The relationship between Δ-forced vital capacity (percent fall in forced vital capacity at the PC₂₀ dose of methacholine) and the maximal airway response in patients who have mild asthma

Airway hypersensitivity is routinely evaluated by measuring the concentration (PC₂₀) of inhaled methacholine or histamine that causes a 20% fall in forced expiratory volume in 1 second (FEV₁). It has been suggested that a percentage fall in forced vital capacity (FVC) measured at the PC₂₀ dose of inhaled agonist (ΔFVC) is a potentially useful clinical measure in patients who have asthma because it provides indirect information about gas trapping and therefore the maximal airway response. The relationships between serum eosinophil cationic protein (ECP) levels and the maximal airway response or ΔFVC are largely unknown. The aims of this study were to determine whether ΔFVC is correlated with the degree of maximal airway response and to examine the relationships between serum ECP and ΔFVC or maximal airway response in patients who have mild asthma. Fifty-eight patients with mild asthma underwent high-dose methacholine challenge testing. The PC₂₀, maximal airway response, and ΔFVC were measured on the methacholine dose–response curves. Serum ECP levels also were determined. Subjects without a maximal response plateau (n = 33) had a significantly higher level of ΔFVC (17.9 ± 4.1%) than subjects with a plateau (n = 25; 14.9 ± 4.8%). A significant correlation was found between ΔFVC and the level of maximal response plateau (r = 0.446; p = 0.026). Not only methacholine PC₂₀ but also maximal airway response or ΔFVC had no relationships with serum ECP levels. Our results suggest that ΔFVC can be used as a surrogate marker of maximal airway response in patients who have mild asthma and that neither maximal airway response nor ΔFVC reflects blood eosinophil activation any more than methacholine PC₂₀.