Peak Inspiratory Flows: Defining Repeatability Limits and a Predictive Equation for Different Inhalers.

BACKGROUND: Peak inspiratory flow (PIF) has been proposed as a measure to assess a patient's ability to use dry powder inhalers (DPI). However, robust quality criteria to determine a repeatability limit for measuring PIF are lacking. RESEARCH QUESTIONS: What are the repeatability limits for measuring PIF? What is the relationship between PIF measured using the In-Check DIAL device at Diskus(R) (PIFD) and HandiHaler(R) (PIFHH) resistances? STUDY DESIGN AND METHODS: Data from a randomized, controlled, phase 3 trial (study 0149) was used to define repeatability limits for PIF. Additionally, a model to characterize the relationship between PIF measured using the In-Check DIAL device at PIFD and PIFHH was defined using data from two randomized, controlled, phase 3 trials (studies 0128 and 0149). RESULTS: In study 0128, the mean values (standard deviation [SD]) for PIF at zero resistance and PIFHH were 84.6 (33.4) and 57.3 (26.1) L/min, respectively. In study 0149, the mean values (SD) for PIFD and PIFHH were 42.4 (11.2) and 29.0 (8.3) L/min, respectively. At the mean level, the mean difference between measurement attempts for PIFD and PIFHH was small, < 5 L/min and < 3 L/min, respectively. The repeatability limit was determined as 10 and 5 L/min for PIFD and PIFHH, respectively. Modeling the relationship between PIFD and PIFHH, after controlling for significant covariates demonstrated that a PIFD value of 60 L/min was approximately equivalent to PIFHH of 40 L/min. INTERPRETATIONS: This analysis demonstrated that the two highest values of PIF using the In-Check DIAL device among three inspiratory efforts met the repeatability limit. Altogether, these data provide guidance for measuring PIF against the simulated resistance of a specific DPI in clinical practice and research studies.