Changes in Pulmonary Blood Flow Affect Vascular Response to Chronic Hypoxia in Rats

We banded the left pulmonary artery in rats to investigate, in the same animal, the effect of both increased and decreased flow on the lung vasculature and to determine how these hemodynamic states modify the structural changes produced by a 2-week exposure to hypobaric hypoxia. In unanesthetized rats, pressures were recorded from the main pulmonary artery and aorta via indwelling catheters, cardiac output was calculated by the Fick principle, and pulmonary and systemic vascular resistance estimated. Technetium-99m macroaggregated albumin was injected and radionuclide activity counted separately over the right and left lungs as a measure of flow. At postmortem, right and left ventricles of the heart were weighed and the lungs injected to permit analysis of arteriograms and morphometric assessment of structural changes in the pulmonary vascular bed. Flow in the left lung was reduced to one-fifth normal in rats with left pulmonary artery bands. In "room air" rats, pressure proximal to the left pulmonary artery band and in the right lung was slightly higher than in nonbanded controls, but not as high as in nonbanded or banded hypoxic rats. Changes in flow and pressure in both lungs of "room air" rats with left pulmonary artery bands were associated with a mild degree of extension of muscle into peripheral pulmonary arteries normally nonmuscular, medial hypertrophy of normally muscular arteries, and reduced arterial density. These three structural changes were present in both lungs of "hypoxic" rats but were much more severe. High flow in the right lungs of "hypoxic rats" with left pulmonary artery bands worsened only the degree of extension. Decreased flow and pressure in the left lungs of these animals prevented both the extension and the medial hypertrophy of hypoxia, but not the severe reduction in arterial density. It seems that the latter may occur as a direct response to low oxygen tension, whereas extension and medial hypertrophy are influenced by altered flow and pressure, respectively.