CASE STUDY #2

Confusing Connectors

This device is a controller for a heated mattress for use during lengthy surgery, to keep the patient warm (see left, front, and right views). Using the controller is simple in concept: plug in and turn on the controller, plug in the cable from the matttress, set the desired temperature.

The functionality of the controller was also conceptually simple: the controller provided power to a heating element in the mattress, a temperature sensor in the mattress allowed the controller to sense and maintain the desired temperature.

In this case the patient, who was lying supine on the mattress during surgery received a burn on the back of her thigh. As is common with medical liability cases, there were many factors to consider; in this case, for example, the patient not only received a burn but the burned area subsequently became infected. The engineering question was the extent to which the controller system contributed to the patient injuries.

It was clear upon initial examination that the location of the patient’s burn coincided with the location of a burned area on the surface of the mattress. Beneath the burned area of the mattress was a small electronic circuit board. Further examination revealed that the temperature sensing component of the circuit board (a thermistor) had overheated, possibly because it had experienced electrical current leveles to cause substantial heating.

The proximate cause was most probably misconnection of the mattress cable to the controller. The cable was supposed to be connected to a receptacle on the left side of the controller. In this configuration the controller worked as intended. However, there was a very similar (but not identical) receptacle on the right side of the controller. It was possible to partially insert the mattress cable into the right-hand receptacle. In that configuration, electrical power was inappropriately directed to the thermistor, causing it to overheat.

Design issues: The use of similar connectors invited misconnection. The lack of labeling on the controller made misconnection more likely. The lack of clear instructions and illustrations in the user manual also made misconnection more likely.

Matt Baretich was deposed and the case was subsequently settled out of court.