CIRCADIAN-RHYTHM OF THE SIGNAL-AVERAGED ELECTROCARDIOGRAM AND ITS RELATION TO HEART-RATE-VARIABILITY IN HEALTHY-SUBJECTS

Objective-To examine the circadian variation in the signal averaged el ectrocardiogram (saECG) and heart rate variability and investigate the ir relations in healthy subjects. Methods-24 hour ECGs were obtained w ith a three channel recorder using bipolar X, Y, and Z leads in 20 hea lthy subjects. The following variables were determined hourly: heart r ate, filtered QRS (f-QRS) duration, low and high frequency components of heart rate variability (LF and HF), and the LF/HF ratio. Results-He art rate, f-QRS duration, HF, and the LF/HF ratio showed significant c ircadian rhythms, as determined by the single cosinor method. Heart ra te and the LF/HF ratio increased during daytime, and f-QRS duration an d HF increased at night. f-QRS duration was negatively correlated with heart rate (r = 0.95, p < 0.001) and the LF/HF ratio (r = 0.94, p < 0 .001) and positively with HF (r = 0.93, p < 0.001). Conclusions-f-QRS duration has a significant circadian rhythm in healthy subjects and is closely related to the circadian rhythm of autonomic tone.