Effect of Lung Expansion on the Fetal Lamb Circulation

Summary The change in pulmonary blood flow as a result of expansion of the lungs with 100‐200 ml of nitrogen was studied in eleven fetal lambs. The fetal circulation was evaluated by simultaneous injection of 5% sodium chloride into the left ventricle and indocyanine green into the right ventricle and by recording the concentration of these indicators in the right carotid artery and right femoral artery. During the control period, prior to lung expansion, most of the right ventricular output bypassed the lungs; in fact, there was no detectable pulmonary blood flow in six of the lambs. As long as the lungs were forcibly maintained expanded there was no increase in pulmonary blood flow, but when the expanding nitrogen was given free outlet again there was a change in the distribution of right ventricular output in nine of the lambs. The portion passing through the lungs increased in eight lambs but decreased in one. In two lambs the lungs were expanded with 100 ml of Dextran. This did not affect pulmonary blood flow although the following expansion with 100 ml of nitrogen in both instances caused more of the right ventricular output to be distributed to the lungs. It is concluded that surface tension in the newly formed air‐liquid interfaces of the expanded alveoli may decrease resistance to pulmonary blood flow.