There are conflicting reports and conflicting concepts in the literature as to the effect of hypoxia on the pulmonary circulation. An array of studies has been done in perfused lungs, anesthetized animals, and unanesthetized humans which indicate a rise in pulmonary vascular resistance in response to hypoxia.1,8 Other studies, which show elevation in pulmonary artery pressure with concomitant rise in cardiac output, indicate vasodilatation and no increased resistance. There are also studies which indicate an irregularity in resistance response.12,13 Boake et al14 produced increased pulmonary vascular resistance in animals breathing 8% oxygen and partially reversed the effect with perfusion of some oxygenated blood into the pulmonary artery. Lanari-Zubiaur and Hamilton15 found "nonsignificant" resistance changes in their anoxia studies and raised the teleological question: "It is doubtful that a vigorous pulmonary constrictive response to anoxic breathing could long survive in evolutionary development."
All observers agree that there