In a patient with a Wolff-Parkinson-White (WPW) syndrome type A mid-right atrial stimulation at a rate of 73/min produced a lesser degree of ventricular pre-excitation than when a slower sinus rhythm was present. This paradoxical effect was not related to tachycardia-dependent block in the accessory pathway because pre-excitation again increased at faster pacing rates. It was partly the result of a (proportionally) greater prolongation of intra-atrial conduction time to the accessory pathway than to the atrioventricular node and partly of a faster atrioventricular nodal conduction time. The latter, in turn, could be attributed either to later-than-normal arrival of excitation at the atrioventricular node, at a time when this structure was more recovered, or to a change in the site or mode of entry into the atrioventricular node. A gap in the atria was present because at a St1-St2 interval shorter than that which A2 had been blocked in the accessory pathway conduction was again possible, but with longer A1-A2 intervals. Finally, at similar, short, coupling intervals the impulse penetrated the atrioventricular node from the mid-right atrium but not from the coronary sinus. The unusual findings in this case support a recent assumption that in patients with WPW type A atrial stimulation should be performed from the coronary sinus to minimize the potential sources of error which can be produced by intra-atrial delay.
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