Hemodynamic effects of caval and uterine venous occlusion in pregnant sheep

In chronically instrumented pregnant ewes we studied the systemic, uterine, and fetal effects of increased uterine venous pressure with and without simultaneous changes in cardiac output, by occluding either the inferior vena cava or both uterine veins. Increased uterine venous pressure with no reduction in venous return of blood to the heart resulted in a reflex increase in arterial blood pressure. Infrarenal obstruction of the inferior vena cava resulted in a moderate reduction in venous return and cardiac output with no change in arterial blood pressure, while suprarenal obstruction of the inferior vena cava caused a major decrease in cardiac output as well as in arterial pressure. Uterine blood flow varied in proportion with perfusion pressure at all levels of obstruction. Uterine oxygen consumption and fetal oxygenation only decreased with suprarenal obstruction of the inferior vena cava, when uterine blood flow fell more than 50%.