pathway constitutes the determining prognostic element of patients presenting an accessory pathway. This type of episode is ultimately clinically reflected by lipothymia, syncope or sudden death. An accessory pathway is considered malignant when its anterograde refractory period is very short, thus exposing the patient to a potentially very rapid ventricular rhythm. It should be noted that the refractory period may vary as a function of the hemodynamic alteration or of a modification of the catecholaminergic state.

On the electrocardiogram, an atrial fibrillation episode due to an accessory pathway is reflected by an irregular ventricular rhythm with alternation of very broad QRS (major pre-excitation), narrow QRS (exclusive nodal conduction) and intermediate QRS (fusion) complexes. The succession of more or less pre-excited cycles with significant cycle-to-cycle variations of the initial portion of the QRS yields an “accordion-like” electrocardiographic pattern. The axis of the QRS does not change which allows differentiating this pattern from that of a polymorphous ventricular tachycardia of the torsade of pointes type.

This patient underwent a radiofrequency ablation of his accessory pathway, the indication appearing indisputable in this setting.

Take-home message: The incidence of atrial fibrillation is considerably increased in patients with Wolff-Parkinson-White syndrome. The ablation of the accessory pathway significantly reduces the occurrence of this type of arrhythmia which exposes the patient to the risk of sudden death.