Atrioventricular sequential pacing: differential effect on retrograde conduction related to level of impulse collision.

Patients with DDD pacemakers who have intact retrograde conduction are known to be at risk of developing ventricular and "endless loop" tachycardia. To address this problem, a pacing protocol was designed in which V2A2 conduction was assessed in 16 patients during ventricular pacing alone (standard method) and during paced atrioventricular (AV) sequential drive (AV sequential method); the results were then compared. In eight of 16 patients who had intact retrograde conduction with both methods (group 1), the V2A2 intervals were significantly shorter (by 60 to 340 msec) with the AV sequential method. In the remaining eight patients, who demonstrated V2A2 block with the standard method, no V2A2 block occurred with the AV sequential method. In this study, two sets of AV intervals were programmed to produce collision of the two impulses (atrial and ventricular), either in the AV node or the His-Purkinje system (HPS). The site of V2A2 facilitation was related to the site of impulse collision. These results can be explained by earlier excitation by the atrial impulse (of AV node and/or HPS) during AV sequential pacing. However, in some cases it was evident that antegrade propagation of the atrial impulse was responsible for subsequent facilitation. The data suggest that assessment of retrograde conduction in candidates for DDD pacemakers can be made most accurately by the AV sequential method.