Improved expiratory airflow dynamics with smoking cessation.

Lung recoil, Pst(1), dependence and gas density dependence of maximum expiratory flow (Vmax) using air and an 80% helium + 20% oxygen (HeO2) gas mixture before and 60 +/- 4 (means +/- SD) days after cessation of smoking were determined in five male and five female subjects (33.4 +/- 6.4 years). Pst(1) dependence of Vmax, a relationship dependent on airway elastance at the "choke", was estimated by measuring the slope and the zero flow intercept using the maximum flow-static recoil (MFSR) curve. MFSR slope was unchanged by abstinence. The intercept showed no difference between air and HeO2 before or after cessation of smoking. Density independence of the zero flow intercept is deducible from wave speed theory if there is no change at the "choke" and the slope truly reflects the elastic behaviour of the "choke". This correspondence of theory and measurement suggests that the "choke" was not changed by abstinence. The reduction of Pst(1) with smoking cessation, without any fall in Vmax at the same lung volume, suggests a complementary reduction in lung recoil and frictional energy loss upstream of the "choke". This would suggest that a decrease in small airway muscle tone was the single cause of a reduction of Pst(1) and peripheral airway resistance.