An inhaler is a very simple device for patients to use and one that is easily identifiable by those who require it. However, concerns with gauging the remaining amount of doses have caused device manufacturers to revisit its design. This article examines the steps manufacturers need to consider when reevaluating the design of familiar medical devices utilizing the inhaler as a real-world example.

By Chris Kadamus
Every year, millions of respiratory illness sufferers will hold their metered dose inhalers (MDI) and attempt to determine the number of effective doses that remain. They will shake, float, weigh, and employ any number of estimation methods that, unfortunately, often provide inaccurate results. For some, the number of effective doses remaining in their MDI will mean the difference between quick relief from an asthmatic episode and a trip to the hospital. In fact, a study published in the July 2006 volume of the Annals of Allergy, Asthma and Immunology, found that 25% of the respondents who used bronchodilators had reached for their MDI during such an episode only to find that it was empty.

Left: The use of a decrementing fuel gauge helps meet the FDA guidance document's requirement that the counting device indicate, in advance, when the MDI is nearing the end of its recommended doses.
Right: Legibility and location can also be factors in the efficacy of the dose counter display mechanism, as in the use of a numbered, rotating disk.

In an attempt to remedy this issue, the FDA released a guidance document for the application of dose counters to MDI devices in 2003. While technically not a regulation, this document has set the stage for the industry to move toward implementing more accurate methods for patients to track usage. Subsequently, a great deal of patent activity has taken place worldwide on MDI dose counting, with the charge led by GSK and the release of the Evohaler. Today, companies attempting to develop dose counting MDIs can have difficulty just navigating through the numerous patented approaches.

Due in part to the patent environment as well as to the inherent engineering difficulties in applying new functionality to an existing product, dose counters have employed various mechanisms and technologies. The emphasis has been on engineering accurate and novel counting schemes, such as systems with rotating gears, linear slides, or even electronic counting methods.However, it is also important to consider how the application of this new functionality affects the usability of a formerly simple device. Without adequate examination of usability, even the most elegant design options may fall well short of their goals, or negatively affect MDI dosing performance.

Six Factors for Maintaining Usability

MDIs have been on the market for 50 years, with the primary functionality of the device remaining predominantly unchanged. Respiratory illness sufferers who use MDIs are familiar with the overall design and operation. Asking patients to adapt their methods of usage introduces another variable into the efficacy of the device. Design engineers should plan any device alteration, even the seemingly simple addition of a dose counter, so that it allows the patient to use the MDI in the same basic manner as he has done for the duration of treatment.

To maximize patient usability, design engineers should address six key factors when creating dose counting systems for addition to MDI devices.

1. Counting of doses should require no additional user time or effort and attempt to provide the user with a new level of convenience and confidence in device reliability.

2. Though not specifically mentioned in the FDA guidance, it is essential that the counting device should not affect the delivered dose. Placing any counting device in the path of the delivered drug has the potential to attenuate or alter the consistency of the dose and thus, is considered undesirable. As such, dose counting solutions have been restricted to operating on valve displacement and canister motion. Relying on force or displacement mechanisms to register counting can have the unfortunate affect of adding to the force necessary for actuation. Any significant change in actuation force could result in increased usage errors, especially for children, the elderly, or the disabled. In addition, adding mechanisms that reduce the smooth actuation of current MDIs could result in the perception of errors or malfunction by the user. Maintaining the simplicity of the MDI actuation functionality is essential to the success of the product.

MDI dose counters can use colors, numbers, and spatial relations to display remaining doses.

3. Designers should attempt to maintain a relatively similar device size when adding dose counting systems to traditional MDIs. The integration of a counting mechanism to a traditional MDI requires a number of physical changes to the plastic housing. The most significant of these changes will be to the overall size of the unit. Yet many MDI users, particularly those who carry bronchodilators every day for the treatment of acute symptoms, would likely find an increase in size to be a serious detriment for portability and ease of use. Adding material and/or parts not only adds costs for the device manufacturer, but also increases perceived complexity on the part of the patient. The ultimate goal should be to ease the patient’s transition between the traditional MDI and the dose counting MDI, not to force the patient to adapt to a new product incarnation.

