Responses of an AV universal (DDD) pulse generator (Cordis 233D) to programmed single ventricular extrastimuli

To evaluate factors playing a role in initiation and perpetuation of pacemaker-mediated tachycardias (PMTs), 22 consecutive patients with symptomatic conduction disorders were studied after implantation of an AV universal (DDD) pulse generator (Cordis 233D). Patients were divided into two groups, depending upon the presence or absence of ventriculo-atrial (VA) conduction during electrophysiological study (EPS) performed before pacemaker implantation. PMTs could be initiated in six of eight patients of Group I and in none of 14 patients of Group II. Initiation and perpetuation of PMTs during DDD pacing were dependent upon the capacity of the patient to conduct ventricular premature beats (VPBs) and subsequent paced ventricular beats retrogradely to the atria, and upon three programmable parameters of the pulse generator (AV delay period, upper rate limit, tachycardia response). Programmed single ventricular extrastimulation demonstrated that: (1) merely the presence of VA conduction during EPS, although necessary, was not sufficient to induce PMTs after DDD pacemaker implantation; (2) VPBs introduced late rather than early in the cardiac cycle initiated PMTs in a different way; (3) the initiation of PMTs could be prevented during study by adjusting the programmable parameters (AV delay period, upper rate limit, tachycardia response); (4) one of the two available tachycardia responses of the pulse generator (gradual fall-back response) was able to terminate and initiate PMTs consistently. These observations helped in understanding the responses of the Cordis 233D pulse generator to ventricular premature beats. They indicate that additional refinement of the pulse generator is necessary to solve the problem of PMT.