Cardiac resynchronization therapy (CRT) has been proven to be a major benefi t to severely symptomatic heart failure patients with widened QRS duration ³ 120 ms, and reduced ejection fraction (EF) ≤ 35%. The prevailing evidence supports that multi-site pacing improves abnormalities of left ventricular (LV) mechanical activation, known as dyssynchrony, as the principal therapeutic effect of CRT. The most common pattern of dyssynchrony is represented by left bundle branch block (LBBB), which is characterized by early septal mechanical activation followed by delayed posterior and lateral wall activation. Although several variations in abnormalities of electrical and mechanical activation may exist, the electrical dispersion manifest by QRS widening is believed to be a surrogate for mechanical dyssynchrony. Current clinical guidelines based on selection criteria used in the CRT trials use QRS widening as a marker for dyssynchrony. Unfortunately, approximately one-third of patients receiving CRT do not seem to benefi t using these routine clinical selection criteria. Since CRT implantation is expensive and associated with a small but signifi cant procedural risk of complications, efforts have been focused on improvement in patient selection. The study of echocardiographic dyssynchrony has led to the important observation that there is a subset of patients with widened QRS duration that do not have signifi cant mechanical dyssynchrony. The reasons for the disassociation of mechanical dyssynchrony from QRS widening are unclear, but most existing information indicates that the patients with QRS widening who lack signifi cant mechanical dyssynchrony do not respond favorably to CRT and appear to have a worse prognosis.
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