Bed sores are something that most people don’t want to think about. Many cannot avoid the subject, however, since more than 10 million people worldwide have bed sores at any one time.
Also known variously as pressure sores, pressure ulcers, or decubitus ulcers, they are a wound consisting of an area of dead tissue, which is rotting away. They may range in size from less than a square inch, to the size of a dinner plate. As they progress, they can go so deep as to reveal bone, and can cause death by septicaemia.

The causes of bed sores are manifold. Usually, the scene is set by factors intrinsic to the individual: age, immobility, circulatory problems, and nutritional status all play a role. With these risk factors in place, the damage is done by pressure, as the soft tissues are squashed between your bones and whatever it is that you are lying on. So, they tend to appear on areas of skin where there is a bony prominence just underneath: the hips, the seat-bones, the sacrum, the heels, the ankles, the shoulder-blades, and the back of the skull.
What is actually happening, as these areas of soft tissue are squashed, is that the tiny blood capillaries which constitute our micro-circulation are cut off. It is this micro-circulation which delivers nutrients, oxygen, hormones, antibodies, and enzymes to our tissues, and which takes away carbon dioxide, waste metabolites, and detritus. Without it, the tissue cannot survive for more than a couple of hours. Once it is dead, it is dead. As it subsequently rots away, it leaves a hole.

Generally speaking, for most of us as we lie down or sit down, the pressure on these risk areas underneath our bony parts are high enough to cut off the micro-circulation. Our body weight transferred through our bones to the upholstery can yield pressures of around 200mmHg, whereas the pressure of blood in our capillaries is in the range 10mmHg to 100mmHg. So, it is normal for our tissues to be starved for a while.

The reason we don’t normally get bed sores, is that we fidget. When we roll over in bed, or fidget into a different position, what we are doing is changing the loading conditions. An area of skin that was under load before, is now off-loaded, at the expense of a new area of skin. This new area now has its blood supply cut off, while the previous one recovers. The higher the pressure on the skin, the sooner we will have to fidget again. Some people are just too frail or unwell, or not sufficiently conscious to keep up the amount of fidgeting required. That’s when the bed sores happen.

In terms of prevention, there is not much we can do about our age, and there may be little that we can do about other intrinsic factors, such as the friability of our skin, or the state of our blood supply. The most promising line of attack is therefore to reduce the pressure on the skin, by using a mattress which is designed to do this.
In the series “how beds work” we looked in some detail at how mattresses can reduce pressure, by accommodating the body shapes, to spread the body weight over a larger surface area. This approach, properly executed, can be quite effective. However, it has certain limitations. Your body weight is always there, regardless of what the mattress does. And there is only a certain amount of surface area available to spread the pressure over: the ‘footprint’ of your body is as big as it gets. Once you have spread the weight of your body uniformly over the whole of that area, that’s as low as you can go, pressure-wise. On a perfect flotation system, such as the “low air loss beds” used in hospitals, costing tens of thousands of dollars, it is still going to be around 20mmHg, which may still enough to cut off the circulation.
There is a different approach, deployed with some success in the world of wheelchair cushions (wheelchair users also frequently get pressure ulcers). Instead of simply pressure distribution (spreading pressure over a larger area), some wheelchair cushions aim for pressure re-distribution (moving pressure to a different place). For example, if a user has a particular problem with pressure under the tail-bone, a cushion can be produced with a moderate cut-out in that area, to re-distribute pressure from that area to the rest of the butt. The pressure does not go away altogether; it just moves to somewhere less problematic.

Recently, developments in mattress technology have started to move beds in the same direction. By carefully zoning the support properties of the mattress, it is possible to fine-tune the distribution of pressure over the body, to preferentially spare critical pressure zones such as the hips and the shoulders, at the expense of less critical areas such as the thighs and midriff.

For somebody at risk of bed sores, this zoning approach can mitigate those risks. For the rest of us, it might lead to a lot less frequent fidgeting.