Metered-dose inhaler

A metered-dose inhaler (MDI) is a device that delivers a specific amount of medication to the lungs in the form of a short burst of aerosolized medicine that is usually self-administered by the patient via inhalation. It is the most commonly used delivery system for treating asthma, chronic obstructive pulmonary disease (COPD) and other respiratory diseases. The medication in a metered dose inhaler is most commonly a bronchodilator, corticosteroid or a combination of both for treating asthma and COPD. Other medications less commonly used but also administered by MDI are mast cell stabilizers, such as cromoglicate or nedocromil.

A metered-dose inhaler consists of three major components: the canister, which is produced in aluminum or stainless steel by means of deep drawing, where the formulation resides; the metering valve, which allows a metered quantity of the formulation to be dispensed with each actuation; and an actuator (or mouthpiece) which enables the patient to operate the device and directs the aerosol into the patient's lungs. The formulation comprises the drug, a liquefied gas propellant and, in many cases, stabilizing excipients. The actuator contains the mating discharge nozzle and generally includes a dust cap to prevent contamination.

To use the inhaler, the patient presses down on the top of the canister, with their thumb supporting the lower portion of the actuator. Actuation of the device releases a single metered dose of the formulation, which contains the medication either dissolved or suspended in the propellant. Breakup of the volatile propellant into droplets, followed by rapid evaporation of these droplets, results in the generation of an aerosol consisting of micrometer-sized medication particles that are then inhaled.

Metered-dose inhalers are only one type of inhaler, but they are the most commonly used type. The replacement of chlorofluorocarbons propellants with hydrofluoroalkanes (HFA) resulted in the redesign of metered-dose inhalers in the 1990s. For one variety of beclomethasone inhalers, this redesign resulted in considerably smaller aerosol particles being produced and increased in potency by a factor of 2.6.

Before the invention of the MDI, asthma medication was delivered using a fragile and unreliable squeeze bulb nebulizer. The relatively crude nature of these devices also meant that the particles that they generated were relatively large, too large for effective drug delivery to the lungs. Nonetheless, these nebulizers paved the way for inhalation drug delivery, inspiring the MDI.

MDIs were first developed in 1955 by Riker Laboratories, now a subsidiary of 3M Healthcare. At that time, MDIs represented a convergence of two relatively new technologies, the CFC propellant and the Meshburg metering valve, originally designed for dispensing perfume. The initial design by Riker used a glass canister coated with vinyl plastic to improve its resilience. By 1956, Riker had developed two MDI-based products, the Medihaler-Ept containing epinephrine and the Medihaler-Iso containing Isoprenaline. Both products are agonists that provide short-term relief from asthma symptoms and have now largely been replaced in asthma treatment by salbutamol, which is more selective.

Metered-dose inhalers are sometimes used with add-on devices referred to as holding chambers or spacers, tubes attached to the inhaler that act as a reservoir or holding chamber and reduce the speed at which the aerosol enters the mouth. They serve to hold the medication that is sprayed by the inhaler. This makes it easier to use the inhaler and helps ensure that more of the medication gets into the lungs instead of just into the mouth or the air. Proper use of a spacer can make an inhaler more effective in delivering medicine.

Spacers can be especially helpful to adults and children who find a regular metered dose inhaler hard to use. People who use corticosteroid inhalers should use a spacer to prevent getting the medicine in their mouth, where oral yeast infections and dysphonia can occur.