Inhaler Resistance, Flowrate and Duration of Inhalation: The Effort to Use an Inhaler Adequately
Mark Sanders & Ashley Green
Clement Clarke International Limited, Edinburgh Way, Harlow, CM20 2TT, UK
As chronic obstructive pulmonary disease (COPD) becomes more severe, the ability of the patient to perform the correct inhalation manoeuvre becomes progressively limited and may fall below that required for correct use. The physiological effort associated with achieving this has seldom been expressed with the focus on peak inspiratory flow and resistance: duration and flow profile are also important. We have quantified the effort of untrained and trained moderate (n=12) and very severe (n=10) COPD volunteers to use seven commonly used pMDI, DPI and SMI devices in a minimally adequate way (n=6: Evohalerâ, Respimatâ, HandiHalerâ, Breezhalerâ, Turbohalerâ, Elliptaâ, and NEXThalerâ). Patient flow profiles, at three resistances (0.04, 0.16 and 1.00 kPa½/L.min-1) were accessed from a database, enabling interpolation and modelling to the devices. Device resistances were measured and enumerated at the device-effective flowrate. Duration of inhalation was determined from the literature, combined with an assessment of time for actuation/coordination, aerosol delivery, ‘chase’ air, and capsule emptying. Energy (airWatts, aW), was calculated as volume (m3.sec-1) x pressure (kPa). To provide a comparable value for each inhaler, Energy (aW) was multiplied by duration (sec) to express the Effort of inhalation. The data show that training helps subjects to exert a greater effort. Untrained subjects who produced insufficient Effort or who were close to failure were able to deliver adequate Effort following training. Devices that propel the medication towards the patient (pMDI, SMI) require less inspiratory Effort. Post-training, all of the devices were within the theoretical capability of the subjects for successful use.