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Differences in the response to methacholine between the tidal breathing and dosimeter methods: Influence of the dose of bronchoconstrictor agent delivered to the mouth

From the Seccion de Alergologia, Hospital Universitario Dr Peset, and Universidad de Valencia(*), Valencia, Spain

prieto_jes{at}gva.es

Abstract

BackgroundIt has been postulated that differences in methacholine PC₂₀ values between the dosimeter (DM) and tidal breathing (TBM) methods might be due to differences in the dose of agonist delivered to the mouth. The aim of the present study was to determine the influence of the dose of aerosol delivered to the mouth on differences in the response obtained with each challenge method.

MethodsThis study measured airway responsiveness to methacholine by DM and TBM in 27 subjects with suspected asthma. The dosimeter was modified to deliver an identical volume to that obtained with the TBM. Concentration-response curves were characterized by the provocative concentration of methacholine causing a 20% fall in FEV₁ (PC₂₀).

ResultsThe DM PC₂₀ was significantly higher than the TBM PC₂₀, with geometric mean (95% CI) values of 4.03 (1.86 - 8.78 mg/ml) and 2.19 (1.32 – 3.64 mg/ml, P = 0.04), respectively. The mean difference in the PC₂₀ value detected with each method was similar in subjects with TBM PC₂₀ values 2 mg/ml (0.77 doubling concentrations) and in those with PC₂₀ values<2 mg/ml (0.96 doubling concentrations, P = 0.83).

ConclusionsThe TBM produces methacholine PC₂₀ values significantly lower than a modified DM which delivers the same volume of aerosol. These results suggest that the discordant PC₂₀ values obtained with the two methods are not due to differences in the dose of agonist delivered to the mouth.

Key Words: asthma • methacholine • airway responsiveness • challenge methods • reactivity