Original Article

Cardiac resynchronization therapy (CRT) has been shown conclusively to reduce morbidity and mortality in many patients with heart failure (HF) and prolonged QRS dura-tion.1–4 However, between 30 % and 50 % of patients have lim-ited or no response to this therapy.5,6 Potential explanations for this include insufficient left ventricular (LV) mechanical dyssynchrony7,8 and excess LV scaring.9,10 Previous studies have shown that a left bundle-branch block (LBBB) pattern is a strong predictor of outcomes compared with the pres-ence of other ECG morphologies.11–19 Several other key clini-cal questions remain, and more data are required to inform these issues.17,20,21 Patients with narrower QRS durations seem to have a lesser response, but is there a QRS duration cut point which can be used to guide patients selection?17 Is the cut point different for different QRS morphologies?17 Is there any role for CRT in patients with right bundle-branch block (RBBB)13,22 and nonspecific intraventricular conduction delay (NIVCD)? The impact of QRS morphology and dura-tion on the effectiveness of CRT has been usually assessed separately.17 However, a few studies have begun to examine these issues together.19,23,2