Retrograde conduction in the His-Purkinje system. Analysis of the routes of impulse propagation using His and right bundle branch recordings.

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Abstract
Routes of retrograde impulse propagation in the His-Purkinje system (HPS) were analyzed in 24 patients with normal intraventricular conduction. Using intracardiac recordings from the His bundle and right bundle branch (RBB), predetermined paced ventricular cycle lengths were scanned with right ventricular extrastimuli (S2). Electrophysiologic findings suggested that retrograde activation of the His bundle after premature stimuli (H2) occurred via the left bundle branch (LBB) in 16 of 24 cases and via the RBB in 3 of 24 patients. The remaining patients demonstrated retrograde H2 activation via both RBB and LBB at different times during the scanning. A linear relationship existed between retrograde conduction delays in the HPS (i.e., S2H2 intervals) and ventricular coupling intervals (S1S2) when retrograde H2 activation consistently occurred via the same bundle branch. Sudden and unexpected increases (30-90 ms) or decreases (10-55 ms) in S2H2 intervals were noted in 11 of 24 patients; these changes occurred with and without concomitant changes in the retrograde His-right bundle (HRB) activation sequence. Retrograde H2 activation during right ventricular premature stimulation occurs exclusively through the LBB in a majority (67%) and through the RBB in a minority (12%) of cases. In some patients (21%), the route of retrograde H2 activation may vary and could occur via both the LBB and the RBB. Recordings from the HRB area should help to delineate the route of retrograde impulse propagation to the His bundle.