Stability over time of circadian rhythm of variability of heart rate in patients with stable coronary artery disease.

Reproducibility of circadian rhythm of variability in heart rate was studied in 40 patients with stable coronary artery disease who underwent 48-hour ambulatory electrocardiographic recordings at baseline (time 1) and after 4 months (time 2). The standard deviation of the R-R interval and the low-frequency (0.04 to 0.15 Hz) and high-frequency (0.15 to 0.45 Hz) components of variability in heart rate were assessed every 5 minutes. In 35 patients a significant circadian rhythm was observed at both time 1 and time 2 in the standard deviation of the R-R interval, with the acrophase occurring at around 5:00 AM, in the high-frequency amplitude with the acrophase around 3:00 AM, and in the low-frequency/high-frequency ratio with the acrophase around noon. In these patients, parameters of circadian rhythm (mesor, amplitude, and acrophase) showed good within-individual reproducibility with an intraclass correlation coefficient of 0.63 to 0.95 (p < 0.001 for all). In the patients who showed inconsistency about the significance of circadian rhythm between time 1 or time 2, the amplitude of circadian rhythm, even if significant, was found in the lowest five values in the distribution. We conclude that the circadian rhythms of cardiac autonomic activity are stable over time within individual patients with stable coronary artery disease.