Pulmonary perfusion and gas exchange in hemorrhage and shock

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Abstract
Pulmonary perfusion and gas exchange were studied during control period, maximum bled volume (MBV), posttransfusion (PT) and circulatory decompensation (CD) in anesthetized, supine, spontaneously breathing dogs. Pulmonary arterial pressure (Pa) did not change from control during MBV and pulmonary venous pressure decreased. The maintenance of Pa was associated with development of metabolic acidosis. Pa increased above control during PT and returned toward control during CD. The gravity-dependent regional pulmonary perfusion measured with labeled microspheres did not change from control during MBV. Regional perfusion became more uniform during PT and returned toward control during CD. Venous admixture (.ovrhdot.Qs/.ovrhdot.Qt) increased from 9.4 .+-. 2.3 to 20.5 .+-. 4.6%, alveolar-arterial PAO2 gradient (PAO2-PaO2) increased from 20.1 .+-. 7.5 to 45.7 .+-. 7.1 torr and dead space (VD) increased from 87.3 .+-. 5.2 to 104.2 .+-. 3.9 ml during MBV. These values returned toward control during PT and remained unchanged during CD. The wet/dry lung weight ratios were normal at CD and there was no histological evidence of embolization, atelectasis and edema. The normal distribution of regional pulmonary perfusion during MBV may be secondary to the acidosis-induced maintenance of Pa at control levels. The shift in perfusion during PT may be due to elevated Pa, and the normal distribution of perfusion during CD may be due to return in Pa to control. The increases in .ovrhdot.Qs/.ovrhdot.Qt, PAO2-PaO2 and VD at MBV suggest ventilation-perfusion inequalities. The mechcanisms for these changes may be related to the development of atelectasis, thromboembolization and edema during MBV. The return in .ovrhdot.Qs/.ovrhdot.Qt, PAO2-PaO2 and VD toward control following transfusion suggests that regional ventilation and perfusion are better matched during PT and CD than during MBV, and that mechanisms underlying the alterations are partially reversible following transfusion.