To evaluate how patients determine that their metered-dose inhalers(MDIs) are empty and to measure doses available of MDIs in differentlaboratory conditions.
Fifty new pediatric patients and their caregivers who used MDIsregularly were asked the question "How do you know when it istime to replace your inhaler?" and then were asked to elaborateon their answers. For the second part of the study, samplesof MDIs (Flovent, Serevent, albuterol, and Qvar) were obtainedfrom the manufacturers and studied in the laboratory. They evaluatedthe MDIs to determine how many actuations could be emitted andobtained weights during the process. They evaluated the usefulnessof floating the MDIs in water to determine if they were fullor empty, as has been suggested in the past for tracking thecontent of MDIs.
The survey revealed that 72% of subjects determined that theirMDI was empty when they could no longer hear a sound when actuated.Another 20% said they replaced it when it was "old" withoutgiving specific details, although most said "within a monthor so" or "after a while." Four patients stated that they weretold to float their MDI in water to determine if it was full(sinks to the bottom) or empty (floats), although none had actuallydone it. The majority (78%) said that they knew they were supposedto shake the MDI before using it, but only half shook the MDIwhen their technique was evaluated later. In the laboratory,MDIs had similar flotation patterns, with mean flotation anglesof 27.6 to 31.7° when empty. Water obstructed the valveor collected near the valve during this procedure 27% of thetime. The chlorofluorocarbon inhalers (Flovent, Serevent, andalbuterol) had a mean of 86% more audible puffs and Qvar 54%more than the stated manufacturer actuations. Shaking the MDIbefore actuation increased the doses available for the chlorofluorocarboninhalers significantly.
Most patients studied did not know how to tell if their MDIwas empty, and many did not shake the MDI before actuation,which can limit the amount of drug delivered. These resultsmay in part explain the poor adherence with refills for MDIs,because patients may not realize that they are not receivinga full dose of active drug (because all of the MDIs studiedhad significantly more actuations than noted on the canister),which the authors termed "pseudo-adherence." The only way totruly track the number of remaining doses in MDIs is to counteach dose. Most MDIs will emit more drug doses if the deviceis shaken before actuation. Floating MDIs in water is not accuratefor assessing remaining doses and often will clog the valve.
This article demonstrates one of the limitations of MDIs inthe inability of patients to accurately assess when they areempty without counting each dose. It illustrates the need forbetter devices to track doses remaining (an advantage of dry-powderinhalers).