Introduction: Cardiac resynchronization therapy (CRT) is a well-established treatment for heart failure (HF) patients with a wide (>120 msec) QRS complex. Despite not meeting current guidelines, some narrow QRS HF patients with mechanical dyssynchrony receive CRT. The effects of CRT on cardiac function, HF symptoms, and outcomes are not clearly known in these patients. Methods: All consecutive CRT recipients between 2003 – 2008 with ejection fraction (EF) ≤ 35% and New York Heart Association (NYHA) class III or IV were studied. There were 146 and 492 patients with narrow and wide QRS complex, respectively. Eighty-five narrow QRS patients in sinus rhythm with suitable images were matched by age and gender to wide QRS patients for a multi-plane tissue tracking and speckle-tracking echocardiographic analysis.
Results: Narrow QRS patients were younger and had less radial dyssynchrony at baseline. Increases in EF of 3.2 ± 9% and 6.8 ± 9% (p<0.05) were seen in narrow and wide QRS patients, respectively. Wide QRS patients had decreased left ventricular size (p<0.01) and increased longitudinal systolic function (p=0.04), while narrow QRS patients had decreased delayed longitudinal contraction (p<0.01), while tending to decrease longitudinal dyssynchrony (p=0.076) and increase radial strain (p=0.086). Both groups improved diastolic function (p<0.01), and had a similar proportion of patients with improved clinical symptoms (p=0.17). Five-year mortality rates were 40% and 46% in the narrow and wide QRS groups respectively (p=0.234); however, wide QRS patients tended to have a more favorable survival free of cardiovascular hospitalization (p=0.056). Conclusion: Narrow QRS HF patients with mechanical dyssynchrony respond favorably to CRT, but to a lesser extent than wide QRS patients.