Apparent bradycardia-dependent block in the accessory pathway in intermittent Wolff-Parkinson-White syndrome.

A case of intermittent Wolff-Parkinson-White syndrome is reported, which is the first report of an apparent bradycardia-dependent block in the accessory pathway. When the P-P interval gradually lengthened to a critical value of 0.89 second or more, preexcitation failed to occur, but thereafter, when the P-P interval gradually shortened to 0.89 second or less, preexcitation again occurred. These findings seem to suggest that bradycardia-dependent block occurred in the accessory pathway. In the electrocardiogram taken on another day, however, when a ventricular extrasystole was followed by a compensatory pause, the ensuing QRS complex showed preexcitation, although the pause was markedly longer than the critical period of 0.89 second. This indicates that this case is not one of true bradycardia-dependent block in the accessory pathway. Rather, the effect occurred because increased vagal tone depressed conductivity in the accessory pathway to a greater degree than automaticity in the sinus node.