Electrophysiologic study of patients with short P-R interval and normal QRS complex.

In order to elucidate the electrophysiological nature of the short P-R interval we have studied 15 patients with this phenomenon and a normal QRS complex in the electrocardiogram; none of them had a history of arrhythmias. His bundle electrography and atrial stimulation were used to study the conductivity and refractory periods of the atrioventricular (AV) junction. In 13 cases the studies were repeated after the administration of verapamil. Baseline recordings showed a shortened A-H interval in 6 cases, a shortened H-V interval in 6 and in 3 more with a borderline H-V interval. During rapid atrial pacing A-H interval prolongation was abnormal in 10 cases that showed small increases or sudden prolongations at certain atrial rates. The effective nodal refractory period was shorter than normal. A double AV nodal pathway was demonstrated in one patient and another developed junctional reentrant tachycardias at fixed extrasystolic intervals. After verapamil the A-H interval increased in all but one patient. Significant changes were recorded for the AV nodal refractory periods. In the patient with dual AV nodal conduction verapamil prolonged conduction time and refractory periods through both pathways, and in the one with reentrant tachycardias upon atrial stimulation it abolished the tachycardia. In the presence of a normal QRS, a short P-R interval may be due to shortened conduction time through the AV node, and/or the His-Purkinje network. Our observations tend to rule out the presence of a complete bypass of nodal tissue. Despite the absence of a history of arrhythmias these patients may have electrophysiological abnormalities that predispose them to reentrant AV nodal tachycardias.