A Metered-Dose Inhaler (MDI) is a small, portable medical device used to deliver aerosolized medication directly into the lungs. It is widely prescribed for managing chronic respiratory conditions, such as asthma and Chronic Obstructive Pulmonary Disease (COPD). The MDI works by releasing a precise, measured dose of medication when activated, ensuring a consistent amount of drug is available for inhalation. This direct delivery provides a rapid and targeted therapeutic effect for patients experiencing breathing difficulties.
Defining the Device and Delivery Mechanism
The MDI device consists of two primary parts: a metal canister and a plastic actuator, which serves as the mouthpiece. The canister holds the medication, typically suspended or dissolved in a propellant and other excipients. When the user presses down on the canister, a specific amount of the mixture is released through a metering valve.
The term “metered dose” refers to the precise, consistent quantity of medication released with each activation or “puff.” The metering valve ensures this uniformity for consistent dosing and treatment efficacy. Propellants are an integral part of the formulation, providing the driving force needed to atomize the drug into a fine mist.
Modern MDIs primarily use a hydrofluoroalkane (HFA) propellant, which replaced the older chlorofluorocarbon (CFC) propellants. When the dose is released, the HFA propellant rapidly evaporates, creating a plume of aerosolized medication. This fine aerosol cloud is then inhaled deep into the lower airways, where the medication exerts its therapeutic effect.
Step-by-Step Guide to Proper Usage
Effective MDI use requires careful coordination and technique to ensure the medication reaches the lungs instead of being deposited in the mouth or throat. Preparation involves removing the cap and shaking the inhaler vigorously several times to properly mix the drug and propellant. If the inhaler is new or has not been used in a while, it must be “primed” by spraying a few test puffs into the air, as directed by the manufacturer.
To prepare for inhalation, the user should sit or stand up straight and exhale fully. This creates space within the lungs to draw in a deep breath of the aerosolized medicine. The user should then place the mouthpiece into their mouth, closing the lips tightly around it to create a seal.
The most challenging step is coordinating the press and the breath: as the user begins to inhale slowly and deeply through the mouth, they must simultaneously press down firmly on the canister to release the dose. The inhalation should be slow and steady, ideally lasting three to five seconds, to pull the medicine deep into the lungs. If the breath is taken too quickly, the medication may impact the back of the throat.
After inhaling the dose, the user should remove the inhaler from their mouth and hold their breath for about five to ten seconds, or for as long as is comfortable. This allows the small aerosol particles to settle onto the walls of the airways.
If a second dose is required, the user must wait approximately 15 to 30 seconds before repeating the entire process, starting with shaking the inhaler again. After using a corticosteroid MDI, rinsing the mouth with water and spitting it out helps reduce the risk of side effects like oral thrush.
The Role of Spacer Devices
A spacer, also known as a holding chamber, is a tube-like accessory that attaches to the MDI mouthpiece. This device significantly improves the drug delivery process. Spacers function by holding the aerosolized medication in a chamber after actuation, allowing the user to inhale the dose slowly.
Using a spacer eliminates the need for perfect hand-breath coordination, which is often difficult for many people, especially children. When the medication is sprayed into the chamber, the large, fast-moving particles slow down. This reduction in particle speed means less medication is deposited in the mouth and throat, decreasing localized side effects.
The device increases the amount of medication that successfully reaches the lower airways. Without a spacer, delivery efficiency is low, but proper spacer use significantly improves this. Valved holding chambers are a specific type of spacer that includes a one-way valve, keeping the medicine inside the chamber until the patient breathes it in.