Circadian variation of haemodynamics in patients with essential hypertension: comparison between early morning and evening

OBJECTIVE: An increased incidence of cardiovascular accidents in the morning has been reported, but the reason why is not clear. We measured 24-h haemodynamics and focused on its change in the morning. DESIGN: To study the circadian variation of haemodynamics, we recorded 24-h direct blood pressure and electrocardiogram using a telemetry method, in 21 untreated inpatients with essential hypertension, and measured cardiac output using the dye-dilution method in the morning, in the evening and during sleep. We also determined the beat-to-beat cardiac output (using the pulse-contour method), the total peripheral resistance and the ratio of low- to high-frequency components (using power spectral analysis of the R-R interval during 24 h), and made comparisons between morning and evening values. RESULTS: Both systolic and diastolic blood pressure increased rapidly in the early morning. Although the comparison of blood pressure between morning and evening showed no difference, total peripheral resistance and low- to high-frequency ratio were significantly higher in the morning than in the evening, but cardiac output was lower in the morning. CONCLUSIONS: Sympathetic nervous activity and vascular resistance seem to be higher in the morning than in the evening, and these haemodynamic changes may stress the cardiovascular system.