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Designing Usable Medical Devices for People with Diabetes

A focus on the design of diabetes-related devices highlights concerns and solutions for devices used by people with other medical conditions and age-related impairments.

DESIGN

Figure 1. (click to enlarge) Images show the how a clear image (a) might appear to a person with retinopathy (b), macular edema (c), glaucoma (d), and cataract (e). Image courtesy of Lifescan (Milpitas, CA).

Care to estimate how many people in the United States have diabetes? The National Institutes of Health pegs the number at 23.6 million.1 That's more than the entire population of Australia or the combined populations of Norway, Sweden, and Finland.

About 24% of the cases are undiagnosed, and 12.2 million cases involve people who are 60 years of age or older. Among adults with diagnosed diabetes, 84% take insulin or oral medications to control their blood sugar levels. The cost of providing direct medical care to this population in 2007 was estimated to be $116 billion, when diabetes ranked as the sixth leading cause of death.1

That means billions of interactions—a scary thought when you consider the consequences of use error. Fourteen percent of the 17.9 million Americans diagnosed with diabetes treat their diabetes with insulin (not including oral medications).2 If each of them interacts with a diabetes care device a hypothetical average of 3 times per day, their interactions would total 2.7 billion per year.2 For example, insulin misadministration caused by an erroneous meter reading, incorrect insulin pen setting, or insulin pump programming mistake can lead to hypoglycemia or hyperglycemia; both of which have serious health consequences. Therefore, companies that develop diabetes-related technology must exercise extreme care in design, particularly with regard to user interface design. A good starting point is to consider the ways that diabetes affects human beings' ability to interact with technology, and then consider the design ramifications.

Deficits

People with diabetes can be symptom free, particularly if they are young and maintaining good glycemic control (i.e., controlling their blood sugar level) through diet, exercise, and medication. However, many diabetic patients will eventually experience ill effects from the disease.

Vision. People with diabetes can develop significant vision problems. The common ones, macular edema, neovascularization, and retinopathy, are the consequence of high blood sugar levels (i.e., hyperglycemia) and high blood pressure. Contamination of the eye's normally clear vitreous humor, retinal swelling, and the growth of abnormal blood vessels can lead to blurred vision, black and blind spots, and total blindness. Figure 1 shows how these conditions might affect someone's view of a glucose meter's numeric readout, possibly leading to reading errors.

Figure 2. (click to enlarge) The left image shows a true-color representation of a device, whereas the photo at the right shows how the same image might appear to a person with deuteranope—a red-green color deficit.

Diabetes can also lead to color blindness, affecting people's ability to differentiate a home dialysis machine's controls by color alone (see Figure 2).

Sense of Touch. Over time, diabetes can rob people of their sense of touch due to progressive nerve damage. One condition within a family of related nerve conditions is called peripheral neuropathy. This malady can cause numbness, tingling, and pain in the toes, feet, legs, arms, hands, and fingers.3 People who have the condition might have difficulty handling devices and perceiving tactile feedback when they perform tasks such as connecting components, moving levers, and pressing buttons.

Mobility. Individuals who have had diabetes for several years can experience impaired physical mobility due to hyperglycemia. Specifically, high blood sugar can lead to a thickening of the tendons, which eventually reduces dexterity and overall flexibility.

Cognition. People with diabetes are vulnerable to hypoglycemia—low blood sugar. As a person's blood sugar level drops, he or she might have difficulty performing cognitive tasks and experience mental cloudiness.4,5 Accordingly, hypoglycemia can interfere with a person's ability to follow instructions without skipping steps, to navigate software menus, to perform mathematical calculations, and to recall displayed values, for example. Research has also shown that low blood sugar levels result in a “significant deterioration” in contrast sensitivity and visual information processing.6

Strength and Stability. For several medical reasons, people with diabetes might feel weak, dizzy, shaky, and twitchy. These physical states can complicate a wide range of physical interactions with medical devices, such as moving a moderately heavy device from a closet shelf to a counter, twisting and pulling apart components, and making fine adjustments to control knobs.

Design Opportunities

Figure 3. (click to enlarge) Suggested changes to insulin pump features as recommended by survey respondents.

