Dosimeter methacholine challenge: comparison of maximal versus submaximal inhalations

Deep inhalation has bronchodilating and bronchoprotective effects, particularly in subjects who are normal or have mild airway hyperresponsiveness (AHR). We have anecdotally observed that the 5 breath to total lung capacity (TLC) dosimeter method reduced the response to methacholine in some subjects with mild AHR.

To compare prospectively submaximal inhalations with TLC inhalations during the dosimeter methacholine challenge.

Sixteen subjects with asthma and a methacholine PC 20 <8 mg/mL performed 2 methacholine challenges in random order; the standard dosimeter method was compared with a modified dosimeter challenge in which methacholine inhalations were performed to approximately 50% to 60% below TLC.

The standard methacholine challenge PC 20 was almost twice that obtained with the modified submaximal inhalation method (geometric mean PC 20, 5.2 mg/mL vs 2.8 mg/mL, respectively; P = 0.0216). In the 5 subjects with the mildest AHR, there was a 2.5-fold to 14-fold difference in PC 20 between methods. The standard (full TLC) PC 20 s were falsely negative (>16 mg/mL) in these 5 subjects with current asthma, 4 of whom required inhaled corticosteroids.

A submaximal inhalation dosimeter methacholine challenge results in a significantly lower PC 20 compared with the standard 5-breath dosimeter method. This effect is limited to the mildly responsive group, probably because of the bronchoprotective effect of the deep inhalation during the standard method, and results in false-negative tests in some subjects.