Impulse Oscillometry in Healthy Nonsmokers and Asymptomatic Smokers: Effects of Bronchial Challenge with Methacholine

Abstract

The clinical application of respiratory impedance measurements by oscillation techniques for monitoring bronchial challenge testing is hampered by the fact that data in healthy nonsmokers and asymptomatic smokers are very limited. The objective of this study was to analyze the changes in impedance to a methacholine provocation test in healthy nonsmokers and asymptomatic smokers, and to investigate whether smokers show a different response compared to nonsmokers. The response to methacholine challenge was assessed by impulse oscillometry (IOS) (resistance R and reactance X at 5, 10, 15, 20, 25, and 35 Hz) and spirometry (FEV₁, MEF₅₀) in 105 healthy subjects (55 nonsmokers: "NS"; 50 asymptomatic smokers: "S") in whom the provocation dose of 2.44 mg methacholine did not result in a fall of FEV₁ below 20% predicted of the baseline value. The baseline reactance X values of S were significantly lower compared to NS from 10 to 35 Hz (p ≤ 0.01), whereas S and NS did not differ in resistance R over the whole frequency spectrum from 5 to 35 Hz. S showed a significantly higher mean baseline resonant frequency X(f₀); i.e., the frequency at which inertial forces are equal and opposite to elastic forces, compared to NS (NS: X(f₀) = 10.8 ± 2.9 Hz; S = 12.6 ± 4.0 Hz, p = 0.01). In both groups methacholine challenge resulted in an increase in R (f) especially at 5 and 10 Hz, and a marked decrease in X(f) over the whole frequency spectrum. In S a significantly higher decrease of postchallenge X_5Hz and X_10Hz was measured compared to NS (NS: ΔX_(5Hz) = -0.044 ± 0.003; S: ΔX_(5Hz) = -0.083 ± 0.013; p = 0.0017. NS: ΔX_(10Hz) = -0.050 ± 0.001; S: ΔX_(10Hz) = -0.082 ± 0.017; p = 0.008). R and X at low frequencies showed a three to four times higher postchallenge reaction compared to FEV₁. Pre- and postchallenge FEV₁ did not correlate with resistance R(f) and reactance X(f) over the whole frequency spectrum. In S perchallenge X(f) values from 10 to 35 HZ were significantly lower, and postchallenge decrease of X_5Hz and X_10Hz values were significantly higher compared to that of NS, whereas pre- and postchallenge R(f) values were similar in both groups. These results can be explained by changes in the elastic properties of the lung induced by a diffuse subclinical respiratory bronchiolitis.