Circulatory Changes following Birth of the Calf and the Effect of Hypoxia.

• Within a few days after birth, the pulmonary circulation changes from a high pressure, low flow system to the low pressure, high flow system which persists throughout life. Several factors are potentially important in causing this decrease in pulmonary vascular resistance: 1) The expansion of the fetal lung allows tortuous pulmonary vessels to straighten. 2) Hypoxia, a known pulmonary vasoconstrictor, is relieved following ventilation of the fetal lung with air/' 3) The lumina of the pulmonary arterioles increase in size as the arteriolar smooth muscle atrophies." 4) The postnatal decrease in alveolar and interstitial fluid' may permit increased alveolar capillary blood volume. Previous reports have indicated that a) initial expansion of the fetal lung with either air or nitrogen markedly decreased the pulmonary vascular resistance,' 8 b) newborn infants 9 and lambs 3> 5j 8 with lungs already expanded with air may again increase their pulmonary arterial pressure to systemic levels by breathing nitrogen or low oxygen mixtures, and c) hypoxia in utero may cause pulmonary vasoconstriction in the fetal lamb. Studies in lambs have involved anesthesia or thoracotomy.' -There are a few measurements of pulmonary hemodynamics in infants."' Serial measurements of pulmonary arterial pressure have been reported for newborn dogs, but blood flows were not measured nor were the dogs made hypoxic. With this background of previous work in mind, we undertook a systematic study of pulmonary