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An asthma spacer is an add-on device used to increase the ease of administering aerosolized medication from a metered-dose inhaler (MDI). The spacer adds space in the form of a tube or “chamber” between the canister of medication and the patient’s mouth, allowing the patient to inhale the medication by breathing in slowly and deeply for five to 10 breaths.
After removing the MDI’s cap, the MDI is inserted into the back-piece. The front part of the chamber is closed off by either a facemask that covers both the patient’s mouth and nose, or simply a mouthpiece that goes in the patient’s mouth. To administer the medication, the patient brings the facemask to the face (or the mouth-piece to the mouth) and depresses the metered-dose inhaler once, resulting in the release of one dose of medication. The medication from the MDI is then briefly suspended in the spacer’s chamber while the patient inhales the aerosolized medication by breathing in and out deeply at a slow rate. Some spacers are equipped with a whistle, which sounds as a warning when the patient is inhaling too quickly.
The facemask on the spacer has valves which insure that the medication suspended in the chamber is inhaled by the patient, and that the exhaled breath exits the device through the exhalation valve mounted in the mask. When using a spacer without a facemask, the patient must inhale through their mouth and exhale through their nose. Spacers with facemasks are used in toddlers and young children because that population is unable to coordinate inhaling through the mouth and exhaling through the nose. However, the facemasks are available in small, medium, and large sizes, and spacers with facemasks may also be used in the adult and elderly population.
The term spacer is often used to refer to any tube-like MDI add-on device. Some spacers utilize a collapsing bag design to provide visual feedback that successful inspiration is taking place. Another type is transparent plastic in two vase-shaped parts that come together forming a barrel shape.
In order to properly use an inhaler without a spacer, one has to coordinate a certain number of actions in a set order (pressing down on the inhaler, breathing in deeply as soon as the medication is released, holding your breath, exhaling), and not all patients are able to master this sequence. Use of a spacer avoids such timing issues. Spacers slow down the speed of the aerosol coming from the inhaler, meaning that less of the asthma drug impacts on the back of the mouth and somewhat more may get into the lungs. Because of this, less medication is needed for an effective dose to reach the lungs, and there are fewer side effects from corticosteroid residue in the mouth.
Valves on a spacer (which technically makes it a holding chamber) cause the patient to inhale the contents of the spacer, but exhalation goes out into the air. The problem of co-ordinating an inhalation with a press of an inhaler is avoided, making use easier for children under five and the elderly. It also makes asthma medication easier to deliver during an attack. For this reason, many advise use of spacer.
Use of spacers may also reduce the risk of developing a yeast infection, known as oral thrush. Alternatively, rinsing the mouth after application of inhaled steroids will prevent oral thrush as well. 
Whereas people with asthma can discreetly keep an MDI close-by at all times, the bulkiness of spacers can limit patient's utilization of spacers outside of the home. That being said, there are now smaller, easily separable spacers now available from various makers, which are more compact and so provide a more portable option.