Effects of alveolar hypoxia on pulmonary capillary beds.

Using open chested dogs (n = 12), we tested the hypothesis that the pulmonary capillary changes its caliber in response to alveolar hypoxia. Animals were placed in a left upright lateral position. Pulmonary perfusion was measured by electromagnetic flow transducers attached to the main and left pulmonary arteries. Systemic artery, pulmonary artery and pulmonary vein pressures were measured via catheters inserted into them. Shunt flow through the pulmonary capillary beds was evaluated by the microsphere method, injecting a mixture of three different size (3, 9 and 15 microm) radioactive microspheres into the inferior vena cava. Right one lung ventilation with left lung atelectasis or left lung insufflation of 5 cmH2O (O2 or He) was achieved by occluding the left main bronchus with a blocker attached to an endotracheal tube. Right one lung ventilation caused redistribution of the perfusion from the left lung to the right lung. Left pulmonary vascular resistance increased significantly, while total pulmonary vascular resistance showed no significant changes. The shunt ratios of the 3 and 9 microm microspheres were not changed by right one lung ventilations with left lung atelectasis or insufflation. The shunt ratio of the 3 microm microspheres through the left lung was significantly higher than that through two lungs during both the two lung and one lung ventilations. We concluded that caliber changes in the pulmonary capillary do not occur in response to alveolar hypoxia.