Minimal Inspiratory Flow from Dry Powder Inhalers According to a Biphasic Model of Pressure vs. Flow Relationship

2 , the slope became steeper when PI was less than a certain value (critical PI, existed between 15-20 cmH2O). When PI was less than critical PI, linear rather than parabolic regression between PI and flow yielded better fits (r > 0.90, p < 0.001). Inspiratory flows at the critical PI were 53.9 (Diskus), 65.8 (Diskhaler), 45.9 (Turbuhaler for Pulmincort), 48.6 (Turbuhaler for Symbicort) and 38.0 l/min (Twisthaler). These find- ings suggested that flow through the DPI becomes laminar rather than turbulent flow in the range below critical PIs. We suggest that patients should inhale from the DPIs with inspiratory pressure higher than critical PI.