Spontaneous resolution of pulmonary edema caused by short periods of cyclic overinflation

Mechanical ventilation with high or even moderate peak inspiratory pressure produces pulmonary permeability edema. Besides the level of overinflation, duration may affect both severity and type of edema. We studied the effect of 2 min of 35-mmHg peak pressure mechanical ventilation (HV) on microvascular permeability and deep lung fluid balance in rats. It resulted in increased extravascular lung water (+50%), bloodless dry lung weight (+25%), and albumin uptake in lungs (+450%). The increase in dry lung weight and albumin uptake compared with that of lung water suggested major permeability alterations. Ultrastructural examination showed the presence of numerous endothelial blebs. Epithelial lining fluid (ELF) volume, its potassium and protein concentrations, and cellular composition were assessed by bronchoalveolar lavage. There was an increase in ELF volume (+180%), a decrease in ELF potassium concentration (-50%), and an increase in ELF protein content (+76%). A few blood cells were recovered, suggesting the presence of a few large epithelial breaks. Some animals were allowed to recover for periods less than or equal to 180 min after HV. Extravascular lung water, dry lung weight, and albumin distribution space returned to control levels within 45 min. ELF volume diminished but remained larger than in controls, and ELF protein concentration increased probably because of alveolar fluid resorption. No further hemorrhage was observed. These results indicate that periods of HV as short as 2 min transiently alter microvascular permeability in rats.