Variability in airway conductance and lung volume in subjects with asthma.

Variability in airway conductance (Gaw) and lung volume (TGV) was studied in 26 subjects with moderately severe asthma during a 9-week period. Specific airway conductance (SGaw) was calculated as Gaw:TGV. There was considerable inter-individual variability in airway conductance, and a smaller variability in TGV. Airway conductance (SGaw) showed an eight-fold difference and TGV a three-fold difference between smallest and largest values. The intra-individual variability was less, with a range of +/- 55% (SGaw) and +/- 12% (TGV) of the grand mean, respectively. The error of the method contributed only marginally to the variations in airway conductance. These data for spontaneous variability of conductance facilitate, for example, the assessment of the clinical importance of changes in lung function seen after exposure to air pollutants in chamber studies. Furthermore, the substantial inter-individual variability in conductance argues against comparing samples of asthmatic subjects in polluted and non-polluted areas, and in favour of prospective studies of cohorts of subjects with asthma.