Effects of Physiologic Pacing vs. Ventricular Pacing
abstract & commentary
Synopsis: Dual-chamber pacing is associated with a higher rate of complications. Clinicians should use considerable judgment when selecting the optimal pacing system for individual patients.
Source: Connolly SJ, et al. N Engl J Med 2000;342: 1385-1391.
The canadian trial of physiologic pacing was a multicenter study designed to test the hypothesis that physiologic pacing (dual-chamber pacing or atrial pacing in patients with intact atrioventricular [AV] conduction) is superior to single-chamber ventricular pacing. A total of 2568 patients were enrolled in 32 participating Canadian centers. Adult patients (> 18 years of age) were eligible if they were scheduled for an initial pacemaker implant to correct symptomatic bradycardia, did not have chronic atrial fibrillation, had an expected life expectancy of more than two years, and had not undergone an AV junctional ablation. Centers chose in advance a randomization ratio of between 2:1 and 1:2 to reflect budgetary limitations at each site. After consent was obtained, patients were randomly assigned to either physiologic pacing or single-chamber ventricular pacing. Rate-adaptive pacing was required when appropriate. The primary end point of the study was a composite of stroke and cardiovascular death. Secondary end points were all-cause mortality, new or recurrent atrial fibrillation, and hospitalization for congestive heart failure.
Due to the unequal preselected randomization ratios, 1474 patients were assigned to ventricular pacing and 1094 to physiologic pacing. The groups were well matched in terms of age (mean, 73 years), gender (58% male), New York Heart Association functional class, indication for pacing, cardiac history, ventricular function, and medications. Complications related to implantation were more common in the physiologic pacing group (9.0% vs 3.8%; P < 0.001), primarily due to difficulties with atrial pacing, sensing thresholds, or lead dislodgment.
During a mean follow-up of three years, the annual rates for stroke or cardiovascular death were 5.5% and 4.9% in the ventricular pacing and physiologic pacing groups, respectively. The relative risk reduction was 9.4%, with a 95% confidence interval of -10.5 to 25.7%. All-cause annual mortality was also not significantly improved with physiologic pacing (6.6% vs 6.3%). There was a significant decrease in the annual risk of developing atrial fibrillation in the physiologic pacing group (5.3% vs 6.6%; P = 0.05), but this effect became apparent only after two years of follow-up. The annual rates of hospitalization for heart failure were also not significantly different (3.5% vs 3.1%; P = 0.52). In a subgroup analysis, age younger than 74 years was associated with a trend toward benefit that approached significance.
Comment by John P. DiMarco, MD, PhD
The introduction of implantable permanent pacemakers almost 50 years ago dramatically changed the prognosis of patients with complete heart block and Stokes-Adams attacks. The early devices were primitive by today’s standard in that they provided only fixed-rate, single-chamber, usually ventricular, stimulation. Numerous advances in technology have resulted in pacemakers programmable with a dazzling array of features, of which many are designed to better mimic normal physiology.
Connolly and colleagues elected to study the value of this additional technology. At least in terms of the primary end points—stroke and cardiovascular death—the short-term answer appears to be that the value is less than might be desired.
Several prior retrospective studies have shown improved survival in subgroups of patients with dual-chamber pacing. These studies were, however, obviously limited by potential for bias since the mode of pacing was not randomly assigned. The Pacemaker Selection in the Elderly study (N Engl J Med 1998;338:1097-1104) also reported no change in mortality but, since all patients received dual-chamber pacemaker systems, crossover was common and might have blunted a real difference. A Danish trial that evaluated atrial, not dual-chamber, vs. ventricular pacing in patients with sinus node dysfunction showed reductions in both stroke and mortality during long-term follow-up with atrial pacing (Lancet 1997;350:1210-1216). This latter trial raises the important issue of the possible negative hemodynamic effects that may occur with pacing from the right ventricular apex.
The data presented here also show that dual-chamber pacing is associated with a higher rate of complications. Although equipment costs are highly variable, in general dual-chamber pacemakers are more expensive than single-chamber units, and cost-effectiveness is an additional consideration.
In the absence of a survival advantage, end points other than stroke and mortality may still provide important reasons for selecting a more complex and expensive mode of pacing for a patient with bradycardia. Our hope is that substudies from this trial will address important issues like quality of life, psychological well-being, and function in society. At present, clinicians should continue to use considerable judgment when selecting the optimal pacing system for any individual patient.
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