Uterine vascular resistance during compression of the umbilical arterial and/or venous circulation in sheep

Abstract

Total umbilical cord occlusion and selective occlusion of the umbilical arteries and veins is associated with changes in uterine blood flow. In the present study, the resistance to uterine blood flow during selective occlusions of the umbilical arteries and/or veins was analysed in five chronically instrumented pregnant sheep in the last third part of pregnancy. An occluding device which allows separate occlusion of umbilical veins and arteries was applied to the umbilical cord. Median uterine artery blood flow was measured using an electromagnetic flow meter. Maternal pressures were measured in a branch of the uterine artery and vein. Two occlusions of the umbilical veins and/or arteries with a duration of 30–60 s were performed in each animal. Selective occlusion of the umbilical arteries resulted in a small increase in uterine blood flow from 637 ± 79 ml/min during control toward 664 ± 77 ml/min at the end of occlusion (p < 0.05). Uterine perfusion pressure (uterine arterial pressure — uterine venous pressure) did not change. No changes were observed in calculated uterine vascular resistance (Poiseuille equation). Selective occlusion of the umbilical veins on the other hand caused a decrease in uterine blood flow from 617 ± 75 ml/min during control to 546 ± 69 ml/min at the end of occlusion (p < 0.001), and a return to control value at 1 min after occlusion. The uterine perfusion pressure increased from 40.1 ± 6.8 mmHg during control to 42.8 ± 7.0 mmHg at the end of occlusion (p < 0.01). Uterine vascular resistance increased from 0.074 ± 0.017 mmHg · ml −1 · min before occlusion to 0.089 ± 0.0020 mmHg · ml −1 · min at the end of occlusion (p < 0.05). Occlusion of both umbilical arteries and veins simultaneously resulted in a decrease in uterine blood flow from 630 ± 66 ml/min during control to 589 ± 66 ml/min at the end of occlusion. Uterine perfusion pressure and uterine vascular resistance did not change, however. It is concluded that short-lasting occlusions of the umbilical veins affect the vascular resistance to maternal uterine blood flow.