Metered-dose inhaler

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A metered-dose inhaler (MDI) is a device that delivers a specific amount of medication to the

A metered-dose inhaler consists of three major components: the canister, which is produced in aluminum or stainless steel by means of

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.^[3]

Uses

Metered-dose inhalers are only one type of

• Asthma inhalerscontain a
  medication
  that treats the symptoms of asthma.
• Nicotine inhalersallow cigarette smokers to get nicotine without using tobacco, much like nicotine gum or a nicotine patch. The US Food and Drug Administration (FDA) has approved some nicotine inhaler products for smoking cessation.^[5] Nicotine inhalers that are marketed as nicotine replacement therapy should not be confused with electronic cigarettes, which produce an aerosol, usually from a tobacco extract, using a heating coil, while nicotine inhalers produce a noncombusted nicotine aerosol.^[5] A study found that "levels of carcinogens and toxins in e-cigarettes typically exceeded those measured in an FDA-approved nicotine inhaler."^[5] Nicotine inhalers are also known by their nickname of “the puffer.” These devices are made of thin plastic, sometimes resembling a cigarette or a cylinder shape. They contain a porous nicotine-filled plug located at the base of the product. When you puff on the inhaler, nicotine aerosol is inhaled and absorbed in the mouth’s lining. Every inhaler delivers almost four hundred puffs of this nicotine vapor. The nicotine inhaler is also temperature-sensitive. In cooler weather, less nicotine is delivered. The nicotine inhaler is considered more straightforward to use than the electronic cigarette as it is usually disposable and contains fewer parts than the electronic cigarette.
  • Nicotine inhalers also exist as non-MDIs; for example, the format for the Nicorette Inhalator is an “inhalation vapour solution.”
• Dry powder inhalers involve micronized powder, often packaged in single-dose quantities in blisters or gel capsules containing the powdered medication to be drawn into the lungs
  by the user's breath. These systems tend to be more expensive than the MDI, and patients with severely compromised lung function, such as those that occur during an asthma attack, may find it difficult to generate enough airflow to get good function from them.
• Metered-dose inhalers can be used to treat COPD, both in a stable state and during lung attacks.[6]

History

Before the invention of the MDI, asthma medication was delivered using a fragile and unreliable squeeze bulb nebulizer.^[7] 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.^[2] 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

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.[9]

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

The deposition of the content of drug formulation on the canister surface can result in a shorter shelf life of an MDI inhaler. Applying a suitable surface coating to the components helps to extend this shelf life. Over the years, various coating processes have been developed that can be applied to both the canister and valve to protect the contents from deposition and degradation. Gas plasma processing is an industrial technique carried out in a vacuum to coat the entire MDI inhaler. It involves constant or pulsed gas excitation by radio frequency (RF) or the microwave field to produce an energetic plasma. This coating ensures that the drug formulation does not stick to the interior wall of the MD inhaler and results in the patient receiving the prescribed dose of medication, extending the product's shelf-life.

A metered dose inhaler contains enough medication for a certain number of actuations (or "puffs") printed on the canister. Even though the inhaler may continue to work beyond that number of uses, the amount of medication delivered may not be correct. It is important to keep track of the number of times an inhaler is used so that it can be replaced after its recommended number of uses. For this reason, several regulatory authorities have requested that manufacturers add a dose counter or dose indicator to the actuator. Several inhalation products are now sold with a dose counter-actuator. Depending on the manufacturer and the product, inhalers are sold as a complete unit or the individual canister as a refill prescription.

Inhaler technique and use

While MDIs are commonly used in the treatment of lung-based disorders, their use requires dexterity to complete the required sequential steps to achieve the application of these devices. Incorrect completion of one or more steps in using an MDI can substantially reduce the delivery of the administered medication and, consequently, its effectiveness and safety. Numerous studies have demonstrated that between 50-100% of patients do not use their inhaler devices correctly, with patients often unaware that they are using their inhaled medication incorrectly.^[11][12] Incorrect inhaler technique has been associated with poorer outcomes.^[12][13] Incorrect maintenance and cleaning of metered dose inhalers is also an issue identified by many users, highlighting the need for clear guidance for patients prescribed MDIs.^[14][15]

Propellants

One of the most crucial components of an MDI is its propellant. The propellant provides the force to generate the aerosol cloud and is also the medium where the active component must be suspended or dissolved. Propellants in MDIs typically make up more than 99% of the delivered dose,^[16] so the properties of the propellant dominate more than any other individual factor. This is often overlooked in literature and in industry because so few propellants are used, and their contribution is often taken for granted. Suitable propellants must pass a stringent set of criteria, and they must:

• have a boiling point in the range of -100 to +30 °C ^[17]
• have a density of approximately 1.2 to 1.5 g cm⁻³ (approximately that of the drug to be suspended or dissolved)^[16]
• have a
  vapour pressure of 40 to 80 psig ^[18]
• have no toxicity to the patient [16]^[18]
• be non-flammable ^[16][18]
• be able to dissolve common additives (active ingredients should be either fully soluble or fully insoluble)^[16]

Chlorofluorocarbons (CFCs)

In the early days of MDIs, the most commonly used propellants were the

In 2008, the

Hydrofluorocarbon propellants have replaced CFC propellants.^[21] Concerns about the use of hydrofluorocarbon propellants have, however, since arisen since these compounds are potent greenhouse gases; propellants released during the use of a single inhaler result in a greenhouse footprint equivalent to greenhouse gases released during a 180-mile car journey.^[22]

Surfactant lipids

• Inhaler
• Dry powder inhaler
• Asthma
• Spray bottle

References