Medical Device & Diagnostic Industry MagazineMDDI Article IndexOriginally Published January 2000DESIGNER'S NOTEBOOKThroughout the design and production process, manufacturers must focus on the needs and requirements of the end-user in the environment where the device will ultimately be used.

January 1, 2000

17 Min Read
User-Centered Design: A Clinician's Perspective

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

Originally Published January 2000

Throughout the design and production process, manufacturers must focus on the needs and requirements of the end-user in the environment where the device will ultimately be used.

In modern medical practice, the requisite amount of knowledge clinicians must absorb and use is enormous and increasing exponentially. The average clinician must diagnose, treat, and counsel growing numbers of patients, usually in an outpatient setting. Concurrently, the medical devices upon which every clinician depends have become ever more sophisticated, with a greater reliance on state-of-the-art science and engineering. As a result, it is becoming increasingly unlikely that clinicians understand the underlying technologies of the complex medical devices they use.

Given these circumstances, it is troubling that many medical device user interfaces are poorly designed, fail to adequately support the clinical tasks for which they are intended, and frequently contribute to medical error. At least until recently, informal surveys suggest that medical device companies give variable attention to human factors issues in device design.1

HUMAN FACTORS ENGINEERING

The successful development of ergonomically sound medical equipment and systems requires that manufacturers consider a device's desired end-user interface throughout the entire design cycle, beginning in the predesign phase. Human factors engineering (HFE) is the application of knowledge about human characteristics and abilities (physical, emotional, and intellectual) to the design and development of tools, devices, systems, environments, and organizations. FDA has become increasingly interested in ensuring that medical device manufacturers use HFE design principles and adhere to standardized good manufacturing practices (GMPs). The Association for the Advancement of Medical Instrumentation Human Factors Engineering (AAMI HFE) committee is currently drafting a national standard for the HFE design process for medical devices. A concurrent effort is under way by the International Electrotechnical Commission (IEC), which is developing a collateral standard to IEC 60601.

The study and involvement of users at every step of the design and development process is a cardinal element of HFE. A device's design focus must be on the needs and requirements of the actual end-users in the context of the environment where they will use the device. This approach, which has been termed user-centered design (UCD), demands close and continuous contact with users during the development process.

Designers should employ a variety of HFE methods to elicit the device's functional requirements, user preferences, and critical interface issues. Valuable user data are generally obtained in one of five ways: interviews and surveys, subject-matter expert exams and testing, laboratory testing (of varying sophistication and realism), qualitative naturalistic observation and assessment (in actual use environment), and quantitative naturalistic testing.

There are many examples—primarily in hospital-based acute-care medicine—of how medical device design flaws have contributed to user error, resulted in clinical inefficiency, or caused adverse patient events. In most circumstances, if manufacturers had employed a greater use of HFE and followed more closely a user-centered approach to the design process, a superior device performance would have resulted. High-workload, high-stress, high-risk environments, such as operating rooms or intensive-care units, benefit substantially from the incorporation of HFE in the design of medical devices.

USER-CENTERED DESIGN

An excerpt from the draft AAMI HFE standard describes the recommended broad scope of a UCD process for medical device design. The user interface comprises "all aspects of a device (including its labeling, instructional materials, and packaging) that users see, feel, and hear when operating the device." Users include "operators, maintainers, cleaners, and other service personnel, as well as other individuals directly affected by the use of the device." Thus, the draft standard continues, "a user may be a caregiver (e.g., anyone who gives a diabetic his/her insulin injection), a patient (e.g., diabetics who can administer their own insulin injections), or someone who provides support for either a caregiver or a patient (e.g., a diagnostic ultrasound technician). A caregiver may be a trained clinician, a layperson (possibly a family member), or the patient."2

Traditionally, FDA has focused solely on the safety and efficacy of medical devices and drugs. A safe device can be used without harm to patients, users, or adjacent personnel. An efficacious device accurately does what it was designed to do. Although clearly interrelated, usability must be distinguished from safety and efficacy. HFE specifically addresses device usability, a property that is determined by a device's user interface. Both safety and efficacy are more likely to be satisfied if the device is also usable. Poorly designed user interfaces promote user error, and thus can lead to adverse patient outcomes.

The AAMI draft standard suggests that medical device users (including physicians, nurses, therapists, technologists, patients, and service personnel) consider usability to be one of a device's most important design characteristics. They understand that a usable device will require less training to learn to operate and will make fewer cognitive demands on them. In addition, a usable medical device will help clinicians be more productive. For home-care devices, usability may be the key determinant as to whether patients will be able to use the device at all. Thus, manufacturers benefit from investing the necessary time and resources to ensure a device's usability.

