Drug delivery from the various types of inhaler device is produced by different methods.
All inhaler devices are designed to deliver drug particles of a certain size to the small airways during inhalation. Particles of this size (generally agreed to be approximately between 2 and 5 microns) are known as the "respirable fraction". The particles are either in aerosol (in a suspension or a solution) or dry powder form.
As the great majority of asthma medication is delivered via inhalers, optimum inhaler technique is an important factor in the management of this disease. Patients require their medication for both short-term relief and long term prevention, and the delivery of these drugs to the lungs is affected by inspriatory flow.
Common pMDI Metered Dose Inhaler, and MDI spacers with low resistance (e.g. AbleSpacer): 30 – 60 L/min
Autohaler® Automatic pMDI: 30 – 60 L/min
Easibreathe Automatic pMD: 20 – 60 L/min
Diskus® Multiple-dose powder inhaler: 30 – 90 L/min
Turbuhaler® Turbulent flow inhaler: 60 – 90 L/min
With most MDI's, the aerosol is delivered under pressure at high speed (often over 90 kilometers per hour). The inhalation should be slow and steady. Inhaling too fast may cause a greater proportion of the aerosol to impact at the back of the throat and be subsequently swallowed, thus reducing the beneficial clinical effect and increasing the potential for local and systemic side effects. With the majority of MDI's, manual depression of the drug canister actuates the drug delivery. However, with breath-actuated metered dose inhalers (e.g. Autohaler®) the aerosol is released by mechanical actuation, triggered by the patient breathing in through the device at any flow rate above a minimum level. At inspiratory flows below this, the patient will not receive any of the medication, because a dose will not be released.
It is recognized that the optimum inhalation technique for using and MSI with holding chamber/spacer is a slow inhalation (30 to 60/min). As the resistance of the valves of most chamber/spacer devices are low, the In-Check DIAL can be set to 'low resistance pMDI' or "Freeflow" to provide an approximate resistance for inspiratory flow measurements to be made.
Drug delivery from DPIs is triggered by inhaling through the device. A metered quantity of powder is drawn into the airflow, and follows a specific route within the inhaler, which creates a resistance that is designed to break up the medication into particles of a respirable size.
As the internal design of each DPI is different, and the formulation of medication is not identical, the resistance the patient encounters when inhaling – and the speed of inhalation at which the optimum performance occurs – will be different from device to device.
The effort required for the patient to achieve a given inspiratory flow will increase as the internal resistance of the device increases. For the same patient effort, the higher the resistance the lower the resulting inspiratory flow.