The underlying mechanisms by which dual chamber pacing is purported to improve outcome is straightforward; in patients with normal sinus rhythm, cardiac output is improved by 15 to 30 percent. In addition, a number of retrospective studies that have compared single chamber with atrial-based or dual chamber pacing have suggested that the latter may prevent adverse clinical events such as atrial fibrillation, congestive heart failure, cerebral vascular accidents, and death. While none of these studies was a randomized trial, the literature is consistent with a concept that dual mode pacing results in improved hemodynamics and a more favorable outcome in patients with sick sinus syndrome. However, the data do not provide definitive answers because of small sample sizes and methodological problems. A major problems with all previous studies is probable selection bias favoring implantation of dual chamber devices in younger, healthier patients.