Log normal distribution of bronchial responsiveness to methacholine in normal young adults

It has not been well known whether or not the distribution of non-specific bronchial responsiveness is log normal in normal subjects. We measured bronchial responsiveness to methacholine using partial flow-volume curves. The expiratory flow at 40% (above RV) from the full forced vital capacity (MEF40) was obtained and used as a reference volume to compare the isovolume flow from the partial flow-volume curve (PEF40). Provocative concentrations of methacholine causing a 20% fall in FEV1 (PC20-FEV1) and a 35% fall in MEF40 (PC35-MEF40) and PEF40 (PC35-PEF40) were measured as indices of bronchial responsiveness in 93 Japanese college students including 36 atopic and 57 non-atopic normal subjects. A 35% or more fall in PEF40 was produced by 160 mg/ml or less of methacholine in all the subjects whereas 20% or more fall in FEV1 did not occur in 4 atopic and 10 non-atopic subjects, and 35% or more fall in MEF40 did not occur in 4 non-atopic subjects at the final concentration of methacholine (160 mg/ml). The geometric mean value of PC35-PEF40 was 2.41 (GSEM, 1.21), 3.74 (GSEM, 1.16), and 3.16 (GSEM, 1.12) mg/ml in atopic, non-atopic, and both subjects, respectively. The distribution of log PC35-PEF40 could not be differentiated at the 5% level from a normal distribution using chi-square test to examine the goodness of fit in each group of subjects and in the whole subjects. These results directly confirm the log normal distribution of non-specific bronchial responsiveness in normal humans.