['QRS duration (QRSd) and prolonged corrected QT interval (QTc) are associated with ventricular arrhythmic events. This study was designed to determine whether CRT by means of biventricular pacing alters the QTc and QRSd, and whether such changes are related to the risk of sustained ventricular tachyarrhythmias (sVTA).', 'A total of 127 patients (102 men, mean age 63.9 +/- 8.9 years) with drug-resistant heart failure and QRS duration > or = 130 ms underwent CRT/CRT-ICD. The aetiology of the heart failure was ischaemic in 41 patients (32.3%). After a median follow-up of 24 months, 42 sVTA occurred in 35 patients (27.6%). Twenty-nine patients had a single sVTA, in five patients two sVTA and in one patient three sVTA occurred. The paced QTc was longer in sVTA patients (505 +/- 55 ms) compared with no sVTA patients (486 +/- 44 ms, P < or = 0.003). Similar responses for paced QRSd were observed (182 +/- 27 ms in sVTA patients vs 167 +/- 27 ms in no-sVTA patients, P < or = 0.03). This effect was independent from intrinsic QTc and QRSd and the aetiology of the heart failure. The mortality rate was significantly higher in patients with ventricular fibrillation and fast VT (P < or = 0.004) who experienced shock therapies. However, the sVTA were not the immediate cause of death.'] Are qRS duration and QTc interval predictors of risk for ventricular arrhythmias during cardiac resynchronization therapy?