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.