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Alterations in pulmonary function and in hemodynamics were studied in dogs with high pulmonary blood flow resulting from systemic pulmonary artery shunts. In order to facilitate elevation in pulmonary artery pressure, the vascular bed was reduced in some cases by obstructing branches of the pulmonary artery with Teflon clips or by lobectomy. Results in 30 control dogs and in 30 animals that survived 5–36 months (average 16) following creation of shunts indicated that pulmonary function was not significantly altered by increased pulmonary blood flow until pulmonary artery hypertension developed. When systolic pulmonary artery pressure exceeded 40 mm Hg, there was a decrease in arterial Po2, an increase in venous admixture percentage of cardiac output, and an increase in the A-a O2 difference during three levels of O2 breathing, indicating both abnormal venous admixture and abnormal diffusion. Possible explanations for these findings are presented. Evidence in one dog suggests that these alterations are reversible. Submitted on August 10, 1960

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