4. The approach used to connect a counting device to an MDI influences the mechanical configuration of the dose counter itself, and thus its usability. Attaching a counting mechanism to the MDI housing and not the drug canister, while a step in the right direction, does not entirely solve the problem of accurate counting. Ideally, the counting device and drug canister should be inseparable so as to eliminate the possibility of using multiple drug canisters with one counter. The housing may then be separated from the canister and counter for cleaning without worry of imparting errors on the dose counter. Permanently attaching a counter to the smooth cylindrical canister may present a significant engineering challenge—yet the patient benefits to such an approach are significant in terms of maintaining counting accuracy and tamper resistance. Integrating the counting mechanism with the canister affords the patient absolute confidence in the information displayed by the counter and addresses the security required in the FDA guidance.

5. The display or indicator showing remaining doses must be intuitive and easy to interpret. While the FDA guidance does not recommend any particular display method, such as color coding, numeric, or the use of a spatial indicator, the guidance document does explicitly state that the counting device must provide an indication that the MDI is nearing the end of its recommended doses far enough in advance such that the patient can acquire more medication. Although the use of a decrementing numerical counter seems like the most straightforward choice, it is debatable whether numbers alone make for the most intuitive display mechanism.

Adding a fuel gauge-type of indicator and colors as well as numbers can enhance the display and provide the user with an immediate signal as to how many doses remain. A fuel gauge is a familiar graphic across all geographies and can indicate a reasonably accurate number of remaining doses, with minimal use of numbers or colors, in a clear, intuitive form. A linear display, especially one that decrements top to bottom, is similarly recognizable by nearly all users, and with the addition of color, may provide a solution that requires no numeric display. However, the most effective display solution would likely combine several or all of these elements, using spatial relations to allow for gross counting at a glance, numeric elements to provide accurate counting, and colors to indicate warnings as the drug depletes to the last recommended dose.

6. Legibility and location can also be factors in the efficacy of the display mechanism.
Printed or displayed characters—numeric or alphabetic—smaller than 2.0 mm in height may be too small for vision-impaired or older individuals to interpret. A character height of 2.5 mm is preferable for the majority of the population. Display location plays a role in the adoption of the device by the patient. A display that is visible every time the unit is used will elicit more conscious attention from the patient than a display that cannot readily be seen. The location of the display on the MDI can act as a visual reminder for the user to be aware of the number of remaining doses. Prominent locations can encourage the user to perceive the display mechanism as an integral part of the unit, rather than seeming like an added function. Again, easing patient transition should be the primary goal.


Beyond the benefit to users, adding dose counters to traditional MDIs may allow significant improvements in the MDI’s functionality. The nature of the MDI valve system allows for partial actuation if the user does not fully compress the valve spring. Partial actuation can lead to insufficient dosing of the drug, especially since inactive components can make up 95% to 99% of the drug formulation. If a dose counter is designed into an MDI, it can also function as a mechanism to prevent partial actuation. By employing the use of gears or ratchets, full actuation can be induced every time the unit is activated. Lockout mechanisms to prevent inadvertent actuation or to lock units once all the recommended doses have been released would also be beneficial additions to many types of MDIs. (Note that the FDA guidance does not recommend lockout mechanisms for “rescue bronchodilators”—MDIs used to treat acute bronchospasm.) If an electronic dose counting method is implemented, units may be used for dose tracking and provide physicians with information regarding the incidence of acute symptoms or warnings of drug misuse or abuse.

MDI manufacturers have been presented with a significant challenge in designing dose counting technology that integrates smoothly with traditional MDIs. However, retooling and redesigning an MDI to add dose counting also presents an outstanding opportunity to create a more efficient, intuitive, and effective device, provided designers address key usability factors. Doing so can significantly improve the patient experience and establish an important competitive edge in the growing inhaled drug market.


Bradshaw D. “Developing dose counters: An appraisal based on regulator, pharma and user needs.” Respiratory Drug Delivery 2006. 2006; 121-131

Guidance for Industry, Integration of Dose-Counting Mechanisms into MDI Drug Products, FDA, March 2003.
For additional information on the technologies and products discussed in this article, visit Cambridge Consultants at

Chris Kadamus is a senior design engineer at Cambridge Consultants. He has over eight years of experience in designing medical devices. Kadamus can be reached at 617-532-4700 or