ROLE OF POSTURAL STATUS IN THE NOCTURNAL HEMODYNAMIC PATTERNS OF NONHUMAN-PRIMATES

We compared the nocturnal hemodynamic patterns of seven tethered monke ys (Macaca mulatta) with those of seven chaired animals to determine w hether the overnight changes are comparable in the two conditions. In both groups, we found a consistent hemodynamic pattern characterized b y an overnight fall in cardiac output and central venous pressure and a rise in total peripheral resistance that maintained blood pressure h omeostasis. The pattern of overnight change occurred despite major dif ferences in response levels: cardiac output and central venous pressur e were significantly elevated, and total peripheral resistance was sig nificantly reduced at all times (from 1800 to 1200 h the following day ) in the chaired animals relative to the tethered animals. This differ ence was probably due to an expanded plasma volume in the chaired anim als, because stroke volume was also significantly elevated. Because th e nocturnal hemodynamic pattern occurred under both conditions, it is likely that it is a stable biologic effect, which is probably related to an overnight loss in fluid volume that is not replaced in animals t hat sleep throughout the night.