Diurnal variations in cardiovascular integration

Beat-to-beat heart periods and intra-arterial blood pressures were monitored continuously for 18 h in each of six monkeys over periods of several weeks. Means, based on 128-s averages of these functions, were highly correlated during early evening and daytime periods; however, heart period was poorly correlated with blood pressure during the evening hours, although systolic and diastolic pressures were highly correlated, and the standard deviations of heart period also were highly correlated with the pressures. These shifts in cardiovascular integration were unexpected and suggest that the circulation is relatively sensitive to the conditions under which it is being assessed, and they suggest further that response relationships observed under one set of conditions may not be comparable to those measured under another set of conditions.