Inferior Vena Caval Pressure in Late Pregnancy

Abstract

THAT the venous pressure in the lower limbs is raised in late pregnancy has been shown by many workers. Two main theories have been advanced to explain this; first, simple mechanical obstruction to the venous return by the gravid uterus (McLennan, 1943) and, second, increased blood flow resulting from the placenta in situ (Burwell, 193 8).Most of these studies were carried out using the femoral venous pressure. This indicates changes in pressure occurring below the uterus, but does not throw light upon the exact site of venous obstruction (if any). Moreover, the great majority of investigations used water manometry. This method, though simple, cannot show rapid pressure changes that may occur and also requires the use of large bore needles. Modern techniques using pressure transducers are extremely sensitive to rapid changes in pressure, require only very fine catheters and allow continuous recording over long periods. The catheters can be inserted percutaneously into the femoral vein and advanced into the inferior vena cava, thus indicating the pressure at various levels up to the right atrium.In carrying out our investigations we were immediately impressed by the fact that the pressure within the lower vena cava in advanced pregnancy was raised in the supine position to a level comparable with that obtained by complete vena caval occlusion in non-pregnant women (Scott, 1963). Moreover, the pressure was raised to some extent throughout the length of the inferior vena cava, only becoming normal above the diaphragm. These results indicated that the venous return from the abdomen and lower limbs was of a very different pattern from that of the non-pregnant state.

Uterine hemodynamics and fetal response to vena caval occlusion in sheep

“…Due to the reduced venous return to the heart maternal BP drops steadily leading to a further slight decrease in UBF. The rise in uterine vein pressure is in consistency with the findings in humans [15]. The influence of V. C. O. on mean arterial blood pressure is of diflerent kind (Fig.…”

“…The same findings were reported in humans. SCOTT and KERR [15] found in pregnant women undergoing caesarean section by V. C. O. a decrease in cardiac Output and no change in maternal blood pressure. In women lying in supine position there is also a variable change in systemic arterial pressure.…”

“…For this fact exists no real explanation. One may speculate if it is due to the variable efficency of the collateral venous channels of the ' · N uterus or due to a para-sympathetic response äs suggested by KERR [15]. The decrease in venous blood returning to the maternal heart results in pregnant women undergoing a caesarean section or turning from lateral to supine position in a decrease in cardiac Output and an increase in total peripheral resistance (TPR).…”

Effect of hypotonic expansion on sodium, water, and urea excretion in late pregnancy: the influence of posture on these results

“…The decrease in filtered sodium has been mentioned as a causal factor (33), but in this study volume expansion before changing position resulted in a number of instances in which supine recumbency was antinatriuretic despite an increased filtered load. The increase in caval pressure induced by supine recumbency has also been implicated (31,(34)(35)(36), since in man congestion of the inferior vena cava induced by an inflatable balloon consistently reduced urinary sodium excretion (37). We have no data to support or deny this sug-'Lindheimer, M. D., and P. V. Weston.…”

The carotid sinus baroreceptor reflex in the pregnant rabbit

“…Although the rabbits were lying supine, it is unlikely that the gravid uterus would have occluded the inferior vena cava in the rabbit as it does in women (Scott & Kerr, 1963; Lees, Scott, Kerr & Taylor, 1967), since in the rabbit the uterus is bicornuate and in the supine position the weight of the fetuses does not press directly on the great vessels. Nevertheless, in case some obstruction might have impaired the reflex response to reduced sinus pressure, the responses of several animals were tested when they were lying both on their side and supine.…”