Supplementary Material for: Getting the Most out of Spirometry: A Tool to Guide Dry Powder Inhaler Use

<b><i>Background:</i></b> Dry powder inhaler (DPI) use requires sufficient peak inspiratory flow over the DPI internal resistance (PIF_R). <b><i>Objectives:</i></b> We examined whether spirometric peak inspiratory flow (PIF_spiro) could serve to predict PIF_R in patients with obstructive lung disease. <b><i>Method:</i></b> Thirty healthy nonsmokers and 140 stable outpatients (70 COPD, 70 asthma) performed spirometry according to the 2019 ERS/ATS spirometry update, yielding PIF_spiro. Using a PIF_R measurement device with varying orifices, all subjects’ PIF_R values were recorded for 5 predefined resistance levels, characterized by 5 orifice cross sections (S_R). A test group including all healthy subjects, 30 of the asthma, and 30 of the COPD patients was used to establish the relationship between PIF_R and both PIF_spiro and S_R by multiple regression. A validation group including the remaining 40 asthma and 40 COPD patients, served to verify whether their predicted PIF_R value corresponded to the measured PIF_R for each resistance level. <b><i>Results:</i></b> The asthma (FEV₁ = 78 ± 17 [SD] %pred) and COPD (FEV₁ = 46 ± 17 [SD] %pred) patients under study had varying airway obstruction. In the test group, PIF_R could be predicted by ln[PIF_spiro] (<i>p</i> &lt; 0.0001), S_R (<i>p</i> &lt; 0.0001), and S_R² (<i>p</i> = 0.006), with an adjusted <i>R</i>² = 0.71. In the validation group, estimated PIF_R did not significantly differ from measured PIF_R (<i>p</i> &gt; 0.05 for the 5 resistance levels). <b><i>Conclusions:</i></b> We propose a simple method to predict PIF_R for a range of common DPI resistances, based on the device characteristics and on the patient’s characteristics reflected in PIF_spiro. As such, routine spirometry can serve to estimate a patient’s specific PIF_R without the need for additional testing.