Contribution of viscoelastic stress to the rate-dependence of pulmonary dynamic elastance.

To further characterize the contribution of the stresses accumulated during inflation in viscoelastic elements of the lungs to the rate-dependence of pulmonary dynamic elastance, we analyzed the changes in the pressures measured at the airway opening and in subpleural air spaces during airway occlusions performed at constant inflation rates of 5, 10, 20, and 40 ml/(kg sec) in 13 anesthetized piglets (mean age = 7 days). The analysis was repeated after saline lavage of the lungs and during intravenous infusion of histamine in 7 and 4 of the piglets, respectively. Viscoelastic stresses dissipated as stress relaxation were solely responsible for the differences between dynamic and static elastance before and after lung lavage and for more than 40% of this difference during histamine infusion (the remainder probably being caused by ventilation inequalities). The viscoelastic contribution to dynamic elastance increased by more than two-fold after lung lavage and was independent of inflation rate and only minimally dependent upon inflation volume. Our results demonstrate that viscoelastic stresses are primarily responsible for the dynamic stiffening of piglet lungs at low rates of inflation. They also support the notion that viscoelastic and elastic stresses are coupled as the lungs inflate.