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

Abstract

Airway constriction is accompanied by folding of the mucosa to form ridges that run axially along the inner surface of the airways. The mucosa has been modeled (R. K. Lambert. J. Appl. Physiol. 71:666-673, 1991) as a thin elastic layer with a finite bending stiffness, and the contribution of its bending stiffness to airway elastance has been computed. In this study, we extend that work by including surface tension and intraluminal fluid in the model. With surface tension, the pressure on the inner surface of the elastic mucosa is modified by the pressure difference across the air-liquid interface. As folds form in the mucosa, intraluminal fluid collects in pools in the depressions formed by the folds, and the curvature of the air-liquid interface becomes 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 the mucosa, and the area of the air-liquid interface remains constant as airway cross-sectional area decreases. In that case, surface energy is independent of airway area, and surface tension has no effect on airway mechanics. If the amount of intraluminal fluid is > 2%, the area of the air-liquid interface decreases as airway cross-sectional area decreases. and surface tension contributes to airway compression. The model predicts that surface tension plus intraluminal fluid can cause an instability in the area-pressure curve of small airways. This instability provides a mechanism for abrupt airway closure and abrupt reopening at a higher opening pressure.

Compliance of peripheral airways deduced from morphometry

“…The mechanics of this are similar to those for the initial buckling of the epithelial membrane (9), but, instead of the epithelial membrane, it is the entire thickness of the airway wall that is involved in the buckling. This buckling has been described by Hill and colleagues (6), who showed that it is governed by the mechanical and geometrical properties of not only the wall but also the parenchyma. They predicted that it could occur at a PL as low as 1 cmH 2 O.…”

Surface wrinkling of mucosa induced by volumetric growth: Theory, simulation and experiment

“…Hill et al (1997) and Heil and White (2002) studied this fluid-solid model with particular focus on airways, but they did not account for the growth effect. Normalize the Cauchy stresses by s rr ¼ s rr =m s and s y y ¼ s y y =m s .…”

The effect of viscoelasticity on the stability of a pulmonary airway liquid layer

“…18 There have been many fluid mechanical models of the statics and dynamics of airway closure. 11,[19][20][21][22][23][24][25][26][27][28][29] Hammond 30 was one of the first to formulate a model to study the stability of an annular thin liquid film coating a rigid tube with circular cross section assuming a passive air-core phase. He found that waves initially amplified but eventually saturated into almost disconnected liquid collars.…”