Determinants of antegrade echo zone in the Wolff-Parkinson-White syndrome.

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Abstract
Forty-five patients with the Wolff-Parkinson-White syndrome and reciprocating tachycardia which utilized the atrioventricular (A-V) node for antegrade conduction and an accessory pathway for ventriculo-atrial (retrograde) conduction were studied. The extrastimulus technique in the right atrium and coronary sinus at multiple pacing cycle lengths was used. Three types of "echo zones" were defined by the relationship of the antegrade effective refractory period (ERP) of the accessory pathway, the longest coupling interval accompanied by an atrial echo, the ERP of the A-V node, and the functional refractory period of the atrium. Eighteen of 45 patients (40%) had more than one type of echo zone demonstrated by changing either pacing site or pacing cycle length or both. Pacing site and intraventricular conduction delay were found to have important effects on the longest coupling interval accompanied by an atrial echo in those echo zones in which the longest coupling interval accompanied by an atrial echo was less than the antegrade ERP of the accessory pathway. The echo zone is a useful concept to explain the physiology of the common reciprocating tachycardia in patients with the Wolff-Parkinson-White syndrome. However, the echo zone is profoundly affected by pacing site and pacing cycle length. When these variables are changed, many patients can be shown to have more than one type of echo zone. Studies of the echo zone have limited value as therapeutic guides.