Relationships among lung extravascular fluid compartments with alveolar flooding

We studied the relationship between lung perivascular cuff fluid and alveolar fluid in dogs with alveolar flooding. In one group of dogs, we produced edema with alloxan. We waited 1 h and injected Evans blue dye intravenously. We then caused additional alveolar flooding by a rapid fluid load. The concentration of dye in perivascular fluid cuffs averaged 40% of that in airway edema fluid. We conclude that alveolar fluid derives from microvascular filtrate in the perimicrovascular compartment rather than from fluid in the loose connective tissue spaces. In a second group of dogs, we produced alveolar flooding by mechanically increasing left atrial pressure with a simultaneous fluid load. After flooding stopped and pressures stabilized, we injected Evans blue dye. Morphological examination of lung sections showed no dye present in alveolar fluid, although dye was present in alveolar septa and perivascular cuffs. Flooding occurs during a transient period consistent with an apparent increase in alveolar epithelial permeability, after which an isolated alveolar reservoir is created.