Abstract & Commentary
Synopsis: Right ventricular apical pacing contributes to heart failure hospitalization and atrial fibrillation in patients with sinus node dysfunction. Ventricular desynchronization caused by right ventricular pacing is responsible for this phenomenon.
Source: Sweeney MO, et al. For the MOde Selection Trial Investigators. Circulation. 2003;107:2932-2937.
Sweeney and colleagues from the Mode Selection Trial (MOST) report on the effects of ventricular pacing in patients with sinus node dysfunction. MOST is a prospective randomized comparison of single-chamber ventricular rate modulated pacing (VVIR) vs dual-chamber rate modulation pacing (DDDR) in patients with sinus node dysfunction. In both groups, the lower rate limit was programmed to at least 60 bpm with an upper rate limit of at least 110 bpm. In the DDDR group, the programmed AV delay was suggested to be between 120 and 220 msec. The primary end point was mortality, and an earlier report had shown no difference between the groups. The frequency of heart failure hospitalizations and the documented occurrence of atrial fibrillation were secondary end points used in the study. At each follow-up clinic visit, the mean percent of ventricularly paced beats over all visits was calculated. A similar calculation was done for atrial pacing in the DDDR group. In MOST, the average age was 73, and there were equal numbers of males and females. The mean left ventricular ejection fraction was 55%, and most patients had mild or no symptoms of congestive heart failure at baseline. Prior atrial arrhythmias had been noted in 54% of the patients. The median cumulative percent of ventricular pacing was significantly higher in the DDDR group vs the VVIR group (90% vs 58%; P = .001). Approximately half of the patients in the DDDR group were ventricularly paced either continuously or near continuously (greater than 90% of the time) compared with only 20% in the VVIR group. There were 1339 patients in MOST who had a baseline QRS duration of less than 120 msec. The influence of cumulative percent of ventricular pacing on heart failure hospitalization was analyzed in this subgroup.
The overall rate of heart failure hospitalization in the 2 pacing modes was similar (10% DDDR, 9% VVIR). However, in the DDDR mode, the risk of heart failure hospitalization increased as the percentage of ventricular pacing increased. Ventricular pacing greater than 40% of the time in the DDDR mode was associated with a 2.6-fold increased risk of an increase in heart failure class compared with pacing less than 40% of the time. The slope of increasing risk was relatively flat above 40% paced beats. In the VVIR mode, the risk was level between 0% and 80% ventricular pacing, with a sharp increase in heart failure risk in those paced more than 80% of the time. When the influence of ventricular pacing on the risk for heart failure hospitalization was adjusted for a history of prior heart failure, the ejection fraction, and use of antiarrhythmic therapy, the hazard ratio was lower but still significant.
A similar analysis was performed for development of atrial fibrillation. The risk for developing atrial fibrillation increased in both pacing modes in parallel with increases in the cumulative percent of ventricular pacing. The overall rate of atrial fibrillation was slightly higher in the VVIR group (24%) vs the DDDR group (21%). There was an increasing risk for atrial fibrillation from 0% up to about 80% or 85% ventricular pacing in both pacing modes. A life-table analysis showed that the risk for atrial fibrillation occurred early in the VVIR mode and somewhat later in the DDDR group.
Sweeney et al conclude that right ventricular apical pacing contributes to heart failure hospitalization and atrial fibrillation in patients with sinus node dysfunction. They postulate that ventricular desynchronization caused by right ventricular pacing is responsible for this phenomenon.
Comment by John DiMarco, MD, PhD
When first introduced, dual-chamber pacing was meant to represent a more physiologic form of pacing than single-chamber ventricular pacing. Current guidelines recommend dual-chamber or atrial pacing in patients with sinus node dysfunction. In the United States, dual-chamber pacing is more commonly used than atrial pacing with a single-chamber device. In controlled clinical trials, however, it has been difficult to show that dual-chamber pacing is superior to single-chamber ventricular pacing in patients with sinus node dysfunction. In contrast, AAI(R) pacing has been shown to be superior to VVI(R) pacing in these patients in well-controlled studies. The observation that right ventricular apical pacing induces ventricular dyssynchrony is an explanation for these observations. This study from a trial looking at DDDR vs VVIR pacing in patients with sinus node dysfunction offers further insights. The risk of heart failure hospitalization and atrial fibrillation are related to the proportion of right ventricularly paced beats. This observation was made even though this patient group had a low baseline incidence of symptomatic heart failure and systolic dysfunction.
There are ways to minimize the negative hemodynamic effects of right ventricular pacing. Certainly, single-chamber atrial pacing would be effective. If patients with abnormal AV conduction are excluded at baseline, the risk of progression to heart block is low and atrial pacing should be the preferred mode. If there is some risk of AV block, new pacing algorithms available in some models allow adaptations in the AV interval that allow normal AV conduction to occur without risking prolonged first-degree or higher-grade AV block.
Another solution to the problem of dyssynchrony caused by right ventricular pacing is the use of biventricular pacing. However, biventricular pacing devices require a more complicated implant procedure, and late problems with the left ventricular pacing lead are common. Therefore, biventricular pacing should still be reserved for patients who need ventricular pacing and are likely to develop heart failure with right ventricular pacing alone.
Dr. DiMarco, Professor of Medicine, Division of Cardiology, University of Virginia, Charlottesville, is on the Editorial Board of Clinical Cardiology Alert.