2.4 Electrocardiographic measurements
Baseline patient characteristics were collected retrospectively. The surface electrogram was recorded by the GE CardioLab Electrophysiology recording system (GEHealthcare Inc, Marlborough, MA) at 100 mm/s. We measured two parameters, which are the intrinsic QRS duration (iQRSd) and the paced QRS duration (pQRSd). The iQRSd was defined from the first deviation from isoelectric line to the last deviation of the QRS complex in the 12 leads. The pQRSd was measured - for SLBBP - from the onset of the first sharp vector of the QRS complexes crossing the baseline and - for NSLBBP - from the onset of the sharpest deflection to the end of the last deflection of the complex in the 12 leads, respectively. All two of these parameters were measured by Cardio Calipers 3.3 (Inconico, Inc, New York, NY). Three continuous QRS complexes on the lead V1 were measured and the averaged values were reported. The electrocardiographic recording and measurements were performed and analyzed by two independent and experienced ECG specialists.