Provocative Dose of Methacholine Causing a 20% Drop in FEV1 Should Be Used to Interpret Methacholine Challenge Tests with Modern Nebulizers

Rationale: The American Thoracic Society guidelines (1999) for methacholine challenge tests (MCTs) using the 2-minute tidal breathing protocol were developed for the now-obsolete English-Wright (EW) nebulizer. In addition, the guideline recommendation to use the provocative concentration of methacholine causing a 20% drop in FEV1 (PC20) rather than the provocative dose of methacholine causing a 20% drop in FEV1 (PD20) for determining the level of bronchial hyperresponsiveness has been challenged. Objectives: To determine if cumulative dose or concentration of methacholine delivered to the airways is the determinant for airway responsiveness and to validate use of the AeroEclipse* II BAN (Aero; Trudell Medical International, London, ON, Canada) nebulizer compared with use of the reference standard EW nebulizer. Methods: Subjects with asthma (10-18 yr old) participated in randomized, controlled cross-over experiments comparing four MCT protocols using standard methacholine concentrations, but varying: (1) methacholine starting concentration (testing for cumulative effect); (2) nebulizer (EW versus Aero); and (3) inhalation time. PD20 was calculated using nebulizer output rate, inhalation time, and preceding doses delivered. ANOVA analyses were used to compare geometric means of PC20 and PD20 between protocols. Results: A total of 32 subjects (17 male) participated. PC20 differed when starting concentration varied (0.46 vs. 0.80 mg/ml; P < 0.0001), whereas PD 20 did not (0.06 vs. 0.08 mg). PC20 differed with the EW versus the Aero nebulzer with 30-second inhalation (1.19 vs. 0.43 mg/ml; P = 0.0006) and the EW versus the Aero nebulizer with 20-second inhalation (1.91 vs. 0.89 mg/ml; P = 0.0027), whereas PD20 did not (0.07 vs. 0.06 mg and 0.11 vs. 0.09 mg, respectively). Conclusions: In MCTs, the cumulative dose (PD20), not the PC20, determines bronchial responsiveness. Modern nebulizers may be used for the test if clinical interpretation is based on PD20.