FACTORS DETERMINING RESIDUAL VOLUME IN NORMAL AND ASTHMATIC SUBJECTS

Abstract

The mechanisms determining the residual volume (RV) were studied by inducing temporary changes in RV through respiratory maneuvres and bronchodilation. Four asthmatic and 5 normal subjects inhaled placebo, salbutamol and ipratropium bromide, and performed maximum expiratory maneuvers after partial and maximal inspiration. RV and static lung pressure-volume measurements were made in each experiment. After bronchodilation, RV increased in both groups after a maximal inspiration while bronchodilation as such decreased RV in both groups. In the asthmatic patients, the static transpulmonary pressure (Pst) at low lung volumes increased after bronchodilatation. These findings can be explained if bronchodilators open-up closed airways, and if a preceding maximal inspiration decreases the elastic recoil pressure of the lungs causing a change of the lung volume at which airway closure occurs. Evidently, Pst at low lung volume is influenced by airway closure and underestimates the elastic recoil prssure of te lungs, even in normal subjects. Apparently, the increase in Pst at low lung volumes after bronchodilatation is due to smalller amount to trapped gas.