Effects of surface tension and intraluminal fluid on mechanics of small airways

Hill, Mark J., Theodore A. Wilson, and Rodney K. Lambert.Effects of surface tension and intraluminal fluid on the mechanicsof small airways. J. Appl. Physiol.82(1): 233-239, 1997.Airway constriction is accompanied byfolding of the mucosa to form ridges that run axially along the innersurface of the airways. The muscosa has been modeled (R. K. Lambert.J. Appl. Physiol. 71: 666-673,1991) as a thin elastic layer with a finite bending stiffness, and thecontribution of its bending stiffness to airway elastance has beencomputed. In this study, we extend that work by including surfacetension and intraluminal fluid in the model. With surface tension, thepressure on the inner surface of the elastic mucosa is modified by thepressure difference across the air-liquid interface. As folds form inthe mucosa, intraluminal fluid collects in pools in the depressionsformed by the folds, and the curvature of the air-liquid interfacebecomes nonuniform. If the amount of intraluminal fluid is small,<2% of luminal volume, the pools of intraluminal fluid are small, the air-liquid interface nearly coincides with the surface of themucosa, and the area of the air-liquid interface remains constant asairway cross-sectional area decreases. In that case, surface energy isindependent of airway area, and surface tension has no effect on airwaymechanics. If the amount of intraluminal fluid is >2%, the area ofthe air-liquid interface decreases as airway cross-sectional areadecreases, and surface tension contributes to airway compression. Themodel predicts that surface tension plus intraluminal fluid can causean instability in the area-pressure curve of small airways. Thisinstability provides a mechanism for abrupt airway closure and abruptreopening at a higher opening pressure.