Left ventricular (LV) dyssynchrony is a pathological condition in which segments of the myocardial wall contract at different times. This dyssynchrony results in a decreased LV ejection fraction (EF) and an increased level of mitral regurgitation. LV dyssynchrony has been linked to higher rates of morbidity, mortality, and arrhythmic susceptibility in patients with congestive heart failure. Cardiac resynchronization therapy (CRT) with biventricular pacemakers has benefited patients with drug-refractory heart failure and signs of ventricular dyssynchrony. Patients are currently selected for CRT therapy if they have a prolonged QRS complex (> 120 msec) on a surface electrocardiogram as well as an EF of less than 35%. However, recent data suggests that these criteria are insufficient, as 30% of patients do not respond to CRT treatments.

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