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 resi stance in eight healthy ambulatory volunteers. Beat-to-beat intra-arte rial blood pressure was recorded with the Oxford system; subjects were ambulant during daytime and slept at night. Beat-to-beat stroke volum e was determined by the pulse contour method from the arterial pulse w ave. During the nighttime, compared with the daytime average, there wa s a decrease in blood pressure (9 mm Hg), heart rate (18 beats per min ute), and cardiac output (29%); stroke volume showed a small decrease (7%), and total peripheral resistance increased unexpectedly by 22%. W hen subjects arose in the morning a steep increase in cardiac output a nd decrease in total peripheral resistance were found. Comparable chan ges were seen during a period of supine resting in the afternoon: wher eas physical exercise caused opposite changes in hemodynamics. This pa ttern was observed in all subjects. We conclude that the circadian pat tern of cardiac output and total peripheral resistance originates from the day-night pattern in physical activity: during the nighttime, blo od flow to the skeletal muscles is decreased through local autoregulat ion, which increases total peripheral resistance and decreases cardiac output compared with the daytime.