Some Effects of Maternal Hyperoxia and Hypoxia on the Blood Gas Tensions and Vascular Pressures in the Foetal Sheep

Abstract

Blood gas tensions, pH and intravascular pressures have been measured at various sites in 22 foetal sheep of gestational age 110-147 days. Umbilical venous oxygen tension did not change significantly with age. The mean pulmonary artery pressure always exceeded that in the aorta and this pressure gradient did not change with age. By contrast the umbilical arterio-venous pressure gradient increased with age. When the ewe breathed room air, the oxygen difference between uterine artery and umbilical vein was 43-62 mm. Hg. When the ewe breathed 100 per cent 02 umbilical venous Po2 rose by 4-15 mm. Hg and umbilical venous pH fell by 0 04-0 07 pH units.There were no consistent changes in foetal heart rate or blood pressures.When the ewe breathed 10 per cent 02 in N2, umbilical venous Po. fell by [7][8][9][10][11][12][13][14] mm. Hg, mean pressures in pulmonary artery and aorta and the pulmonary artery/ aortic pressure gradient increased as did the size of the right to left shunt across the ductus arteriosus. Heart rate invariably increased. The oxygen saturation difference between ascending and descending aorta was maintained or increased.These results suggest (1) that factors other than a diffusion barrier to oxygen are responsible for the maternal/foetal oxygen gradient and (2) that, with maternal hypoxia, there is a redistribution of blood flow in the foetus so that as high a tissue Po, as possible is maintained in coronary and cerebral circulations.

PO2, PCO2 and pH levels in the umbilical and uterine blood of the mare and ewe

“…The relatively small changes in Po2 in both uterine and umbilical venous blood during oxygen administration to the ewe reported in both past 596 PLACENTAL EXCHANGE IN MARE AND EWE (Parker & Purves, 1967) and present experiments, were found during the latter part of gestation. However, between 100 days and term the placental circulation is fully developed and uterine blood flow, expressed in terms of unit weight of the uterus and its contents, does not change markedly, whereas in early gestation the cotyledons are small or even rudimentary (D. H. Steven, unpublished observations) and the rate of uterine blood flow per kg uterus is higher than at any other time in gestation (Huckabee, Metcalfe, Prystowsky &; Barron, 1961 Fig.…”

“…The majority of evidence indicates in the sheep that a large and stable Po2 gradient exists between the uterine vein and the umbilical vessels (Barron, 1951;Parker & Purves, 1967). Furthermore, changes in maternal arterial Po0 have little effect on foetal or uterine venous Po0.…”

“…Furthermore, changes in maternal arterial Po0 have little effect on foetal or uterine venous Po0. Thus foetal hypoxia occurs only after a marked reduction in the maternal arterial blood Po2 and attempts to raise the foetal Po2 have met with little success despite the administration of pure oxygen to the mother (Comline, Silver & Silver, 1965;Kirschbaum, Lucas, De Haven & Assail, 1967; Acheson, Cook & Reynolds, 1967;Parker & Purves, 1967). The Po2 gradient across the placenta is still present even when the foetus is supplied with oxygen by other means (Campbell, Dawes, Fishman, Hyman & James, 1966).…”

Effects of Chronic in Utero Hypoxia on the Pulmonary Vasculature of the Newborn Guinea Pig

“…While it has been shown that the fetal lamb responds to maternal hypoxia with a considerable decrease in PO2 (26), there is little information concerning smaller animals. From previous studies in our laboratories we know that young and adult guinea pigs respond to chronic alveolar hypoxia by developing pulmonary hypertension like most other animals.…”

Formation of Liquid in the Lungs of the Foetal Lamb

“…Taking as a representative value a pulmonary artery pressure of 70 cm. H 20 [21] and assuming that the pressure drop along the capillary is small, a capillary pressure of 22 cm H20 would indicate that about a third of the pulmonary vascular resistance was in post capillary vessels.…”