Effects of biorhythms on blood flow distribution in the pregnant uterus (Macaca mulatta).

Ten rhesus monkeys were studied between 143 and 161 days of gestation. The unanesthetized animals were confined to restraining chairs and maintained in a controlled environment with continuous monitoring of spontaneous uterine dynamics. Forty-five determinations of blood flow distribution were made using radionuclide-labeled microspheres. The microsphere injections confirmed a circadian pattern of uterine blood flow. These biorhythms are related to alterations in distribution of blood flow to placental and nonplacental portions of the uterus. Myometrial blood flow is highest during the period of light. It is correlated positively to aortic blood pressure (r = 0.490, p less than 0.05) and inversely to uterine artery blood flow (r = -0.508, p less than 0.05). Placental blood flow is highest during the period of darkness. The flow rates are significantly correlated to intra-amniotic pressure (r = -0.602, p less than 0.05) but not to aortic blood pressure (r = -0.185, p less than 0.05). The data indicate that blood flow distribution in the primate uterus is modulated by factors in addition to physiologic pressure-flow relationships. Extrapolation to the human has potential significance.