MDIs are designed for at-home administration of aerosolized drugs and are the preferred devices to deliver maintenance glucocorticoid
and bronchodilator medications in people with asthma or chronic obstructive pulmonary disease (COPD). Particles delivered
by MDIs are larger than those created by nebulization and, thus, do not penetrate as deeply into the respiratory tract.1
How they work. A traditional pressurized MDI consists of a mouthpiece and an actuator (holder) into which a canister of medication is inserted.
Manually depressing the canister (actuation) releases a single dose of medication (sometimes called a puff). People shake the canister, exhale deeply, insert the mouthpiece, and simultaneously depress the canister and inhale as
deeply as possible. They then hold their breath for as long as possible, exhale, and rinse the mouth and expectorate to remove
most of the drug deposited in the oropharynx (only about 10% of each dose reaches the airways). Since dogs and cats cannot
use a MDI in this way, spacer devices designed to fit MDIs have allowed MDIs to be adapted for use in animals.
Equipment needed. Several types of spacers are available, from simple tubes inserted between the MDI and the mouth and nose to holding chambers
with one-way valves activated by inhalation. Spacers were designed for young children or debilitated adults with less than
ideal coordination, so simultaneous canister depression and inhalation is not necessary. The spacer also has the advantage
of allowing the largest particles to fall out and not enter the patient's mouth. In people, spacers improve drug delivery
by about 10%, nearly doubling the amount of drug reaching the target site.1,2
Some spacer devices are made specifically for veterinary patients, such as Aerokat and NebulAir Small Animal Chamber. Other
devices for people can be adapted for veterinary use, such as OptiChamber Advantage Valved Holding Chamber (Philips Respironics)
(Figure 3). The devices for people can be purchased inexpensively over-the-counter from any pharmacy. Face masks can be purchased (anesthetic
masks are about $40) or fashioned. The MDI fits directly in one end of the spacer.
3. Propellant-driven MDIs can be adapted for use in cats or dogs by attaching a spacer device and a face mask. Spacers made
for people can be adapted (as seen here), but spacer and mask combinations are also available specifically for use in cats
Recent developments. Until the last several years, most MDIs used chlorofluorocarbons as propellants. Chlorofluorocarbons are being phased out
because of concerns about the ozone layer. The drugs previously used in MDIs must be reformulated either to use alternative,
ozone-safe propellants or new technologies including breath-actuated inhalers.
Breath-actuated inhaler devices contain no propellant but instead rely on the patient's inhalation through a reservoir containing
the drug dose. The most common types include dry powder inhalers, such as disk inhalers and turbohalers (Figure 4). These breath-actuated inhalers do not use a spacer device and require a voluntary inhalation of a minimum force to deliver
the drug, so they are not readily adapted for dogs or cats.
4. Many drugs designed for inhalant delivery in people are available in breath-actuated delivery systems not readily adapted
for administration to pets.
Because breath-actuated inhalers are not readily used by all people (e.g. infants) either, some drugs are still available in pressurized MDIs using alternative propellants such as hydrofluoroalkanes.
The recent change to newer formulations of MDIs has dramatically increased the cost of some medications formerly available
as inexpensive generic preparations.