A SYSTEMS APPROACH TO DESIGN

A systems approach must be used in the design of safe and effective medical devices. When an adverse patient event occurs in connection with the use of a medical device, it is rarely due solely to either user error or a poorly designed device-user interface. Most adverse medical events are a consequence of several interrelated events that are best understood as part of a complex interactive system comprising the user, the device, and the use environment.

Medical systems are characterized by complex and irregular structures, multiple interdependent components, diverse interactions, context-sensitive meanings, and the absence of any unifying representations. Thus, a medical device cannot be properly evaluated outside its context of use. The use environment includes the physical environment (e.g., the operating room, intensive-care unit, ambulatory clinic, etc.); the social and political environment; the institutional structure, including policies and procedures; the professional, organizational, and personal culture; and the legal and governmental constraints.

User requirements must be emphasized during the design of equipment and devices. The goal should be to produce devices that are easily maintained, have an effective user interface, and are tailored to users' abilities. This is best accomplished during the early phases of system and equipment design, when the ergonomics and engineering specialists can work together with end-users to produce a safe, reliable, and usable product.

Norman eloquently presented the rationale and principles of UCD in The Psychology of Everyday Things, a useful book for all managers, engineers, programmers, and designers involved in developing new medical devices.3 Norman provides several key recommendations for user-interface design, including: make things visible, provide good mapping, create appropriate constraints, and design for error. Although many of these principles may be routinely ignored or violated by some designers and manufacturers of today's medical devices, they are nevertheless vital to successful interface design.

Make Things Visible. A well-designed human-machine interface conveys to the user the purpose, operational modes, and controlling actions for the device. If the design of the device or system is based on a clear conceptual model, its purpose will be readily apparent to the user. Most devices have several operational modes, and the user must be able to determine rapidly and accurately whether the system is in the desired mode and when the mode changes. With most devices, a number of user actions are possible at any given time; with complex systems, the allowable commands often depend on the current operational mode. The user should be able to tell which actions are possible at any given instant and what the consequences of those actions will be. After each user action, feedback must be provided; it should be readily understandable and match the user's intentions.

The user's understanding of the function and operation of a device will critically influence the effectiveness of the system. The function and operation of many common devices is learned by experience within one's culture. People expect certain objects to function in a particular manner (e.g., knobs are for turning, buttons are for pushing). With other devices, the function can and often should be implied by the device itself. That is, by design, the purpose and operation of a particular control or display should be as intuitive as possible for the user (e.g., the sturdy horizontal handle on the side of a large medical device on wheels is obviously for pulling the device from one location to another).

Intuitive operation may be more difficult to attain with complex, microprocessor-controlled, multifunction devices. When the design requires the user to learn and retain specialized knowledge to operate the system, the need for training increases and there is a greater chance of system-induced user error, especially under stressful, unusual, or high-workload conditions.

Provide Good Mapping. Mapping is the relationship between an action and a response. It may be natural or artificial. Natural mappings are intuitive; artificial mappings must be learned. Artificial mappings that have been learned so well that the relationship between action and effect is recognized at a subconscious or automatic level are called conventional mappings. On an anesthesia machine, squeezing the bag in order to inflate the lungs is a natural mapping. Turning the oxygen flow control knob counterclockwise to increase gas flow is an artificial mapping. Because this design follows the conventional function of valves, however, users do not typically have difficulty adjusting the flow of oxygen on the anesthesia machine. Unfortunately, for many medical devices, the methods for activating alternate modes of action, adjusting alarm limits, or manipulating data are created using artificial, unique, or nonstandard mappings.

Create Appropriate Constraints. Constraints are limitations to the user's available options or actions. They can be physical, semantic, cultural, or logical. The provision of a control that can be oriented only in specific ways is a physical constraint (e.g., a switch can be either on or off). With a semantic constraint, the meaning of a particular situation controls the set of possible actions. For example, the sounding of an alarm is meant to indicate the need to take some kind of action. Cultural constraints are sets of allowable actions in social situations (e.g., signs, labels, and messages are meant to be read). Natural mappings typically work by logical constraints. When a series of indicator lights are arranged in a row, each with a switch beneath it, the logical constraint dictates that the switch underneath a particular indicator light controls, or is associated with, that light. Devices, particularly their human interface components, should contain constraints that facilitate simple, logical, and intuitive operation.

