Pulmonary edema

The lungs are marvelously designed to handle fluid. The mechanical properties of the lungs and the lymphatics act simply and efficiently to drain fluid out of the pulmonary interstitium. Despite the enormous blood flow through the pulmonary and bronchial circulation, a dynamic equilibrium is maintained between fluid fluxing out of the vasculature into the pulmonary interstitium and fluid being drained out of the lungs by the lymphatics. This is obviously important because maintaining "dry" air spaces is essential for normal pulmonary function. Fluid accumulates in the lung when flux across the vascular endothelium exceeds lymphatic drainage. Two different types of abnormalities will result in accumulation of fluid, an increase in pulmonary microvascular pressure and an increase in the pulmonary vascular endothelial permeability to protein. Regardless of the type of abnormality causing pulmonary edema, fluid tends to accumulate in the lungs in a predictable pattern based on the same mechanical properties that normally keep the lung dry. Only by understanding the normal process of fluid handling will the clinician truly appreciate the consequences of pulmonary edema.