WESTPORT, CT (Reuters Health) – There appears to be little support for using the position of a pressurized asthma medication canister when placed in water as an accurate means of determining the number of remaining doses, according to New Jersey researchers.
Drs. W. Travis Cain and John J. Oppenheimer of Pulmonary and Allergy Associates, Springfield, note that although counting the number of actuations of a metered-dose inhaler (MDI) may be accurate, it is "an impractical means of keeping track."
One alternative method is to use the floating position of the canister to assess when the medication is exhausted and propellant alone remains. However, they say, new hydrofluoroalkane alternatives to chlorofluorocarbon propellants may affect buoyancy and floating characteristics.
To investigate, the researchers examined the properties of 15 albuterol sulfate, 15 triamcinolone acetonide and 15 fluticasone propionate MDIs. The albuterol device used an HFA propellant. The findings were published in the November issue of the Annals of Allergy, Asthma and Immunology.
The full canisters were floated in water, their attitude was noted and the actuators were then re-attached. This procedure was repeated following 25%, 50%, 75% and 100% of the prescribed number of actuations. The canisters were then actuated until completely discharged and immersed once more.
Each type of canister showed a distinct floating pattern. For example, the HFA canisters maintained a vertical floating position just under the surface from full to 75% of actuations. However, although 12 tilted on to their sides at the prescribed 100% of actuations, the remaining 3 did not do so until they were completely discharged.
When full, the fluticasone canisters adopted a horizontal position on the bottom of the container. At 25% of actuations they moved to a vertical position at the bottom of the container. However, at 50% they adopted a vertical floating position, which did not change at 75 and 100% of actuations.
The attitude of the triamcinolone canisters showed more content-related variation. Nevertheless, they maintained the same position after 50 and 75% of actuations.
In light of these findings, the researchers conclude that for both CFC and HFA MDIs, "the floating method is not an accurate means by which patients can identify the amount of medication contained within an inhaler device."
Dr. Oppenheimer told Reuters Health that in some cases, MDIs may give 100 extra actuations–probably consisting only of propellant–beyond their rated delivery. "Overall," he pointed out, "this study reinforces the difficulty in determining when an MDI is empty. This is an important issue, as it likely represents a major impediment in chronic asthma care. This problem could likely be overcome if dose counters were available on MDIs."