Specific airway conductance and airway conductance-lung volume curves in normal and asthmatic subjects

Airway conductance (Gaw) depends on lung volume (TGV). An approximate correction for this volume dependence can be obtained by calculating specific airway conductance (sGaw = Gaw-TGV). In this study, Gaw-TGV curves were compared with sGaw in 30 healthy and 20 asthmatic subjects who were studied by body plethysmography. Gaw, TGV and sGaw were measured five times at three to five different lung volumes. sGaw was dependent on TGV, the regression having a negative slope (-0.24 and -0.27 kPa-1.s-1.l-1, in the group without and with asthma, respectively). A change in TGV by 1 l caused a 9 and 11% decrease in sGaw, respectively. Bronchial obstruction induced by histamine inhalation in the asthmatic subjects increased the dependence on TGV by sGaw, now with a positive slope. Thus, a change in TGV by 1 l caused a 20-100% increase in sGaw, depending on the degree of airway obstruction. The Gaw-TGV curve was approximately linear around the resting lung volume. The coefficient of variation in determining the slope of the Gaw-TGV curve was as high as 110 and 153% in health and asthmatic subjects, respectively. It is concluded that sGaw, although rapidly determined, has a systematic error in its correction of lung volume dependence, which the Gaw-TGV curve does not. The Gaw-TGV curve therefore has advantages in research work, but since its construction is time consuming it is hardly suitable in clinical practice.