QTc measurement
The methods of computerized QT analysis and their reproducibility have been described in detail previously.9,10 For this study we used a 3 lead Lifecard CF (OSI systems) Holter and data were analyzed using the Spacelab Pathfinder SL software. The Bazett formula was used to obtain heart rate-corrected values of parameters of QTc interval. Mean QTc was given hourly. All measurements were adjudicated by electrophysiology fellow (AY) and by internal Emergency Medicine resident (NN). If a change between two consecutive measurement [(QTc second – QTc first)/QTc first]*100 was >10%, a manual validation/correction was performed. Furthermore, manual adjustment was made for patients with pacemaker during pacing periods. In order to minimize the errors that may occur during short tachy-arrhythmic events, or secondary to diurnal variability, the median of every 4 hours values was used. Accordingly, each patient had 6 measurement per day. Examples are listed in the online data supplement, eFigure B. In order to validate the results further, 10 random Holter strips were printed and evaluated blindly by a senior electrophysiologist (RB), and a fellow (AY), and the results were compared with the computerized results. The matching rate was 99.7%. Examples are listed in the online data supplement, eFigures C-D.