Abstract & Commentary
Synopsis: Dual chamber pacing in patients with sinus node dysfunction does not improve the rate of stroke-free survival, but does decrease the occurance of atrial fibrillation.
Source: Lamas GA, et al. N Engl J Med. 2002;346: 1854-1862.
This paper describes the results of the mode Selection Trial in Sinus-Node Dysfunction (MOST). This study compared single chamber ventricular rate modulated pacing with dual chamber, rate-modulated pacing in patients with symptomatic bradycardia due to sinus node dysfunction. Patients were eligible for the study if they were scheduled for implantation of a dual chamber, rate-modulated pacing system for sinus node dysfunction. All patients received a dual chamber pacing device at baseline and then were randomized to either rate-modulated dual chamber pacing or rate-modulated ventricular pacing. Quality-of-life assessments were performed 3 and 12 months after enrollment and yearly thereafter. The primary end point was death from any cause, or nonfatal stroke. Secondary end points included death from any cause, stroke, hospitalization for heart failure, and changes in the Living with Heart Failure or the health-related quality-of-life scores and appearance of pacemaker syndrome.
A total of 2010 patients were entered into the trial. Their mean age was 74 years; 48% were women. Hypertension was reported by 62% and diabetes by 22%. During the course of the trial, death or nonfatal stroke, the primary composite end point occurred in 447 patients. There was no difference in the primary end point between patients receiving dual chamber pacing and those receiving ventricular pacing (21.5% vs 23%; P = 0.48. There is also no difference in the rate of stroke, death from any cause, or death from cardiovascular cause. A significant difference was, however, seen in the occurrence of atrial fibrillation. A hazard ratio of 0.79 for the occurrence of atrial fibrillation favored the dual chamber-pacing group. This difference was most marked among those who had no prior history of atrial fibrillation. A trend toward a decreased number of hospitalizations was seen in the patients receiving dual chamber pacing (10.3% vs 12.3%; P = 0.13). There were small, but significant, improvements in quality of life associated with dual chamber pacing. Subgroup analysis showed trends toward improvement in all subgroups with dual chamber pacing but no subgroup showed a statistically significant improvement in the primary end point.
Crossover to dual chamber pacing from ventricular pacing was common; 37.6% of the patients in the ventricular pacing group were reprogrammed to dual chamber pacing. Most of these crossovers occurred in the first 3 months of the study.
Lamas and colleagues conclude that dual chamber pacing in patients with sinus node dysfunction does not improve the rate of stroke free survival. However, dual chamber pacing is associated with a lower incidence of atrial fibrillation, small reductions in the signs and symptoms of heart failure and slight improvements in the measures of quality of life.
Comment by John P. DiMarco, MD, PhD
Dual chamber pacing was first introduced about 25 years ago. However, studies demonstrating benefits with dual chamber pacing have been inconclusive. The largest prior study, The Canadian Trial of Physiologic Pacing (N Engl J Med. 2000;342:1385-1391) showed no differences in the rates of death, stroke, or hospitalizations for heart failure among 2568 patients with both sinus node dysfunction and atrioventricular block. That study and several retrospective studies, however, did note a reduction in the incidence of atrial fibrillation with dual chamber pacing. Several studies by Anderson and colleagues (Lancet. 1997;350:1210-1216) also showed a benefit of atrial pacing as opposed to ventricular pacing in patients with sinus node dysfunction. The MOST trial confirms that benefits from dual chamber pacing are small in terms of the most serious end points, death, and stroke, but benefits become apparent when more subtle clinical changes such as quality of life are examined.
The major limitation of this trial is the high rate of crossover. Prior studies randomized patients to single chamber vs. dual chamber pacing at the time of implant and crossover to a dual chamber pacing mode was possible only if repeat operation for atrial lead placement was performed. In this study, all patients received dual chamber pacemakers and crossovers required only simple reprogramming. This led to a higher rate of crossovers than seen in previous studies and potentially minimized the possibility that real changes in end points might be demonstrated since the analyses presented all used an intention to treat principle.
Technological advances in lead and pacemaker design have made the differences between dual chamber and single chamber pacemaker systems less dramatic. At the present time, there is little added risk to implanting a dual chamber system. There is, however, a difference in cost of the devices that may be as large as several thousand dollars. However, only small differences in outcome would be necessary to justify the relatively small initial cost of the procedure. Therefore the data reported in this trial support the current US practice of implanting dual chamber or atrially-based pacemakers whenever possible even if they do not result in improvements in mortality or risk of stroke.
Dr. DiMarco is Professor of Medicine, Division of Cardiology, University of Virginia, Charlottesville.
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