Is narrowest QRS the best? A case of cardiac resynchronization therapy in a patient with left anterior fascicular block

The efficacy of cardiac resynchronization therapy (CRT) in patients with a narrow QRS duration has not been established. We present a patient with a narrow QRS duration and left anterior fascicular block in which CRT was effective. Left ventricular lead implantation at the optimal site and appropriately-timed left ventricular pacing (LVP) resulted in left ventricle reverse remodeling. Left ventricular dyssynchrony did not improve with LVP at a timing that resulted in narrower QRS than an intrinsic QRS duration. The optimization of LVP timing in CRT for patients with a narrow QRS duration is discussed.