ABSTRACT
In New York Heart Association (NYHA) class III-IV patients with left ventricular (LV) ejection fraction (LVEF) 35%, QRS duration 120 ms, and sinus rhythm, cardiac resynchronization therapy (CRT) consistently improves cardiac function, quality of life, functional capacity, and survival.1-7 The effects of CRT, however, have not been thoroughly investigated in several patient populations that could potentially derive similar benefit. Early reports indicated that the benefit of CRT was greater in patients with non-ischemic than in those with ischemic cardiomyopathies. In more recent trials, however, CRT effects were independent of heart failure etiology.8 The evidence that CRT can partially reverse myocardial remodeling mediating heart failure progression was largely obtained in NYHA class III patients.1-7 It remains unknown if CRT can prevent heart failure progression in less symptomatic (NYHA class II) patients or meaningfully improve cardiac function in patients with advanced (NYHA class IV) heart failure. Enrollment in all major CRT trials required a QRS duration 120 ms.1-7 Therefore, the effects of CRT in patients with a narrow QRS but with evidence of mechanical dyssynchrony have not been explored in depth. Similarly, the
role of CRT in patients with atrial fibrillation (AF) remains largely unknown, because patients with this arrhythmia were excluded from most of the pivotal CRT studies.1,3-5,7 The vast majority of CRT trial subjects had a left bundle branch block (LBBB) QRS configuration, and therefore scant data exist on the effects of CRT in patients with a prolonged QRS duration but a right bundle branch block (RBBB) QRS configuration.1-7 Lastly, because right ventricular (RV) apical pacing produces a pattern of LV conduction similar to LBBB, the concern exists that persistent or even intermittent RV pacing may produce functional abnormalities similar to those induced by intrinsic LBBB.9,10 Few data exist on whether the addition of a LV pacing lead will negate the deleterious effects of apical RV pacing in implantable cardioverter-defibrillator (ICD) recipients with underlying LV dysfunction.11