Design for Error. Human performance is prone to error.4 Slips—actions that do not go as planned—are a common form of human error arising from interactions with devices. Accidentally pushing the wrong button is an example of a slip. It is the device designer's responsibility to anticipate user errors and to minimize the risk that these inevitable errors will produce ill effects. Actions with potentially undesirable consequences should be reversible. The designer can also implement a forcing function, a type of constraint that prevents the user from performing an action that is clearly undesirable. An example of a forcing function is the oxygen/nitrous oxide interlock mechanism that prevents the delivery of a hypoxic gas mixture on an anesthesia machine.

These principles of good design are not limited to the interface between user and machine. A well-designed device is also easy to clean, maintain, and repair, and its documentation is organized and understandable.

Many currently available commercial devices violate these basic design principles. For example, an HFE evaluation of a microprocessor-controlled respiratory gas humidifier found that the device had hidden modes of operation, ambiguous alarm messages, inconsistent control actions, and complex resetting sequences.5 One respiratory gas analyzer had a hidden calibration mode that rendered it unusable if the sampling tubing was not attached when the unit was initially powered up.6 At least two different tourniquet controllers had no indicator that the cuff was inflated, although this impression was mistakenly given by a display of "cuff pressure" and a running timer on the front panel.

AVOIDING USER ERROR

Crisis situations tend to generate user errors that might not occur during less-stressful times. For example, during simulated crisis situations, many subjects forgot to coordinate the setting of the manual "bag/ventilator" selector switch, which determines whether a mechanical ventilator is attached to the breathing circuit or if the clinician must continue to manually ventilate the patient by regularly squeezing a breathing bag. As a result, the patient was not receiving any breaths at all, despite the audible indication that the mechanical ventilator was working.7

Devices that demand increased cognitive workload may affect performance on other job-critical tasks.8 Some events that at first glance appear to be caused by human error can be traced back to poorly designed interfaces between human and machine. Norman more strongly suggested that "the real culprit in most errors or accidents involving complex systems is, almost always, poor design."3 Devices are often used inefficiently or incorrectly as a consequence of poor design. When the device acts in unexpected ways, the user develops erroneous or inconsistent mental models of its operation. This problem can be exacerbated when the user has not received adequate instruction before using the device.

In addition to good operating practice, there are four other crucial components to help minimize the potential for user error:

  • The device design must be fundamentally sound.

  • The device must be properly constructed and implemented.

  • The operators must be thoroughly familiar with the device.

  • Ongoing quality control must ensure that use is appropriate over the full range of possible conditions.

Often a design is appropriate for one venue, but is transferred to another without taking into account the unique attributes of the new environment. For example, most of the original integrated physiological monitors were designed for use in the intensive-care unit and provided trend displays using a scale of six or more hours. For most anesthetic cases, the resulting display was far too compressed. The needs of anesthesiologists in the operating room are substantially different from those of nurses in the intensive-care unit or on the wards. This poses substantial design and marketing problems for drug infusion device manufacturers.

CONCLUSION

The optimal design of highly complex medical equipment requires a delicate balance between developing a device that is too complicated for the operator to understand and one that is overly simple. If there are too many displays, or if the displays are confusing, performance may be suboptimal, and errors can result during crisis situations. On the other hand, even well-intended attempts to simplify a device can produce equally poor results.

Cook et al. described a study involving a humidifier that had been redesigned from a manual to an automatic device.5 The clinicians liked the newer device, the study found, because it was "simpler." Yet they did not understand the device's underlying operation. For example, they did not know the procedure required to reactivate the device after an alarm, nor did they know that the heating elements were turned off after an alarm. To reset the device after an alarm, they simply turned it off and then on again. This device was not intuitive in its design or functionality.

There are increasing attempts to automate medical tasks and processes. Under some circumstances, automation can actually introduce more errors than it prevents. Conditions leading to error-prone automation include an inadequate system description; the automation of inappropriate tasks; clumsy implementation; excessive complexity or simplicity; failure to keep the user in the loop (loss of essential situation awareness); or tighter system coupling (removal of slack in the system), whereby minor errors are more likely to have catastrophic consequences.

The ultimate performance of a device is the most important consideration. Poor performance may be related to poor design, implementation, or construction; inappropriate use; or inadequate supporting equipment. Mosenkis suggests that excellent design should allow clinicians to use a device correctly the first time they interact with it, preferably without reading the manual; to use that device well, however, may require practice.9 Mosenkis goes on to assert that healthcare providers use medical devices in the same way they use automobiles: they expect that a new device will work similarly to equivalent older devices. This cognitive model of how medical devices should work can only be supported by the ubiquitous practice of user-centered design.

