Accessory-pathway block on alternate beats in the Wolff-Parkinson-White syndrome: supernormal conduction as the mechanism.

The Holter monitor electrocardiograms were taken from 2 patients with intermittent Wolff-Parkinson-White syndrome. In these patients, when the heart rate was increased, accessory-pathway block on alternate beats was found and was maintained for a considerably long period. In one patient, when accessory-pathway block on alternate beats was found, a ventricular extrasystole occurred. After the long compensatory pause after that extrasystole, a sinus impulse was blocked in the accessory pathway, showing that the effective refractory period of the accessory pathway is markedly long. These findings strongly suggest that alternate sinus impulses fell in the supernormal period of the accessory pathway. An attempt was made to explain the mechanism of accessory-pathway block on alternate beats by using the concept of supernormal conduction in the accessory pathway, in the same way as in bundle-branch block on alternate beats.