Changes in the Pulmonary Circulation after Bronchial Occlusion in Anesthetized Dogs

The effects of bronchial occlusion on the pulmonary circulation were studied in anesthetized cats and dogs. Immediately after occlusion, blood flow fell rapidly at normal pulmonary arterial pressure (—71% cats, —54% dogs), and in constant flow perfusion experiments, perfusion pressure rose (+50% cats, +28% dogs). These resistance changes were reversed by vasodilator drugs, alkali, or perfusion of the lung with arterial blood. We concluded that an active mechanism, probably an increase in vasomotor tone, was involved; a mechanical process would not be reversed in this way. Pulmonary venous Po2 was the factor most closely related to the increase in resistance. In collapsing "oxygen-filled" lobes, compared with "air-filled" lobes the resistance change was delayed until the pulmonary venous Po2 fell. Hypoventilation and ventilation with hypoxic mixtures causing a fall in pulmonary venous Po2 similar to that due to collapse caused equivalent changes in blood flow. Changes in pH, Pco2 and lung volume played a relatively minor role in resistance changes following collapse. The increase in resistance may be caused by a mechanism regulating ventilation-perfusion ratios in both normal and diseased lung. ADDITIONAL