The effect of intrapleural pressure on the hypoxic pulmonary
vasoconstrictor (HPV) responses to
atelectasis and
hypoxia were measured in two groups of anesthetized closed chest dogs. The right lung was continuously ventilated with 100% O2. The left lung was initially ventilated with 100% O2 (
hyperoxia) but subsequently underwent either reabsorption
atelectasis (
atelectasis; group I) or ventilation with a
hypoxic gas mixture (
hypoxia; group II). The mean intrapleural pressure in the left hemithorax was 5.4 cm H2O during
hyperoxia, but with left lung
atelectasis decreased significantly to -3.8 cm H2O by 15 minutes and to -4.2 cm H2O by 90 minutes. Venous admixture (% VA) increased significantly from 10.3% during
hyperoxia to 33.2% at 15 minutes of left lung
atelectasis and to 34.6% at 90 minutes. However, after
sternotomy with the left lung still atelectatic, the %VA decreased significantly to 25.4% For the
hypoxia group, %VA increased significantly from 9.2% during
hyperoxia to 29.9% at 15 minutes of left lung
hypoxia and 25.1% at 90 minutes. HPV diverted blood flow away from both atelectatic lung and hypoxic lung. However, due to the negative intrapleural pressure generated during left lung
resorption atelectasis when the chest was closed, HPV was less effective during
atelectasis than during
hypoxia.