Understanding of the causes of pulmonary oedema must be based on knowledge of the mechanism responsible for fluid exchange between the several compartments of the normal lung. Recent physiological studies have clarified the main features of these mechanisms. However in three areas knowledge is still incomplete--the magnitude of the hydrostatic and oncotic forces responsible for fluid movement within the lung, the means by which
protein leaks across the wall of small pulmonary vessels and the routes by which fluid and
protein pass between the interstitial tissues of the lung and the alveolar space. Further work is needed in these areas. On the basis of this physiological knowledge the mode of development of hydrostatic oedema, the role of lymphatics in pulmonary oedema, and the several stages of pulmonary oedema development that may culminate in alveolar flooding are now clearly understood. Knowledge is less complete about oedema due to increased vascular permeability. In some experimental models, such as
alloxan, leakage is due to irreversible injury to the alveolar wall; in other models, including
ANTU, oedema formation has been shown to depend upon minor and reversible changes in pulmonary vascular endothelium similar to those that cause exudate formation in areas of acute
inflammation. In no instance is detailed information available of both the rate and magnitude of
protein leakage and of the morphological basis of increased vascular permeability. Further work is required in this area. Present knowledge allows an adequate explanation of the changes that occur in many clinically important types of pulmonary oedema, including
cardiac failure and neurogenic pulmonary oedema. Other types of oedema, notably that which may complicate
traumatic shock or extrapulmonary
sepsis and high altitude pulmonary oedema, are more complex and the details of their pathogenesis are still obscure.