His Bundle Electrograms in Patients with Short P-R Intervals, Narrow QRS Complexes, and Paroxysmal Tachycardias

His bundle electrograms were recorded in three patients with short P-R intervals, narrow QRS complexes, and a history of paroxysmal tachycardias. During sinus rhythm or atrial stimulation with long cycle lengths, the shortening of the P-R interval was due to a decrease in the low right atrium-His (LRA-H) interval (representing A-H conduction time). The latter was also short during retrograde (V-A) conduction. These findings support the existence of an A-V nodal bypass operation in both directions. In one patient, the LRA-H interval did not lengthen when the atrial rate was increased. Intermittent atrial pacing was performed in the two other patients. The LRA-H interval was short at long coupling intervals, but it started to increase (progressively) at a given Stimulus 1 -Stimulus 2 interval. Apparently, the refractory period of the accessory bundle was encountered so that the impulse was propagated, with various degrees of delay, through the A-V node. A James bundle need not be present in all patients with similar electrocardiograms. Abnormalities of unknown origin could cause this phenomenon. Reciprocating tachycardias were induced by stimulation of the atria in one patient. The triggering beat consistently had a long A-V conduction time. Although in this case retrograde (V-A) propagation most probably occurred through the accessory communication, the possibility of a functional intranodal dissociation of a single anatomical pathway could not be excluded