Contribution of Mucosal Edema to Airway Obstruction

Whether an increase in vascular permeability results in mucosal edema depends on the balance between the amount of leakage into the mucosa and the rate of clearance from the mucosa, either through the lymphatics or across the epithelium into the airway lumen. The increase of vascular permeability produced by inflammatory stimuli can result in the bulk flow of plasma into the airway mucosa (Renkin, 1992). The amount of plasma leakage depends upon the number of gaps that form in the endothelium of the leaky vessels, the duration of the gaps and the intravascular pressure that drives the extravasation (Clough, 1991 Taylor and Ballard, 1992). The movement of plasma proteins and other osmotically active solutes into the mucosa can increase the interstitial oncotic pressure, which favors the net movement of fluid out of vessels and further increases the amount of leakage (Taylor and Ballard, 1992). 

Fluid can leave the mucosa through lymphatics or by entering the airway lumen. In this respect the airway epithelium serves as a gate across an escape route for fluid in the interstitium. This escape route has been demonstrated in the airways of the rat by Persson and colleagues (Persson, 1990, 1991 Greiff et al. 1993)  

The effect of plasma leakage on airway resistance depends on the amount of mechanical obstruction caused by the increase in mucosal thickness, accumulation of intraluminal fluid and possible reflex bronchocon-striction. In addition, the impact on airway conductance depends on where in the tracheobronchial tree the fluid accumulates. Theoretical models predict that increased mucosal wall thickness itself may have little effect on airflow, but it could exa erate the luminal narrowing caused by bronchoconstriction (James etal., 1989 Wiggs et al., 1990 Yager et ai., 1991). Although this effect probably would be negligible in the trachea and large bronchi, it could be important in peripheral airways. 

2 Release of Inflammatory Mediators Associated With Plasma Leakage 

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