Circadian variations of systolic time intervals.

In order to test the reality of a circadian evolution of systolic time intervals, an experiment was conducted on six subjects resting in a supine position for 24 h. Heart rate (HR), systole (S), diastole (D), presystole (PS), isovolumetric contraction time (IVCT), normal and corrected ventricular pre-ejection and ejection time (LPEP, LPEPc, LVET, LVETc) were computed by electric rheoplethysmography every 3 h in supine and sitting positions. Evidence of the existence of a circadian rhythm was clear in supine subjects; especially LPEP and LVET acrophases were, respectively, 4.03 +/- 2.36 h and 4.31 +/- 1.15 h. In sitting subjects, a circadian rhythm was evidenced for IVCT, LVETc and LPEP/LVET, and on the difference of resting-sitting for PS, IVCT and LPEP, suggesting a decrease in orthostatic tolerance at the end of the night. However, values for characteristics of circadian rhythms (mean, amplitude, phase) strongly depend upon experimental conditions (position, activity, diet). A phase difference between systolic times and HR was also observed. This phenomenon was due to an advance in phase of D relative to S. Finally, all these results show that it is always necessary to take circadian factors into consideration during cardiovascular studies, especially for the preparation of protocols and the interpretation of results.