New concepts about the pathophysiology of pulmonary edema

Three new concepts concerning lung liquid and protein exchange are considered. The first is that the microvascular surface area is as important as the microvascular hydrostatic pressure in assessing filtration in the lung. One of the problems in differentiating hemodynamic from increased permeability edema is the inability to determine whether the microvascular surface area has changed. Several agents, as well as exercise, affect liquid filtration. A new, dynamic procedure that is more sensitive for the detection of increased permeability than static measurements of lung water content is described, along with its limitations. The second concept is that water and electrolytes are cleared from the alveoli by a separate mechanism from protein. Water clearance is fast and occurs mainly by an active process, which can be inhibited by amiloride or phloridzin and accelerated by beta-agonists. The mechanism appears to depend on metabolically regulated sodium transport across the alveolar epithelium. Protein clearance is very slow and is relatively independent of alveolar concentration. The protein clearance mechanism is unknown but may involve transcytosis. The third concept is that during edema formation there are two pathways for liquid clearance in addition to the lymphatic systeminto the pleural space and along the bronchovascular connective tissue into the mediastinum. During recovery from edema, reabsorption into blood is important if the edema liquid has a low protein osmotic pressure. Clearance into the mediastinum may be the major pathway for liquid sequestered in the loose, binding connective tissue.