Future directions in cardiac resynchronization therapy.

Cardiac resynchronization therapy (CRT) reduces symptoms and improves mortality in patients with moderate to severe chronic heart failure (New York Heart Association class III-IV) in sinus rhythm with reduced left ventricular ejection fraction and with wide QRS on the surface electrocardiogram as evidence of ventricular dyssynchrony. CRT's benefit in patients with atrial fibrillation, mild heart failure, or a conventional indication for antibradycardia pacing is being assessed. Moreover, the PROSPECT trial assessed whether the response rate to CRT might increase if, in addition to wide QRS, mechanical dyssynchrony criteria were required. The RethinQ study examined whether patients with narrow QRS but with mechanical dyssynchrony benefit from CRT. Finally, whether patients with heart failure with preserved left ventricular function also may benefit from CRT remains to be studied.