Obstruction of Uterine Venous Return

Obstruction of the inferior vena cava by the gravid uterus has been known as the cause of the “supine hypotensive syndrome of pregnancy” since at least 1953. But the occurrence of this syndrome is highly variable. Some women experience it, while many do not. This variability may have to do with 1) variations in the collateral lumbar, vertebral venous, and azygos venous return pathways which bypass the inferior vena cava and 2) variability in the response of the aortic arch and carotid sinus baroreceptors and sympathetic nervous system to maintain adequate maternal blood pressure despite a reduction in cardiac output. This maintenance of blood pressure by cardiovascular compensation—when it occurs—is achieved by an increase in systemic vascular resistance. Obstruction of the inferior vena cava in the absence of adequate collateral pathways for venous return obstructs venous return from the lower half of the body and reduces cardiac output. But even more significant for the well-being of the fetus, placenta and myometrium may be the fact that such obstruction decreases intervillous and myometrial perfusion and increases uterine venous and intervillous space pressure. This reduction in perfusion and increase in pressure may impair fetal, chorionic villous, and myometrial oxygenation much earlier in gestation than has been previously considered, contributing to the development of preeclampsia, fetal growth restriction, preterm birth, dysfunctional labor, and uterine atony. Since venous return equals cardiac output in a steady state, positional decreases in maternal cardiac output can be used to detect and prevent obstruction of uterine venous return and any harmful consequences of that obstruction. Research should be undertaken to evaluate whether “cardiac output-guided maternal positioning” during the second half of pregnancy might improve pregnancy outcomes.