Although the list of possible physical and mental impairments associated with diabetes is large, device designers can take relatively simple steps to accommodate a sizable proportion of people with the disease (see Figure 3).

Ensure Information Legibility. Human factors guidelines suggest that important information, such as blood glucose readings and bolus dose amounts, should be formed of characters that subtend a visual angle of 20–22 minutes of arc to ensure that people with normal vision can read them.7 The rule applies to information displayed on low- and high-resolution displays (e.g., a seven-segment LCD display and computer screens, respectively) as well as printed material (e.g., instructions, warnings, component labels).

Additional typographical traits, including character proportions, boldness, style, color, and spacing affect legibility. Line thickness should be about one-sixth of the character's height8,9; simple or sans-serif fonts and proportional spacing are preferable. However, assuming the use of simple typography (e.g., black, plain, Helvetica text on a white background), overall size matters most.

Visual angles (or preferred character heights) can be calculated using the following formula:

Visual Angle = Arctangent (Character Height ÷ Viewing Distance) × 60

Character Height = Tangent (Visual Angle ÷ 60) × Viewing Distance

In most cases, these trigonometric equations yield values similar to those as follows:10

Recommended Character Size for Critical Information = Maximum Viewing Distance ÷ 150

In addition, the recommended character size for key, noncritical information is equal to the maximum viewing distance over 300.

Based on this formula, critical text and numbers read at an arm's reach (22 in.) should be at least 0.147 in. tall (approximately a 10.5-point-size font). However, this size could be too short for people with visual impairments. Calculating a maximum height would pose a dilemma because a small proportion of users with poor eyesight (e.g., central visual acuity ≤20/200 and legally blind) would call for extremely large characters.

This accommodation might not be feasible while also producing an acceptably compact device for use by the general population. For example, one might need to draw the line at a 50% increase in character height and call upon users with particularly poor eyesight to get closer to the display (e.g., 12 in. instead of 22 in.) and perhaps even use a magnifying glass. These are admittedly less-desirable accommodations by comparison to design enhancements.

One solution is to produce multiple devices, one with a large display to accommodate large text. This approach is comparable to printing books in normal and large text. Another solution is to enable users to set the character size from normal to large to very large, for example, to accommodate individual reading ability. The latter approach might permit less information to be placed on a given display or screen at once and, therefore, require additional user interface design and programming.

Facilitate Handling. Accommodating people who have reduced tactile sensation and manual dexterity is largely a matter of common sense, such as making push buttons bigger, spacing them farther apart than usual, and increasing tactile feedback. On spacious control panels, push buttons might be 0.5 sq in. or round with their center points spaced 0.75 in. apart.11

However, compact devices, such as insulin pumps, might not allow for such sizing and spacing. Therein lies the potential problem: users struggling to locate the correct push button (sometimes without looking) and pressing it without accidentally striking others. One solution is eliminating unnecessary push buttons to make more room. Another is to move some push buttons off the main control panel—typically a flat surface—and onto an adjacent surface (e.g., one of the four sides on a device shaped like a box of cough drops). The same principle, making components bigger and spacing them farther apart, applies to many other physical components, such as battery compartment door releases and screw-on caps.

Here are some more accommodation strategies:

  • Give components easy-to-distinguish shapes. For example, use a combination of rectangular, circular, and triangular push buttons.
  • Add Braille, pillow-embossed icons, or other types of tactile features that can make product features more identifiable by touch as compared with flat, featureless surfaces.
  • Design components to emit distinctive sounds when actuated (e.g., a natural or electronic click). The aural feedback will make up for any lack of tactile feedback.
  • Make a control position visually obvious, supplementing tactile cues with visual feedback.
  • Design components to produce more tactile feedback (e.g., a more definitive click into place) and increase the potential for sensation. For example, one might increase the amount of key travel to at least 0.125 in. (3 mm).12
  • Limit the force required to actuate controls and other moving components (e.g., release a battery door).
  • Make finger and hand contact surfaces larger than usual, reducing the need for a pinch-type grip, for example, that might be difficult for people who have arthritic hands to achieve or maintain.

