Effect of deep inspiration on impulse oscillometry responses to methacholine in healthy adults

Background Impulse oscillometry (IOS) detects small-airway hyperresponsiveness during methacholine challenge testing (MCT). Deep inspiration (DI) may attenuate these responses, but the magnitude and interpretive impact of this effect in healthy adults remain uncertain.Methods We conducted a prospective two-period crossover study in 38 healthy, non-smoking adults (median age 34.8 years; 53% female). Each participant completed two standardized five-dose MCT protocols: (i) IOS-only and (ii) IOS immediately followed by spirometry (IOS + DI). The primary endpoint was the within-subject difference in total resistance at 5 Hz (Delta R-5) at the maximum methacholine dose. Secondary analyses included R-5-R-2(0), AX, Fres, and bootstrap-derived upper limits of normal (ULNs).Results No participant met the PD20FEV1 criterion. At the final dose, the IOS-only protocol elicited larger responses than IOS + DI (median Delta R-5% + 87% vs. +60%; paired difference +27%, 95% CI +12 to +44; p = 0.002). Similar attenuation was observed for R-5-R-2(0), AX, and Fres (all p <= 0.001), while central resistance (R-2(0)) remained unaffected. Baseline R-5 correlated with Delta R-5 under both protocols (rho approximate to 0.5, p < 0.01). Bootstrap-derived ULNs exceeded manufacturer thresholds, particularly under IOS-only conditions.Conclusion Deep inspiration substantially attenuates methacholine-induced IOS responses, underscoring the need for protocol-specific diagnostic thresholds. IOS-only MCT provides sensitive, effort-independent information and may be particularly valuable when spirometry is impracticable. Clinical Trial Registration: ClinicalTrials.gov NCT05012345.Conclusion Deep inspiration substantially attenuates methacholine-induced IOS responses, underscoring the need for protocol-specific diagnostic thresholds. IOS-only MCT provides sensitive, effort-independent information and may be particularly valuable when spirometry is impracticable. Clinical Trial Registration: ClinicalTrials.gov NCT05012345.