ACKNOWLEDGMENTS

Some material in this article is derived from the AAMI HFE draft standard, "The Human Factors Design Process for Medical Devices," a consensus standard under development by a committee of users, HFE experts, and industry representatives. It has been reprinted with the permission of AAMI, and may not be reproduced in whole or part without the express permission of AAMI.

Robert Loeb, MD, an associate professor of anesthesiology at the University of Arizona, contributed to this article.

REFERENCES

1. R Botney and DM Gaba, "Human Factors in the Design and Evaluation of Anesthesia Equipment: A Survey of Anesthesia Equipment Manufacturers," Journal of Clinical Monitoring 11 (1995): 263.

2. "The Human Factors Design Process for Medical Devices," ANSI/AAMI HE-XX. (In preparation), (Arlington, VA: Association for the Advancement of Medical Instrumentation, 1998).

3. DA Norman, The Psychology of Everyday Things (New York: Basic Books, 1988).

4. MB Weinger and CE Englund, "Ergonomic and Human Factors Affecting Anesthetic Vigilance and Monitoring Performance in the Operating Room Environment," Anesthesiology 73 (1990): 995–1021.

5. RI Cook et al., "Evaluating the Human Engineering of Microprocessor-Controlled Operating Room Devices," Journal of Clinical Monitoring 7 (1991): 217–226.

6. SS Potter et al., "The Role of Human Factors Guidelines in Designing Usable Systems: A Case Study of Operating Room Equipment," Proceedings of the Human Factors Society 34 (1990) 391–395.

7. A DeAnda and DM Gaba, "Role of Experience in Response to Simulated Critical Incidents." Anesthesia and Analgesia 72 (1991) 308–315.

8. MB Weinger, OW Herndon, and DM Gaba, "The Effect of Electronic Record Keeping and Transesophageal Echocardiography on Task Distribution, Workload, and Vigilance during Cardiac Anesthesia," Anesthesiology 87 (1997): 144–155.

9. R Mosenkis, "Human Factors in Design," in Medical Devices, ed. CWD van Gruting (Amsterdam: Elsevier, 1994), 41–51.

BIBLIOGRAPHY

Gurushanthaiah, K, MB Weinger, and CE Englund. "Visual Display Format Affects the Ability of Anesthesiologists to Detect Acute Physiologic Changes. A Laboratory Study Employing a Clinical Display Simulator." Anesthesiology 83:1184–1193, 1995.

Howard, SK, et al. "Anesthesia Crisis Resource Management Training: Teaching Anesthesiologists to Handle Critical Incidents." Aviation Space and Environmental Medicine 63:763–770, 1992.

"Human Factors Engineering Guidelines and Preferred Practices for Medical Devices," ANSI/AAMI HE48. Arlington, VA: Association for the Advancement of Medical Instrumentation, 1993.

Loeb, RG. "A Measure of Intraoperative Attention to Monitor Displays." Anesthsia and Analgesia. 76:337–341, 1993.

Loeb, RG, MB Weinger, and CE Englund. "Ergonomics of the Anesthesia Workspace." In Anesthesia Equipment: Principles and Applications, eds. J Ehrenwerth and JB Eisenkraft. Malvern, PA: Mosby Year Book, 1993.

Mackenzie, CF, et al. "Comparison of Self-Reporting of Deficiencies in Airway Management with Video Analyses of Actual Performance." Human Factors 38:623–635, 1996.

Weinger, MB. "Human-Use Medical Device Interactions in the Anesthesia Work Environment." In Human Automation Interaction: Research and Practice, eds. M Mouloua and JM Koonce. Mahwah, NJ: Lawrence Erlbaum Associates, 1997.

Weinger, MB, et al. "An Objective Methodology for Task Analysis and Workload Assessment in Anesthesia Providers." Anesthesiology 80:77–92, 1994.

Weinger, MB, et al. "Incorporating Human Factors in the Design of Medical Devices." Journal of the American Medical Association 280:1484, 1998.

Weinger, MB, and NT Smith. "Vigilance, Alarms, and Integrated Monitoring Systems." In Anesthesia Equipment: Principles and Applications, eds. J Ehrenwerth and JB Eisenkraft. Malvern, PA: Mosby Year Book, 1993.

Xiao, Y, et al. "Task Complexity in Emergency Medical Care and Its Implications for Team Coordination." Human Factors 38:636–645, 1996.

Matthew B. Weinger, MD, is a professor of anesthesiology at the University of California, San Diego, and a staff physician at the San Diego Veterans Administration Medical Center. He is also co-chairman of the AAMI Human Factors Engineering Committee.


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