Mode Selection Trial in Sinus Node Dysfunction (MOST)

BACKGROUND:

Permanent pacing is estimated to cost one billion dollars annually in health care costs in the United States. Initially, pacing was primarily confined to ventricular pacing with limited sensing, programming and pacing capacity. Tremendous growth has occurred in pacing technology, making available dual chamber pacing with sophisticated sensing, pacing, and rate control. These more advanced pacemakers are more costly and complicated to place surgically.

One of the most common indications for pacing is sick sinus syndrome. Initial therapy is usually medical to inhibit the tachyarrhythmias (most commonly paroxysmal atrial fibrillation). However, if symptomatic bradycardia results, then permanent pacing is commonly employed. The appropriate type of pacing in this setting is not clearly defined and is controversial.

The development of atrioventricular pacing was principally aimed at improving cardiac hemodynamics and creating a more physiological heart rate control. Hemodynamic studies have clearly shown the benefit of this approach in many patients, particularly those with decreased left ventricular compliance in whom atrial activity contributes significantly to cardiac output. Lack of synchronization between the upper and lower chambers of the heart caused by pacing the ventricle alone can result in a constellation of symptoms commonly referred to as "pacemaker syndrome".

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.