Circadian variation of hemodynamics and baroreflex functions in patients with essential hypertension.

It is well known that cardiovascular accidents such as myocardial infarction frequently occur in the morning, but their triggering mechanisms are not clear. The present study investigated circadian variations of hemodynamics and baroreflex functions. Twenty-three patients with essential hypertension were studied. Direct blood pressure (BP) and ECG were recorded by telemeter over 24 h, and then computer-analyzed. The pulse-contour method was used to measure cardiac output (CO) and total peripheral vascular resistance (TPR). The ratio of low to high frequency components (LF/HF) of the RR-interval on ECG was calculated by power spectral analysis. The baroreflex sensitivity index (BRI) was measured on the basis of the ratio delta RR/delta Ps (delta Ps = spontaneous decrease in systolic BP, delta RR = change in RR). Furthermore, 24-h BP changes were transformed algebraically into positive load component (PC) and negative load component (NC) by using a Windkessel model. The circadian variation of hematocrit (Ht) was also measured. The least squares method was used to determine the time at which the maximum and minimum value of each measurement occurred. Whereas the maximum values for BP and CO occurred in the evening (18:30, 17:00), the maximum values for TPR and LF/HF occurred between 06:30 and 08:00, and the minimum value for BRI occurred at 08:00. PC significantly correlated with Ps, heart rate, and CO (r = 0.81, 0.92, 0.67), and NC significantly correlated with BRI and LF/HF (r = 0.71, 0.64). PC (related to cardiovascular function) reached a maximum and NC (related to baroreflex function) reached a minimum in the late morning (11:00). Ht was highest immediately after the subjects got out of bed. These hemodynamic imbalances may negatively influence coronary blood flow in the morning.