Primary (High Permeability) Pulmonary Edema

Although edema, that is, accumulation of excess fluid in the lungs, occurs when filtration pressures are increased in the lung microcirculation and in response to lung injury, pathogenetically the two forms of edema are different. The previous two articles provide the basis for examining the differences. The diagnosis of high permeability pulmonary edema is made clinically by inference. If pulmonary edema is evident on chest radiograph and pulmonary arterial wedge pressure is normal or low, it is inferred that the edema must be a result of increased permeability of exchange vessels in the lung. Additional criteria usually required for diagnosis of the adult respiratory distress syndrome (ARDS) include severe hypoxemia and decreased lung compliance. Other data collected from patients who meet these criteria support the inference of increased vascular permeability: protein concentrations in edema fluid are high; equilibration of intravascular proteins with edema fluid is rapid. Normally, the lungs are protected against accumulation of fluid by increasing lung lymph flow (draining away excess filtered fluid) and decreasing interstitial oncotic pressure (counteracting the effects of increased vascular hydrostatic pressure). When permeability of lung microvessels is increased, these protective mechanisms are compromised.