Circadian profile of systemic hemodynamics.

We determined the continuous 24-hour profile of mean arterial pressure, heart rate, stroke volume, cardiac output, and total peripheral resistance in eight healthy ambulatory volunteers. Beat-to-beat intra-arterial blood pressure was recorded with the Oxford system; subjects were ambulant during daytime and slept at night. Beat-to-beat stroke volume was determined by the pulse contour method from the arterial pulse wave. During the nighttime, compared with the daytime average, there was a decrease in blood pressure (9 mm Hg), heart rate (18 beats per minute), and cardiac output (29%); stroke volume showed a small decrease (7%), and total peripheral resistance increased unexpectedly by 22%. When subjects arose in the morning a steep increase in cardiac output and decrease in total peripheral resistance were found. Comparable changes were seen during a period of supine resting in the afternoon, whereas physical exercise caused opposite changes in hemodynamics. This pattern was observed in all subjects. We conclude that the circadian pattern of cardiac output and total peripheral resistance originates from the day-night pattern in physical activity: during the nighttime, blood flow to the skeletal muscles is decreased through local autoregulation, which increases total peripheral resistance and decreases cardiac output compared with the daytime.