Assessment of mid flow rate measurements in patients undergoing methacholine challenge

The objective of this study is to assess the FEF(25-75) and FEF(25-75)/FVC in relation to the FEV1 in patients who have had a methacholine inhalation challenge study for a variety of clinical indications. The study is a retrospective review of methacholine challenge results at the university medical center. One hundred twenty-one consecutive patients who had a methacholine challenge performed for clinical indications were included in the study with no intervention. Methacholine was administered in successively increasing twofold concentrations in doses from 0.62 mg to a final concentration of 10 mg. A 20% drop in FEV1 compared to the prechallenge value was considered a positive test. We considered > or = 25% decrease in FEF(25-75) as a significant change. The > or = 25% decrease in FEF(25-75) occurred sooner than the 20% drop in FEV1 with a positive response occurring at least one full dose sooner in 23 of the 55 subjects. Thirty two subjects reacted at the same dose. The dose at which the FEF(25-75) decreased by > or = 25% was significantly different from the corresponding dose causing a 20% decrease in FEV1. The FEF(25-75) decreases more per mg methacholine. There were no subjects in whom there was > or = 20% decrease in FEV1 without a > or = 25% decrease in FEF(25-75). The mean FEF(25-75)/FVC after diluent inhalation = 0.87 +/- 0.27 standard deviation with a range of 0.23 to 1.67. The doses at which the FEF(25-75)/FVC decreased by > or = 20% and by > or = 30% was significantly lower than the corresponding doses causing a 20% decrease in FEV1. FEF(25-75) and the FEF(25-75)/FVC are more sensitive but less specific than the FEV1 as indicators of a positive response to a methacholine challenge. The FEF(25-75)/FVC does not provide additional information to that provided by the FEF(25-75).