Holter ECG
A digital ECG recording device (Kenz Cardy 303 pico+; SUZUKEN Co., Ltd., Nagoya, Japan) with a sampling rate of 125Hz was used with an automatic measurement system (Kenz Cardy Analyzer 05®; SUZUKEN Co., Ltd., Nagoya, Japan). Consecutive sinus beats every 15 seconds were averaged, and each parameter was measured. The rate-corrected QT interval (QTc interval) was determined according to the Bazett formula. The QTe interval was defined as the time between the QRS onset and the point at which the isoelectric line intersected a line tangent to the maximal (or minimal) downslope of the positive (or negative) T-wave. The QTa interval was defined as the time between the QRS onset and the apex (or nadir) of the T wave (Figure 1 ). The linear regression slopes of the QTa interval and the QTe interval plotted against RR intervals (QTa/RR and QTe/RR slopes, respectively) were calculated by the least-squares method. The QTe-QTa interval was defined as the time between the QT apex and the QT end, and was also plotted against RR intervals (QTe-QTa/RR slope). These data were compared between a non-sleep period (daytime) and an actual sleep period (night time).
The study protocol was approved by the Institutional Review Board of each participating institution, and written informed consent was obtained from all patients.