The pulmonary alveolar capillary membrane during hemorrhagic hypotension in dogs

Hemorrhagic hypotension is associated with increased lung water accumulation, secondary to loss of integrity of the alveolar capillary membrane. Whether this increased permeability of the alveolar capillary membrane occurs during hypotension or after resuscitation with intravenously administered fluids remains controversial. In an attempt to answer this question, we measured the movement of various molecular weight substances from pulmonary capillaries to the fluid filled alveolus using an in vivo saline solution filled dog lung model. During hemorrhage, T1/2--time to 50 per cent equilibration between blood and alveolar liquid--decreased for urea and dextrans of 3,000 and 10,400 molecular weight. There was no change compared with the base line figures for dextran 20,000 molecular weight and albumin, 69,000 molecular weight. In addition, during hemorrhage, lung liquid--but not blood--histamine levels increased significantly. We conclude that hemorrhagic hypotension leads to increased permeability of the alveolar capillary membrane to molecular weight substances less than 20,000 molecular weight. This increased permeability may be mediated by histamine.