Another hospital bed entrapment concern: 'power down' versus 'gravity down'

Patient entrapment in side rails, or off the end of the mattress, has received a great deal of attention in the last 15 years, but it is not the only bed entrapment issue that should be of concern.

Another potential entrapment area occurs when the head- or foot end of the mattress is being raised or lowered, especially when this is by electric motor in an electric or semi-electric bed. In this case, body parts can be caught between the moving portions of the bed and the fixed portions, especially between the mattress deck and the bed frame or the head- or footboard.  What can make this type of entrapment particularly hazardous in some bed designs is that the drive motor is powerful enough to cause considerable bodily damage when a control button is not released, or the bed direction reversed, in a timely manner.

It also has been reported that a bed could undergo a “runaway” condition in which the pendant buttons no longer control ongoing motion. In these instances, there is nothing to stop the movement of the mattress deck relative to the bed frame (or the head or foot board) other than the body parts that may be in the way, or the deck reaches the end of its run. At least one death has been reported to a person who was in the space between the raised head end of the mattress deck and the head board and who then came trapped and crushed as the mattress deck lowered and caught them in that position.  Lowering the bed sections is typically far more potentially dangerous than raising them since this in general involves closing pinch points rather than opening them.

An important distinguishing feature between bed designs in this regard is whether lowering the mattress deck occurs by a direct connection between the mattress deck and the motor (“power down”), or whether the mattress deck simply follows the removal of support as the motor operates (“gravity down”).

There are several versions of gravity down designs, and at least one bed manufacturer promotes its gravity down design as a safety feature. In one design, the drive tube connecting the motor to the mattress deck can easily slide out of the motor drive component if a counteracting (upward) force is applied while the bed is being lowered. In another design, the mattress deck rests on a roller rather than being directly attached to the drive mechanism. In this case, as the motor lowers the support linkage, the mattress deck simply follows the withdrawing linkage, but the mattress deck can be easily lifted up and off the roller at any time.

Another possibility for a gravity down design is for the drive bar itself to have a telescoping feature such that it will readily extend when an upward force is applied to the mattress deck, even if the motor is lowering the bed section. This approach could be a relatively simple retrofit to some current designs that are power down. In any of these gravity down designs if a body part were the source of the resisting force, the body part would stop the mattress deck from further movement even though the motor continued to withdraw support. In this case, the only continuing force on the body would be the pivoted weight of the mattress deck, which is considerably less than the force that the drive motor is capable of applying in a power down design. In addition, the mattress deck could then be easily lifted up and off the intervening body part to remove it from harms way.

A simple test

There is a simple test of whether a bed is power down or gravity down. This is to apply by hand an upward force to the partially raised mattress deck when the motor is not operating. If the mattress deck cannot be moved because of its rigid attachment to the drive mechanism, then it also could not be readily stopped without injury by a body part if that body part got in the way while the motor was trying to lower the mattress deck. This then is a power down design.

If, on the other hand, it is easy to lift up on the mattress deck, then there is a separating (as in the roller case) or slip component somewhere in the drive mechanism. This is demonstration of a gravity down design. Further observation in this case also would reveal where the release is occurring, i.e. at the motor linkage, between telescoping components of the drive bar, or as a result of a roller system.

Risk management

The two classic elements of risk management are the consideration of severity of the potential harm and the likelihood of its occurrence. A third important consideration is the availability of alternative designs and/or the ease with which a hazard can be eliminated or mitigated. Since a power down bed can cause death, the severity is maximum and it is poor risk management practice to allow low probability of occurrence to offset a possible death, especially when alternative and feasible designs are readily available.

In this regard, the ideal time to reject a power down design is when purchasing or renting a bed. Simply don’t buy or rent one that has this potentially dangerous condition.  What to do with existing beds is more problematical. One immediate approach is to ask the manufacturer about it. They may have a later design that can be retrofitted to existing beds at a lower cost than replacing the entire bed.

Since in some beds the parts are interchangeable between manufacturers, a different manufacturer may have an alternative part that can be used on the existing bed. If necessary, a shop-made retro fit could be considered. For example, it is relatively easy to replace a non-telescoping drive bar with one that telescopes, especially since in some cases the existing drive bar is already two mating components.

It also may be possible to screen off the dangerous entrapment areas to keep body parts out of them. Ultimately, a decision may need to be made to replace older beds that are more dangerous than other available designs. If that is the case, also be sure that the new beds have well designed side rails that at least meet the FDA’s guidelines, as attested to by the manufacturer.

The author is a professor in the Department of Biomedical Engineering at Texas A&M University in College Station, TX. Check the McKnight’s online archive, or the contents of our “Guest Column” section, for his previous posting on bed entrapment issues.