Cardiac resynchronization therapy: from creation to evolution--an evidence-based review.

In the past decade, cardiac resynchronization therapy (CRT), achieved by simultaneous left and right ventricular pacing, has emerged as a potent therapeutic option for patients with congestive heart failure. Electrical dyssynchrony, most often manifested by left bundle branch block on the surface 12-lead electrocardiogram, results in mechanical dyssynchrony of the left ventricular septum and free wall, which decreases cardiac efficiency. In patients with ejection fractions <30%, New York Heart Association (NYHA) class III or IV, and QRS width >120 ms, CRT improves clinical parameters such as 6-minute walk distances, quality-of-life scores, and NYHA functional class. Long-term reverse remodeling of the failing ventricle results in reductions in congestive heart failure hospitalizations and mortality independent of defibrillator therapy. While most patients show significant improvement, a small proportion fail to respond. Appropriately identifying patients who will benefit most from CRT and timing the initiation of resynchronization therapy remain areas of intense investigation.