Affordances and constraints

In human factors, one of the areas of interest is human-object interaction. Some objects are extremely easy to interact with, often because they have been designed from the beginning with the human user in mind. Examples might include the iPhone (other smartphones are available), and Dyson vacuum cleaners (other vacuum cleaners are available). Other objects can be more difficult to work out. Anybody who has had to fold up a child's buggy or change the time on an oven clock knows what I'm talking about.

In human-object interaction, an affordance describes the actions that a human can readily perceive are possible (Figure 1). Well-designed objects make it readily apparent how they should be used. For example, looking at two LEGO® bricks it is pretty clear that they are made to stack on top of one another.

Figure 1: The handle on a coffee mug
suggests that it can be used to hold the mug. 

A constraint is a design feature which stops an undesirable action (Fig. 2). The constraint may be physical or, for more complex objects, software-driven. Using LEGO® bricks again as an example, they tend to fit together in only very limited ways.

Figure 2: Some devices require post-manufacture
constraints to be added.

In healthcare, many devices are much more complicated than LEGO®. However good design, using affordances and constraints, plays a strong part in minimising errors.

The TCI pump

The TCI pump is used to provide a target-controlled infusion of an anaesthetic (propofol) or a potent painkiller (remifentanil). The pump has a number of useful constraints to minimise errors. For example if you inadvertently press the power off button, the pump will display "LOCKED" (Fig 3) and forces the user to carry out a sequence of steps to ensure that this was the intended action. 

Figure 3: OFF button "slip" error prevented

This sequence includes two additional safety steps. An "OK" button press (Figure 4) needs to be followed by a separate "CONFIRM" button press (Figure 5), preventing an inadvertent double press. 

Figure 4: OK button

Figure 5: Confirm button

Lastly the "power off" button requires a continuous press (Figure 6).

Figure 6: Power off

Unfortunately the Alaris PK also has a few shortcomings. If one forgets to prime the pump when using remifentanil, it takes 7 minutes and 22 seconds before the pump alarms to tell you that a downstream clamp is still on. This means that there is a significant amount of time during which one may think that the pump is delivering a drug when it isn't. The video is a time-lapsed to show that after 5 minutes the pump is informing us that both the plasma concentration and effect site concentration have reached the set levels, however not a single drop of remifentanil has been delivered. A simple design change would involve the pump not allowing an infusion to be started without the pump first being primed.

The suction on the anaesthetic machine

The suction on an anaesthetic machine is used to remove body fluids such as airway secretions or gastric contents. It may have to be used in an emergency if the patient regurgitates gastric contents. On some anaesthetic machines the suction is placed next to the anaesthetic machine ON/OFF switch (Figure 7).

Figure 7: Tempting to switch the anaesthetic machine off by mistake

This means that a person may inadvertently switch off the anaesthetic machine when they meant to switch on the suction. Some anaesthetic machines allow you a grace period when you switch them off in case you have made a mistake and you can quickly switch them back on without them powering down. The anaesthetic machine pictured does not have this function. The manufacturer has instead installed a cover on the anaesthetic machine ON/OFF switch to ask as a physical constraint (Figure 8).

Figure 8: The clear plastic cap acts as a physical constraint

The value of simulation

Many devices undergo only limited testing by carefully selected end users. Very few devices are tested under stressful or crisis conditions. This means that devices can be released without ensuring that they will be used as intended by the manufacturer. Simulation could be used to test products in realistic conditions during the design stages, without the risk of patient harm.

In addition, simulation could be used to train personnel in the correct use of the equipment, ensuring that the actions are maintained under crisis conditions.

Simulation for equipment design and training is greatly under-utilised.  If manufacturers collaborated with simulation centres, their devices could integrate affordances and constraints which would minimise human-equipment interaction errors.