The output and size distribution of
aerosols from
dry powder inhalers are dependent on the flow rate through the device. Therefore, in an in vivo study, we examined the flow-dependency of the effect of
formoterol when delivered from a
dry powder inhaler, the Aerolizer, in a flow range relevant to schoolchildren. In a preliminary study comprising 126 asthmatic children aged 3-10 yrs, the relationship between age and peak inspiratory flow (PIF) rate through the Aerolizer was determined. Mean PIF was 104 L.min-1 and all children aged > 5 yrs performed a PIF > 60 L.min-1. Sixteen children aged 8-15 yrs with
exercise-induced asthma (EIA) took part in the main trial comparing the protective effect of 12 micrograms
formoterol inhaled at 60 and 120 L.min-1. The effect from high and low inspiratory flow was judged from the protective effect against EIA 12 h after
drug administration. The decrease in forced expiratory volume in one second (FEV1) after exercise was 34% on the placebo day, but only 15% when
formoterol was inhaled at the high flow rate. This difference was statistically significant. The decrease in FEV1 was 23%
after treatment with
formoterol inhaled at the low flow rate, that was not significantly different from placebo or from high-flow
formoterol treatment. These clinical findings correspond with the in vitro findings of flow-dependent fine particle mass from the Aerolizer, and corroborate the relationship between fine particle mass of
aerosol and clinical effect. The results indicate a flow-dependent effect of
formoterol dry
powder inhaled from the Aerolizer, within the range of inspiratory flow rate obtainable by school-children. This questions its applicability in children with
asthma.