Page 57 - Safety Risk Management for Medical Devices

36 Safety Risk Management for Medical Devices

Design factors that could cause use errors include:
• Insufficient visibility, audibility, or tactility
• Complex or confusing control system which could lead to dissonance between
user’s mental model and reality
• Ambiguous or unclear device state, settings
• Poor mapping of controls to actions, or of displayed information to the actual
state of the device
In most cases, Use Errors are the result of an incongruity between the mental model
that a patient/User has of the System and the actual behavior of the System. For exam-
ple, in most houses the light switches that are mounted on walls are oriented in such a
way that an up action causes the lights to come on, and a down action causes the lights to
go off. Now imagine if you enter a room where the electrician has installed the light
switch upside-down. It is easy to imagine that with the mental model that was described
earlier, you would flip the switch up to turn on the light. And then when that fails, you
flip the switch down to get the desired effect.
Not all Use Failures result in a Hazard. The BXM method employs the principles
of usability engineering at the service of identification of Hazards that are the result of
Use Failures. Specifically, a Failure Modes and Effects Anslysis of the uses and misuses
(UMFMEA) of the medical device is performed, to this end. The input to the
UMFMEA is the medical device use-scenarios and task analyses. See Section 12.8 for
further details on UMFMEA.

8.4 USE FAILURES

In this section, we delve more deeply in Use Failures. As defined in Section 8.1,a
Use Failure is the failure of a User to achieve the intended and expected outcome
from the interaction with the medical device. How can this happen?

1. The user is unable to perform the task.
This could be due to confusing UI, physically not doable action by the User,
etc.
2. The User completes the task but with significant difficulty.
For example, due to a confusing UI, the user struggles, makes error(s), but notices
the error(s) and corrects it, and in the end completes the task. Although by the
strict definition of Use Failure, this would not be a Use Failure, because the user
eventually completes the task, it is advisable to count this type of interaction as Use
Failure so that it receives the appropriate attention.
Close-calls, or near-misses where a user almost fails a task, should also be trea-
ted as Use Failures so that they could inform design decisions for
improvements.