metered-dose inhaler
A metered-dose inhaler, or MDI, is a small pressurized canister that puffs out a fixed dose of medicine as a fine spray each time you press it — a bit like a tiny aerosol can aimed at your lungs instead of the air. One press, one measured puff.
Inside, the drug is dissolved or suspended in a liquid propellant under pressure. Pressing the canister releases a precisely metered volume that flashes into a fast-moving aerosol of droplets small enough to be carried deep into the airways. The challenge is timing: you must press and breathe in slowly at the same moment, then hold your breath, so the spray follows the air into the lungs rather than splattering on the back of the throat.
Good technique matters enormously. Many people press too fast, breathe too hard, or forget to coordinate, so most of the dose lands in the mouth and is swallowed instead of reaching the airways. Adding a spacer chamber removes most of the coordination problem and improves lung delivery, which is why a spacer is often recommended, especially for children and during flare-ups.
A child with asthma is prescribed a salbutamol MDI with a spacer; the nurse demonstrates shaking the canister, pressing once, and breathing slowly through the spacer for several breaths.
MDI plus spacer is the standard combination for reliable dosing in children.
Older MDIs used CFC propellants; modern ones use HFA propellants, which is why the spray feels softer and less cold than the inhalers some people remember.