Control of the Pulmonary Circulation in the Perinatal Period

Abstract

In the fetus, gas exchange occurs in0 the placenta and pulmonary blood flow requirements are therefore low. In the undisturbed near-term fetal lamb (the species in which accurate measurements have been made), pulmonary blood flow is 90–110 ml min^-1 100 g^-1 wet lung tissue or 30–40 ml min^-1 kg^-1 fetal body weight. This represents about 8% of total fetal cardiac output, which near term is normally about 450 ml min^-1 kg^-1 fetal body weight (Rudolph and Heymann 1970; Heymann et al. 1973). The low pulmonary blood flow at this stage of gestation is maintained by a high pulmonary vascular resistance. Shortly after birth, with the initiation of pulmonary ventilation, pulmonary vascular resistance and pulmonary arterial pressure fall rapidly. There is an associated eight- to ten-fold increase in pulmonary blood flow, which in lambs reaches 300–400 ml min^-1 kg^-1 body weight shortly after birth (Kuipers et al. 1982). In the sheep, pulmonary arterial blood pressure falls towards adult levels within several hours; however, in humans at 24 h of age, normal mean pulmonary arterial blood pressure may still be about twice normal adult levels (Moss et al. 1963). After the initial quite rapid fall in pulmonary vascular resistance and pulmonary arterial blood pressure, there is a slow progressive fall to near-adult levels by 3–6 weeks after birth (Lucas et al. 1961) related to growth of new vessels, to the involution of the large amount of medial smooth muscle found normally in the small pulmonary arteries in the fetus, and also possibly to changes in viscosity as hematocrit falls.