Observations in patients showing A-V junctional echoes with a shorter P-R than R-P interval

Single test stimulation of the ventricle revealed initiation of echoes with a supraventricular QRS complex with a shorter P-R than R-P interval in 28 of 300 patients consecutively studied with programmed electrical stimulation of the heart because of documented or suspected tachycardias. In all 28 the initiation of echoes was related to a discontinuity in the retrograde conduction curve. In 10 patients a different atrial activation sequence in the endocavitary leads was present before and after the discontinuity in the retrograde conduction curve. In five of these a sustained tachycardia with a shorter P-R than R-P interval could be initiated, and in all five patients an accessory pathway with a long conduction time as the retrograde arm of the tachycardia circuit could be demonstrated. In these five patients spontaneous initiation of tachycardia was observed during sinus rhythm or after atrial premature beats. Tachycardia accelerated after the administration of atropine. In the remaining 23 patients the initiation of echoes showing a shorter P-R than R-P interval was nonsustained. In these patients spontaneous initiation of such echoes during sinus rhythm or initiation by atrial premature beats was not observed, and echoes with this relation of the P-R and R-P intervals systematically disappeared after administration of atropine. It is postulated that in these patients a slow atrioventricular (A-V) nodal pathway is used in the retrograde direction during echoes showing a shorter P-R than R-P interval. Sustained A-V junctional tachycardia showing this relation between P-R and R-P intervals favors incorporation of an accessory pathway with slow retrograde conduction in the tachycardia circuit.