REGIONAL DISTRIBUTION OF EXTRA-VASCULAR WATER AND HEMATOCRIT IN THE LUNG

We measured regional pulmonary blood flow (Qr), extravascular water (PEWr), and gas volume (Vr) to test the hypothesis that the decrease in blood flow, reported to occur in the dependent lung, was due to an accumulation of extravascular water in this region. In one group of five anesthetized intact dogs, 99mTc-labeled macroaggregates of albumin were injected as a marker of Qr. PEWr was initially calculated assuming that pulmonary artery hematocrit was representative of small vessel hematocrit in the lung. In another group of five dogs we calculated PEWr having measured regional lung hematocrit (Hctr). Results from the first group of animals showed that PEWr was highest at the top of the lung, and decreased down the lung. When PEWr was calculated taking Hctr into account, the regional differences in extravascular water were abolished and its distribution was relatively uniform throughout the lung. Regional lung volume was measured using the lung density method, and predictably was highest at the top of the lung and decreased down the lung. We conclude that the observed decrease in Qr in the dependent lung is not related to excess extravascular water in that location. It seems likely that the decrease in blood flow is caused by an increase in resistance of the extra-alveolar vessels due to a decreased tethering effect of the lung parenchyma at low lung volumes.