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

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Abstract
Reciprocating tachycardia in a patient with a left sided 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: retrograde atrial activation of the left atrium (LA) 60 ms or more before the low and high right atrium during reciprocating tachycardia and during V-A conduction; the absence of refractory-dependent delay in V-A conduction time with progressively premature ventricular stimulation, characteristic of retrograde conduction through an AP; and 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 2 events seemed to favor the initiation of the tachycardia: shortening of the atrial cycle length causing a decrease in the refractory period of the LA and/or the AP and 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.