Atrioventricular conduction in secundum atrial septal defects.

The atrioventricular (A-V) conduction time in patients with secundum ASD who had left-toright shunts of 40-65% of their pulmonary flow was evaluated utilizing intracardiac electrograms. The A-V conduction time was divided into three components: (1) P-A interval, from the onset of the P wave to the time of excitation of muscle in the vicinity of the A-V node, (2) A-H interval, from the time of excitation of low right atrial muscle to that of the His bundle, and (3) H-V interval, from the time of the His bundle deflection to the onset of ventricular activation. The mean P-R interval of the ASD group (154 msec) was significantly longer (P < 0.01) than that of the control group (135 msec). The internodal conduction time (mean P-A interval) of the ASD group (52.2 msec) was significantly longer (P< 0.001) than that of the control group (29.1 msec). The A-H and H-V intervals of the two groups were not significantly different. Both the increased size of the atrium and the increased distance for internodal conduction produced by the defect itself can account for the prolonged internodal conduction time.