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Heart. 1998 May; 79(5): 493–496.
PMCID: PMC1728677

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 electrocardiogram (saECG) and heart rate variability and investigate their relations in healthy subjects.
Methods—24 hour ECGs were obtained with a three channel recorder using bipolar X, Y, and Z leads in 20 healthy subjects. The following variables were determined hourly: heart rate, filtered QRS (f-QRS) duration, low and high frequency components of heart rate variability (LF and HF), and the LF/HF ratio.
Results—Heart rate, f-QRS duration, HF, and the LF/HF ratio showed significant circadian rhythms, as determined by the single cosinor method. Heart rate and the LF/HF ratio increased during daytime, and f-QRS duration and 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.

Keywords: signal averaged ECG;  f-QRS duration;  circadian rhythm;  heart rate variability

Figure 1
Circadian rhythm of the hourly means of heart rate (A), f-QRS duration (B), ln HF (C), and ln LF/HF (D) in 20 subjects. Solid lines represent curves fitted to the data by the single cosinor method. LF and HF, low and high frequency components, ...
Figure 2
Signal averaged electrocardiogram recordings from a healthy 24 year old woman at 05:00 (A) and at 11:00 (B). Note that the increase of the heart rate from 63 (A) to 76 beats/min (B) was associated with an increase in ln LF/HF ...
Figure 3
Correlations between the hourly means of the f-QRS duration and heart rate (A), ln HF (B), and ln LF/HF (C) in 20 subjects. LF and HF, low and high frequency components, respectively, of heart rate variability; LF/HF, LF to HF ratio.

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