Regional distribution of extravascular water and hematocrit in the lung

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Abstract
Regional pulmonary blood flow (.ovrhdot.Qr), extravascular water (PEWr) and gas volume (Vr) were measured 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 1 group of 5 anesthetized intact dogs, 99mTc-labeled macroaggregates of albumin were injected as a marker of .ovrhdot.Qr. PEWr was initially calculated assuming that pulmonary artery hematocrit was representative of small vessel hematocrit in the lung. In another group of 5 dogs PEWr was calculated having measured regional lung hematocrit (Hctr). Results from the 1st 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. The observed decrease in .ovrhdot.Qr in the dependent lung is apparently not related to excess extravascular water in that location. The decrease in blood flow is possibly 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.