Mechanism of reciprocating tachycardia initiated during sinus rhythm in concealed Wolff Parkinson White syndrome. Report of a case

Reciprocating tachycardia in a patient with a leftsided atrioventricular accessory pathway (AP) (Kent bundle, type A) capable only of ventriculo atrial (V A) transmission is described. The V A AP is established as an essential link of the tachycardia circuit, as evidenced by: 1) retrograde atrial activation of the left atrium (LA) 60 msec or more before the low and high right atrium during reciprocating tachycardia and during V A conduction; 2) the absence of refractory dependent delay in V A conduction time with progressively premature ventricular stimulation, characteristic of retrograde conduction through an AP; and 3) the absence of antegrade conduction through the Kent bundle during sinus rhythm, reciprocating tachycardia, pacing from either atrium, or during induced atrial flutter fibrillation. The onset of the tachycardia was unique in that it could be initiated and perpetuated during sinus rhythm, without a triggering mechanism of an atrial or ventricular extrasystole. The interplay of the following two events seemed to favor the initiation of the tachycardia: 1) shortening of the atrial cycle length causing a decrease in the refractory period of the LA and/or the AP; and 2) the development of rate dependent left bundle branch block, delaying impulse arrival at the ventricular end of the AP. These observations described an additional mechanism of reciprocating tachycardia in patients with the Wolff Parkinson White syndrome.