Notably, people with diabetes are especially prone to developing arthritis. Using Behavioral Risk Factor Surveillance System (BRFSS) data, the Centers for Disease Control and Prevention (CDC) recently estimated that 52% of adult Americans with diabetes also have some form of arthritis, as compared with 27% of adult Americans who do not have the disease.13

Ensure Comprehension. For various reasons, people with diabetes might experience temporary periods of confusion. Hypoglycemia—an abnormally low level of glucose in the bloodstream—is a common cause.
Besides impairing cognitive function, hypoglycemia can lead to unconsciousness. Imagine someone who has become hypoglycemic, perhaps because he or she has forgotten to perform a detailed operating procedure. Fair to say, the person is more likely to commit an error than if blood glucose level were in the normal range, allowing for greater clearheadedness versus being in a “mind fog.” Anticipating such potentials, medical device operations should not place excessive demands on memory or logical processing. Here are some related design strategies:

  • Direct users to perform procedures in a discrete, step-by-step fashion, as opposed to directing users to perform multiple steps at a time.
  • Number steps so that users can stay on track and not skip an essential action.
  • Integrate additional confirmation screens (i.e., safety checks) into critical device interactions. Although additional screens might annoy cognitively stable users, they are likely to reduce the chance of inadvertent insulin delivery.
  • Provide short, simply worded prompts.
  • When possible, display information needed to accomplish a task, rather than requiring users to recall it.
  • Limit the use of symbols—particularly abstract ones with no real-world resemblance to a familiar object—in place of text, because symbols require interpretation while words are unambiguous.

One additional strategy is to require users to confirm critical actions, such as deleting glucose meter test data. However, this strategy applies equally to interactions involving all types of users, not just those that might be performed by an individual with briefly or chronically diminished mental acuity.

Device User's Perspective


Sidebar:
The Need for Improved User Interfaces

People with diabetes are usually eager to help manufacturers optimize their devices' user interfaces because the devices play such a large role in preserving their good health, and they use them so frequently. Therefore, device manufacturers should get users involved in the design process. Not only will such involvement help guide user interface designs in the right direction, but it will also help to satisfy device regulators who look for evidence of good human factors engineering (see the sidebar, “The Need for Improved User Interfaces”).

The challenge is to obtain useful feedback. Effective ways to collect such feedback include the following:

  • Establish a user advisory committee that meets frequently to review and comment on designs in progress.
  • Conduct group interviews (a.k.a. focus groups) with targeted user populations.
  • Observe users performing tasks with existing devices and new prototypes.
  • Conduct usability tests early and throughout the development process.

Kate (name changed on request to maintain patient confidentiality), a 26-year-old special education teacher who was diagnosed with type 1 diabetes as a child, concurs that people like her should have a strong role in developing device design. She says, “My visual impairment is so severe, I can't read my insulin pump's screen without a strong, 12× magnifier. I have to navigate the pump by memorizing the menus and counting button presses because the on-screen text is so small…I think insulin pumps should have a speech output feature. This would allow the pump size to shrink substantially without reducing accessibility for visually impaired users.” Kate suggests that insulin pump manufacturers network with impaired and disabled users to ensure usability and accessibility.

Roseanne, a 48-year-old who has had type 1 diabetes for almost 40 years, struggles to read her insulin pump's screen due to retinopathy, third nerve palsy, and double vision. A couple of strokes in February further weakened her eyesight. Regarding her glucose meter, she says, “The screen is not bright enough, so it's hard to read—even with the greenish backlight.” Accordingly, Roseanne hopes manufacturers develop future insulin pumps with brighter screens that increase legibility and enable use in suboptimally lit environments by all users.

Howard Wolpert, MD, director of the Insulin Pump Program at the Joslin Diabetes Center (Boston), believes that the medical device industry has made great strides in recent years, noting that devices such as glucose meters are becoming simpler and more accommodating to users with physical impairments. He says, “On-screen text size is an important issue for many users, and we're finally seeing new glucose meters and insulin pumps featuring brighter, backlit, high-resolution screens that are easier to read.”

Wolpert explains that most devices lack the safety features needed to ensure appropriate use by individuals who are feeling fuzzy-headed due to low glucose levels. I've had two patients over the past two months who accidentally delivered extra insulin through their pumps during hypoglycemic episodes. This is the exact opposite of what people should do.” Wolpert adds, “Manufacturers have traditionally regarded diabetes management devices as medical products, when in fact they are consumer products. The interface is the key element for user interactions, and to ensure that these products are widely usable and that they provide maximum benefit to users, there needs to be a greater focus on human factors.”

