There are many highly efficacious medications and a range of delivery devices available today for the control and treatment of asthma. However, despite this range of options, management of asthma is still less than satisfactory for many people.¹ This is largely due to a combination of failure to take prescribed medications at the right time and failure to take them correctly. Delivery of therapeutics for asthma is typically via inhalation. There is a good history of using this route for asthma medications, even as far back as ancient Greek and Roman times. Inhaled aerosol medications have advantages over oral and parenteral routes as they allow for selective treatment of the lungs while reducing systemic adverse effects.² By and large, the benefits of aerosolized medications outweigh the drawbacks and, consequently, the National Asthma Education and Prevention Program states that the major advantages of delivering drugs directly into the lungs via inhalation are that higher concentrations can be delivered more effectively to the airways and that systemic side effects are lessened.³ Furthermore, some drugs are therapeutically active only when inhaled (e.g. most corticosteroid preparations, cromolyn, and salmeterol). Manually operated squeezy nebulizers were developed in the early part of the 20th century, but devices that we would recognize today did not emerge until the middle of the century. In the 1950s, the Wright nebulizer was the precursor of the modern hand-held jet-venturi nebulizer; Riker Laboratories (now 3M Pharmaceuticals) developed the first pressurized metered-dose inhaler (pMDI) in 1956; and the 1960s saw the birth of the modern dry powder inhaler (DPI). Today, while nebulizers, MDIs, and DPIs are the mainstays of asthma treatment, there are many variants that require different inhalation techniques and timings on the part of the patient, and all of them have their unique benefits and drawbacks. This presents the physician with a confusing array of choice. This review will hopefully make the picture a little clearer.

Differences and Similarities Between Devices
The main types of inhalers are: nebulizers—jet and ultrasonic; pMDIs— chlorofluorocarbon (CFC; less common) or hydrofluoroalkane (HFA) propellant, breath-actuated or manually operated, and with or without spacer/holding chamber; and DPIs. Ideally, an inhaler should be easy for a patient to use; it should also be easy for a physician to instruct a patient on its correct use. It should be portable and easy to inhale, regardless of the strength of the patient’s inflow. It should also ensure that the correct amount of medication is delivered to the right part of the lungs, and reassure the patient that this had been done. The device should also report the number of remaining doses it holds. While a number of devices on the market currently come close—and are getting closer all the time—to this ideal, none are yet there.

Nebulizers
Jet, or compressed air, nebulizers function using the Bernoulli principle. Compressed gas passes through a small aperture into a larger volume. This reduction in pressure increases its velocity, and the gas picks up drug solution from a reservoir in droplets of around 15–500um in size. These droplets hit baffles, producing smaller droplets, which are inhaled or returned to the reservoir. In contrast, ultrasonic nebulizers create sprays using a high-frequency ultrasonic signal (around 1–3MHz) generated by a piezoelectric transducer. While the newer ultrasonic technology allows such devices to be smaller, faster, and quieter, nebulizers are still, as a rule, larger and less portable than other types of inhalation device. They are also more time-consuming to use and require maintenance and cleaning. Furthermore, this type of delivery is inefficient and quite expensive per dose, although it does not require any co-ordination between inhalation and actuation. Consequently, nebulizers are reserved for use in emergency situations and for those patients unwilling or unable to use other devices, such as the very young and the very old.⁴

Metered-dose Inhalers—Press-and-breathe
More convenient for carrying around and for swift use, pMDIs are the most widely prescribed inhaler devices and can be used with the greatest variety of asthma therapies. The original MDIs developed more than 50 years ago used CFC for propellants, but since the Montreal Protocol these will be gradually replaced with other gases, such as HFAs. More than simply a stand-in for CFCs, pMDIs using HFAs deposit drug more efficiently in the lungs, have a less forceful impact on the back of the throat, and reduce the ‘cold freon’ upon inhalation, all of which are improvements over the old pMDIs.⁵ pMDIs deliver the drug in either solution or suspension that also contains a surfactant such as oleic acid or lecithin to reduce particle agglomeration. Traditionally, the devices delivered the dose when the cylinder was pressed, meaning the patient had to co-ordinate his or her action and inhalation; this makes these devices very difficult to use, particularly for younger patients.⁶ Furthermore, there are recommendations governing the inspiratory flow rate and holding of breath post-inhalation, both of which add to the difficulty of use. Adults, while typically more able to effectively coordinate their actions and hold their breath, can still make mistakes using a normal pMDI.⁷ To overcome some of these dexterity drawbacks, it is possible to fit a spacer or holding chamber to an MDI, which for a short while will retain the aerosolized suspension to be breathed in more normally. These have the additional benefit of reducing drug deposition in the oropharynx, thereby reducing the risk of local side effects such as oral candidiasis and dysphonia. On the downside, for the common plastic spacers, electrostatic forces cause particles to stick to the walls, which can considerably lower the amount of medication delivered to the lungs. Washing the spacer and letting it drip-dry can help with this, but does add a layer of complication to its use. Spacers need to be replaced when damaged or worn and should be checked every three to six months, further adding to the cost of treatment.⁸ There is a plethora of spacers on the market, all of which have different flow dynamics that affect the amount and size of the inhaled medication.