Lung Distensibility and Airway Function in Asthmatic Children

Lung distensibility and airway mechanics were evaluated in 24 asthmatic children and adolescents, ages between 7 and 21 years, by quasi-static pressure-volume curves and by the static recoil-lung conductance relationship. The measurements were obtained by the stepwise inflation technique and the pressure-volume curves were analyzed by a new sigmoid exponential curve-fitting model of the form: V L = V m + [ V M / (1 + b e - K . P s t L )], where V_L is lung volume, P_stL is static recoil pressure, V_M and V_m are the upper and lower asymptotes, and K and b are shape constants. The shape constant K serves as index for lung distensibility, whereas the slope of θ of the static recoil-lung conductance plot represents the flow-resistive behavior of the airways. The combined evaluation of these two parameters (K and θ) shows that some asthmatic children have a very high lung distensibility and normal airway function, whereas others have nearly normal lung elasticity but grossly reduced airway distensibility. Sigmoid exponential analysis of static pressure-volume curves and an evaluation of the static recoil-lung conductance relationship in asthmatic children enable a distinction of these two types of functional derangements. Increased pulmonary distensibility consistent with an increase of alveolar air space seems to indicate an involvement of tissue elements. In contrast, decreased airway distensibility indicates a defect in the conducting airways. Sympathomimetics not only have a positive effect on airway mechanics, but seem to increase lung distensibility both in patients with hyperinflation and to an even greater degree in patients whose lungs are already too compliant.