Diagnostic Methods Electrophysiology

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 ofAV 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 (ofAV 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. Circulation 68, No. 1, 23-32, 1983. RESTORATION of normal atrioventricular (AV) synchrony in patients who need permanent pacing is not a new idea. ' However, the development of stable transvenous atrial electrodes2' has rekindled interest in AV sequential pacing. Programmable pacemakers capable of pacing and seinsing both the atrium and ventricle are now available,4 and one highly desirable feature of such pacemakers is the ability to sense the physiologic variations of atrial rate and to pace the ventricle accordingly, i.e., to accomplish atrial tracking (as in DDD, VDD, and VAT modes). The major clinical problem with these devices has been the development of pacemaker tachycardias. These are usually initiated by a premature ventricular beat (PVB), which produces a retrograde P wave. This is sensed and commits the pacemaker to stimulate the ventricle, which in turn produces another retrograde P wave, closing the From the Natalie and Norman Soref and Family Electrophysiology Laboratory, University of Wisconsin-Mount Sinai Medical Center, Milwaukee. Supported in part by AHA Grant-in-Aid 83-GA-06. Address for correspondence: Rehan Mahmud, M.D., Electrophysiology Laboratory, Mount Sinai Medical Center, P.O. Box 342, Milwaukee, WI 53201. Received Jan. 5, 1983; revision accepted March 10, 1983.