Peak inspiratory flow measured at different inhaler resistances in patients with asthma

BACKGROUND: Patients' peak inspiratory flow rate (PIFR) may help clinicians select an inhaler device.

OBJECTIVE: To determine the proportion of patients with asthma who could generate correct PIFRs at different inhaler resistance settings.

METHODS: During a UK asthma review service, patients' PIFR was checked at resistance settings matching their current preventer inhaler device, at R5 (high resistance dry powder inhaler (DPI)) and at R0 (low resistance, pressurised metered dose inhaler (pMDI)). Correct PIFR ('pass') was defined for R5 as 30-90 L/min and for R0 as 20-60 L/min. A logistic regression model examined the independent predictors of incorrect PIFR ('fail') at R5 and R0. Asthma severity was assessed retrospectively from treatment level.

RESULTS: A total of 994 adults (female 64.3%) were included, of whom 90.4% currently used a preventer inhaler (71.5% pMDI). PIFR pass rates were: 93.7% at R5 compared with 70.5% at R0 (p<0.0001). All patients failing the R0 PIFR breathed in too fast (>60 L/min), and 20% of patients currently using pMDI failed for this reason. Independent risk factors for failing R5 were: female gender, older age group and current preventer pMDI; and for failing R0 included: male gender, younger age group, current preventer DPI and mild versus severe asthma.

CONCLUSIONS: This study demonstrates that most patients with asthma can achieve adequate inspiratory flow to activate high resistance DPIs, whereas approximately a third of patients breathe in too fast to achieve recommended inspiratory flows for correct pMDI use, including one fifth of patients who currently use a pMDI preventer.