The pulmonary vascular response to breathing 5 % oxygen in nitrogen was studied in anesthetized, artificially ventilated dogs. The pulmonary arterial (Pa), pulmonary arterial wedge (Pw), left atrial (LAP) pressures and pulmonary blood flow (Q) were monitored. The pulmonary arterial catheter was wedged at the mid-lung level. At 10 min of hypoxia, Pa-Pw pressure gradient increased while the Pw-LAP gradient did not change significantly. The Pw and LAP were significantly correlated during room air breating (r = 0.832) and during hypoxia (r = 0.980). The calculated resistance from the pulmonary artery to the wedged catheter (Pa-Pa/Q) increased and the calculated resistance from the wedged catheter (Pa-Pw/Q) did not change significantly. These findings indicate that the mean LAP and Pw pressures are not significantly different during severe hypoxia, and that the increase in pulmonary vascular resistance during hypoxia is due to constriction of the large precapillary vessels.

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