Metered-dose Inhalers—Breath-actuated
Breath-actuated pMDIs are another way of overcoming the difficulties of hand–lung co-ordination posed by press-and-breathe MDIs.⁹ Clinical studies have demonstrated that the use of the pirbuterol acetate breathactuated inhaler (Maxair Autohaler™, Graceway Pharmaceuticals) is easy to teach,¹⁰ and delivers drug to the lungs at least as well as a properly used conventional MDI and better than a poorly used conventional MDI.¹¹ It helps reduce the number of prescriptions required per patient.¹² Moreover, it can be used effectively by patients with poor lung function¹³ and limited manual dexterity,¹⁴ as well as by the elderly.¹⁵ The majority of MDIs do not carry dose counters, making it difficult for the patient to know how much medication remains. One study of 50 consecutive patients attending a children’s asthma center found that nearly threequarters did not know how many doses their device contained, and all used it until they could no longer ‘hear’ the medication when actuating. Furthermore, half did not shake the inhaler before use to evenly distribute the drug in the canister. All this means there is a risk that asthmatics will use their device for twice as long as intended, well beyond the time it is empty.¹⁶

Dry Powder Inhalers
DPIs are all breath-actuated and, unlike breath-actuated MDIs, rely on energy from the patient’s inhalation to create the small-particulate suspension. Therefore, DPIs do not contain propellants, making them not only more environmentally friendly than most MDIs, but also very portable, durable, and easy to use; it is also easy to teach the technique. Like breathactuated pMDIs or MDIs with spacers, DPIs do not require hand–lung coordination, but unlike those devices most do carry dose counters. DPIs keep the medication either in discrete dose units or in a reservoir. However, without the propellant, DPIs on average require a higher inspiratory flow to work. This makes them less suitable for younger patients or those with low levels of lung or cognitive functioning.⁶ DPIs can also lead to high levels of pharyngeal drug deposition.

Choosing Between the Different Devices
While there are obvious differences between and within the types of asthma inhalers, there is actually very little evidence that they differ in terms of efficacy. This is the conclusion of a large systematic metaanalysis taking in Medline, Embase, and the Cochrane Library.¹⁷ The study included a total of 254 randomized controlled trials that compared different types of devices against each other, primarily concerning beta2- agonists. Some types of device were under-represented in the metaanalysis, notably breath-actuated pMDIs. Nevertheless, the authors concluded that “none of the pooled meta-analyses showed a significant difference between devices in any efficacy outcome in any patient group. Each of the devices studied can work equally well in patients who can use them appropriately.” The emphasis, however, is on correct use of each device, and it is important that proper training be given regardless of the device chosen. Thus, when a clinician needs to select a device for a patient’s regular use, the main factors to consider include personal preference, practicality, and cost. The primary consideration is which devices are available with the medication required; not all drugs are available with all types of inhaler. If multiple medications are required, selecting a device that is common to them all will minimize errors; patients are more likely to get confused when they have to use different types of inhalers.¹⁸ Age and the mental and physical capabilities of the patient are clearly limiting factors in device selection as well. National guidelines and recommendations for device selection are generally based on a patient’s age.^4,3,19 For very young children under four years of age, most guidelines recommend use of a pMDI with a spacer and a facemask or a nebulizer with a face-mask. Between four and five or six years of age, the face-mask can be replaced by a mouthpiece, while after five or six years of age the child can generally cope with a DPI or one of the types of pMDI.^3,21

Summary
Choice of device is as important as choice of medication in treating asthma. While targeting the lungs directly with inhaled medication is not a new concept, device technology has come a long way in the last 60–70 years. While the perfect inhaler has yet to be made, newer devices are coming closer. Until it arrives, there is still a huge array of choice in terms of inhalers, all of which, if used correctly, are equally effective. Thus, patient choice, cost, and practicality issues will generally determine the right one to use.