Many parents can’t tell when a child’s asthma inhaler is empty, a study released Monday suggests.

Metered-dose inhalers, which send asthma medications directly to the lungs, are an important part of preventing and treating asthma attacks. But the devices’ medication canister must be stored, used and replaced properly to be effective.

The new study, of 50 asthmatic children and their parents, found that while more than three-quarters of the parents knew they were supposed to shake the medication canister before letting their children take a “puff,” only half actually did so when asked to demonstrate how an inhaler should be used.

What’s more, 72 percent of the parents said they relied on their ears to tell them when the medication canister was empty, waiting until it no longer made a sound when a child took a puff.

However, this is a dubious measure, according to the study authors, led by Dr. Bruce K. Rubin of Wake Forest University School of Medicine in Winston-Salem, North Carolina.

When they tested several metered-dose inhalers containing various asthma medications, they found that there were far more audible puffs than the maximum number listed by the manufacturers - 54 percent to 86 percent more.

“If patients are not taught to recognize when a (metered-dose inhaler) is empty, they may continue to use the medication canister long past its intended duration,” the researchers report in the medical journal Chest.

According to experts, the only reliable way to judge whether a canister is empty is to subtract the number of doses used from the total available number listed by the manufacturer.

The new findings support this advice, according to Rubin and his colleagues, who also debunked another way some parents in the study used to define “empty”: seeing whether the medication canister floats in water. If it sinks, some parents believed, there is still medication left. If it floats, it’s empty.

Not only was this method ineffective when the researchers tested it, but it also allowed water into the canisters’ valve opening 27 percent of the time, making the practice “potentially dangerous,” they note.

Keeping track of doses by counting can be difficult and prone to error, the authors acknowledge. However, they add, until effective built-in dose counters are added to metered-dose inhalers, counting doses the old-fashioned way remains “the only accurate method” for judging when a canister is empty.

SOURCE: Chest, October 2004.