Machine Design

Lessons of hospital accidents

Here is a sobering statistic: As a safety expert, I am usually called to investigate an accident at a hospital as many as 12 times a year. The problems I find often result because of poorly trained employees, but there are also lessons in these incidents for designers of medical products.

Some of the accidents I’ve looked at involve unfortunate interactions between medical equipment and seemingly innocuous room furniture. One such case happened when a man went into the hospital for an annual checkup. He was in perfect health. An ultrasound machine was used in part of his checkup. A student nurse was trying to adjust it properly over his chest, but was having no luck. She was trying to lower it closer to the man, but the device was not moving.

The nurse didn’t notice that the high back of her chair was interfering with the heavy machine she was trying to lower. As she got up to go get help, the heavy machine slipped off the back of the seat and fell onto the man’s chest. He died before anything could be done to help him. The man’s wife was in the waiting room to take him home after the checkup. The corrective action was nothing more than to use only low seats in this examination room.

In addition, designers of medical equipment would be well served to envision what can happen to their equipment when maintenance is poor. In one case, a woman had just had a hysterectomy and was in her bed in a hospital room. A student nurse raised the back of the bed so the patient could eat. Just as the back of the bed was approaching the proper vertical position, it suddenly collapsed to the flat position.

In a panic, the student nurse raised the back of the bed again, and again it collapsed to the flat position. By this time, the patient was screaming in pain and was bleeding from her torn stitches. The student nurse ran down the hallway crying for help.

After the incident, the hospital bed in question disappeared, an event that complicated the accident investigation. Fortunately, the hospital had purchased only two styles of beds, and we were able to identify the style involved. An examination of the bed revealed that a power screw rotated by an electric motor controlled the back of the bed, and the motor ran both forward and backward.

The failure mode revealed itself after about 16 hours of experimentation with a similar bed. The power screw employed recirculating ball bearings in its nut. When the snap washer was removed that retained the ball bearings in the system, the ball bearings could fall out slowly. When all of the ball bearings were gone, the power screw fell against the outer nut housing and the back of the bed could still be raised and lowered, to a point.

When the back of the bed approached the vertical position, the weight of the person in the bed created a downward force on the outer housing and the power screw could slide inside the housing. With the failure mode identified, I could demonstrate it on command. This was simply a case of poor housekeeping and poor bed maintenance at the hospital. MD

Lanny Berke is a registered professional engineer and Certified Safety Professional involved in forensic engineering since 1972. Got a question about safety? You can reach Lanny at [email protected].

Edited by Leland Teschler

© 2012 Penton Media, Inc.

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