Stephansen C, et al. - Given the predictive value of QRS narrowing following CRT for patient outcome and that interventricular pacing delay (VVd) optimization can narrow it further, researchers focused on inter and intraobserver differences in manual measurements of QRS duration. In 40 intrinsic 12-lead ECGs, QRS duration (defined as widest QRS in any lead) was assessed by six observers and in 20 of these ECGs, the measurements were repeated by two observers. Recording of five paced ECGs was done at different VVds in 20 CRT patients. The optimal VVd was defined as the most often selected VVd(s) that projected to cause the narrowest QRS in each patient. The measurements and VVd selections were repeated by two observers. Findings indicated clinically satisfactory and similar interobserver and intraobserver disparities in manually measured paced and intrinsic rhythm QRS duration in a cohort of CRT patients. Given the good inter and intraobserver reproducibility for selecting the optimal VVd, manual VVd optimization for QRS narrowing in CRT is justified.