The Mode Selection Trial (MOST) in sinus node dysfunction: Design, rationale, and baseline characteristics of the first 1000 patients

Background More than 200,000 permanent pacemakers will be implanted in the United States in 2000 at a cost of more than $2 billion. Sick sinus syndrom e (SSS) will likely account for approximately half of all cases necessitati ng implantation. Pacemaker technology permits the selection of ventricular (single-chamber) or dual-chamber devices. However, clinical and outcomes da ta are inadequate to support a clear recommendation that one or the other t ype of device be used. Methods The Mode Selection Trial (MOST) is a single-blind study supported b y the National Heart, Lung, and Blood institute designed to enroll 2000 pat ients with SSS. All patients will receive a DDDR pacemaker programmed to WI R or DDDR before implantation. The average time of follow-up will be 3 year s. MOST has a >90% power to detect a 25% reduction in the primary end point -nonfatal stroke or total (all cause) mortality-in the DDDR-treated group. Secondary end points will include health-related quality of life and cost e ffectiveness, atrial fibrillation, and development of pacemaker syndrome. P respecified subgroups for analysis will include women and the elderly. Enro llment was completed in October 1999, with a total of 2010 patients. Results The median age of the first 1000 enrolled patients is 74 years, wit h 25% of patients 80 years or older. Women comprise 49%, and 17% are nonwhi te, predominantly black (13%). Before pacemaker implantation, 22% of patien ts reported a history of congestive heart failure, 11% coronary angioplasty , and 25% coronary bypass surgery. Supraventricular tachycardia including a trial fibrillation was present in 53% of patients. A prior stroke was repor ted by 12%. Antiarrhythmic therapy was in use in 18% of patients. Conclusions MOST will fill the clinical need for carefully designed prospec tive studies to define the benefits of dual-chamber versus single-chamber v entricular pacing in patients with SSS. The MOST population is typical of t he overall pacemaker population in the United Slates. Thus the final result s of MOST should be clinically generalizable.