Airway narrowing and internal structural constraints.

A computer model has been developed to simulate the movement restriction in the lamina propria-submucosa (L-S) layer (sandwiched by the basement membrane and the muscle layer) in a cartilage-free airway due to constriction of the smooth muscle layer. It is assumed that the basement membrane is inextensible; therefore, in the two-dimensional simulation, the perimeter outlining the membrane is a constant whether the airway is constricted or dilated. The cross-sectional area of the L-S layer is also assumed to be constant during the simulated airway narrowing. Folding of the mucosal membrane in constricted airways is assumed to be a consequence of the L-S area conservation and also due to tethering between the basement membrane and the muscle layer. The number of tethers determines the number of folds. The simulation indicates that the pressure in the L-S layer resulting from movement restriction can be a major force opposing muscle contraction and that the maximum shortening of the muscle layer is inversely proportional to the number of tethers (or folds) and the L-S layer thickness.