Gas Exchange in the Lung Deprived of Pulmonary Arterial Supply *

Upon ligation of the left branch of the pulmonary artery in the dog, there is exposed to the gases in the alveoli of this side a capillary bed supplied solely by systemic arterial blood. In such a lung, blood from the bronchial arteries has been demonstrated to gain direct access to the capillaries of the alveoli through large pre-capillary anastomoses that develop with branches of the pulmonary arteries which remain patent despite the ligature at the proximal end.' As the collateral circulation expands, the bronchial vessels carry a flow enormously greater than the normal-equal in volume at the end of eighteen months to approximately one-third of the output of the right ventricle.! The burden of this flow falls solely upon the left side of the heart whose output exceeds that of the right by an amount equal to that of the collateral. The situation is analogous to that of an arterio-venous fistula-systemic artery to pulmonary vein. Not only does this blood maintain the structural integrity of the parenchyma of the left lung, but when not fully saturated, it can also function in the absorption of oxygen. Thus a lung deprived of its pulmonary arterial supply may still possess respiratory function. The observations reported here consider some quantitative aspects of the exchange of gases between such a capillary bed and the alveoli. Two studies,' each of a single patient, have recently appeared, but there has not been published, to our knowledge, a detailed experimental investigation. The observations to be reported were made on dogs under sodium pentobarbital anesthesia under four conditions: (A) Both lungs supplied with room air; (B) as in A, but exchange across the carina at the tracheal bifurcation interrupted by means of a bronchospirometric cannula; (C) right lung supplied with room air, bronchospirometric cannula in place, and left lung re-breathing into a spirometer or rubber bag; (D), as in C, but right lung connected to a spirometer containing oxygen.