Effects of Acute Hypoxia on the Pulmonary Vascular Bed of Patients with Acquired Heart Disease

Abstract

The nearly simultaneous changes in pulmonary blood flow, volume, and pressures during acute hypoxia in 18 cardiac patients were studied. Indicator-dilution curves were inscribed from a brachial artery after rapid sequential injections of indocyanine green into the main pulmonary artery and the left atrium (transseptal technique). Pulmonary blood volume was estimated from the product of cardiac output and mean transit time from the pulmonary artery to the left atrium. The distending or transmural pressure was calculated by averaging the mean pulmonary arterial and the mean left atrial pressures. During acute hypoxia the presence of active vasoconstriction was demonstrated by an increase in the pulmonary distending pressure associated with a decrease or no change in the pulmonary blood volume. The increase in the pulmonary distending pressure was mainly due to an augmented pulmonary arterial pressure, no appreciable alteration in the left atrial pressure being noted. Acetylcholine infused into the pulmonary circulation may counteract the hypoxic vasoconstriction and cause active vasodilation despite the continuation of hypoxia. The vasodilating effects were evidenced by a decrease in pulmonary distending pressure and an increase in pulmonary blood volume. When the period of acute hypoxia was prolonged and a considerable amount of blood was removed, there was a uniform reduction of pressures in both systemic and pulmonary circuits associated with a decrease in pulmonary blood volume. These changes were probably due to peripheral vasodilation, decreased venous return and diminished cardiac output.