Estimate of the subepithelial hydrostatic pressure that drives inflammatory transudate into the airway lumen.

Inflammatory diseases of the upper respiratory tract are characterized by flow of plasma filtrate across the epithelium into the airway lumen ("transudation"). Elsewhere, we have proposed that extravasation from microvessels causes edema, and this is associated with elevated subepithelial hydrostatic pressure that drives transudation. To test this hypothesis, we have attempted to block transudation by elevating luminal hydrostatic pressure. We measured the appearance of plasma markers into the lumen of an isolated perfused segment of rat trachea in vivo and found that stimulation of one vagal nerve caused a rapid (half-time approximately 5 min) and nonselective increase in the flow of markers from blood to airway lumen. Leukocyte migration also caused transudation that developed much more slowly (half-time = 2-3 h). In both cases, transudation was blocked by application of luminal hydrostatic pressures. The critical luminal pressure needed to block vagally induced transudation was approximately 4.5 cmH2O, and, to block epithelial transudation induced by leukocyte traffic, it was 3 cmH2O, and we conclude that these are the subepithelial pressures that drive inflammatory transudation into the airway lumen.