Conclusion


For more than 10 years, Michael Wiklund has written for MD&DI. You can read a selection of his articles online.

This article has focused on the user interface design needs of people with diabetes, who number in the millions in the United States alone. But many of the design concerns and solutions discussed here have wider application to devices used by people with other medical conditions and age-related impairments, such as multiple sclerosis, arthritis, and low-grade dementia. The overarching message is to design user interfaces to match the needs of “worst-case users,” rather than the average or idealized user.13 Moreover, designers should not design devices such as insulin pumps so that they only work well for able-bodied 20-year-olds. Rather, they should energetically study the full gamut of users as a basis for formulating design requirements. Such diligence can lead manufacturers to design solutions that truly serve the users' needs and preferences, which isn't so bad for business either.

Michael Wiklund is founder and president of Wiklund Research & Design Inc. (Concord, MA). Contact him via www.wiklundrd.com. Allison Yale is managing human factors specialist at Wiklund Research & Design. She can be contacted at [email protected].

References

1. National Diabetes Information Clearinghouse (NDIC), statistics for 2007 [online] (Bethesda, MD: NIH, 2008 [cited 5 August 2008]); available from Internet: http://diabetes.niddk.nih.gov/dm/pubs/statistics/#7.

2. NDIC, statistics for 2007 [online] (Bethesda, MD: NIH, 2008 [cited 15 August 2008]); available from Internet: http://diabetes.niddk.nih.gov/dm/pubs/statistics/#treating and http://diabetes.niddk.nih.gov/dm/pubs/statistics/#allages.

3. NDIC, Diabetic Neuropathies: The Nerve Damage of Diabetes [online] (Bethesda, MD: NIH, 2008 [cited 15 August, 2008]); available from Internet: http://diabetes.niddk.nih.gov/dm/pubs/neuropathies/.

4. NDIC, Information about Hypoglycemia [online], (Bethesda, MD: NIH, 2008 [cited 5 August 2008]); available from Internet: http://diabetes.niddk.nih.gov/dm/pubs/hypoglycemia/index.htm.

5. Bulletin #44 [online] (Ashton, MD: Hypoglycemia Association Inc., 2008 [cited 5 August 2008]), available from Internet: www.fred.net/slowup/habul44.html.

6. RJ McCrimmon et al., “Visual Information Processing during Controlled Hypoglycaemia in Humans,” Brain 119 (1996): 1277–1287.

7. AAMI HE75 (Committee Draft), “Human Factors Principles for Medical Device Design” (Arlington, VA: Association for the Advancement of Medical Instrumentation, 2008), Section 6.2.3.5 Minimum Visual Angle, 61.

8. MS Sanders and EJ McCormick, Human Factors in Engineering and Design, 7th ed. (Boston: McGraw Hill, 1993).

9. AAMI HE75 (Committee Draft), “Human Factors Principles for Medical Device Design” (Arlington, VA: Association for the Advancement of Medical Instrumentation, 2008), Section 16.3.2.1.9 Text, 298

10. AAMI HE75 (Committee Draft), “Human Factors Principles for Medical Device Design” (Arlington, VA: Association for the Advancement of Medical Instrumentation, 2008), Section 19.5.5.2 Text Size, 376.

11. AAMI HE48, “Human Factors Engineering Guidelines and Preferred Practices for the Design of Medical Devices,” 2nd ed., (Arlington, VA: Association for the Advancement of Medical Instrumentation, 1993), Figure 21: Design and separation: legend (touch/push buttons) switches, 55.

12. “Arthritis as a Potential Barrier to Physical Activity among Adults with Diabetes—United States, 2005 and 2007,” Morbidity and Mortality Weekly Report [online] (Atlanta: U.S. Centers for Disease Control and Prevention (CDC), May 9, 2008, 57 (18), 486–489; available from Internet: www.cdc.gov/mmwr/.

13. M Wiklund, “Defining and Designing for Worst-Case Users,” Medical Device & Diagnostic Industry 28, no. 7 (2006): 52–59.



Copyright ©2008 Medical Device & Diagnostic Industry
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