Adaptation of the pulmonary circulation to extra-uterine life in the pig and its relevance to the human infant.

The normal structural and functional development of the pulmonary circulation has been studied in the pig, from fetal life to six months of age, with emphasis on the first 2 weeks. Pulmonary and systemic arterial pressures were measured in 50 animals, in 21 of which pulmonary vascular resistance was determined. After post mortem arterial injection, lung structure was analysed using quantitative morphometric techniques. Changes in the pulmonary circulation consisted of three overlapping phases: 1 Dilatation and recruitment of small arteries within the acinar region, beginning during the first 5 min and associated with a reduction in pulmonary arterial pressure. These changes continued during the first 24 h, associated with a loss of arterial muscle. 2 Between 24 h and 2 weeks, a significant reduction in the amount of arterial muscle was associated with a reduction in pulmonary: systemic vascular resistance ratio from 0.58 to 0.18. 3 Functionally, the pulmonary circulation appeared mature at rest by 2 weeks but growth and remodelling of the pulmonary arteries continued until an adult pattern was reached by 6 months of age.