Pressure ulcers are most feared by bedridden person and wheelchair users.
Recently, various areas in the reserach of pressur ulcers have made an advancement. Accordingly, there has been many new evidences reported. On this page, we explain in a simple manner of the "threat of shear force". It will be our pleasures if this information helps your questions and be usuful for effective practice for pressure ulcer prevention.
Former professor at the College of Systems Engineering and Science of the Shibaura Institute of Technology.
Former member of board of directors at the Japanese Society of Pressure Ulcers (JSPU)
Former vice chairman of Pressure Ulcer Countermeasures Standardization Committee at JSPU.
His research on pressure ulcers from engineering standpoint over the past 20 years had achieved various practical accomplishments. His collaboration with medical experts in compatibility between engineering experiment results and clinical trial results drew attentions at home and abroad.
For a long time, pressure has been said as the main cause of pressure ulcers.
"32mmHg or beyond will break capillary vessel
＝result in pressure ulcers" was the leading theory, and hence pressure redistribution was believed to be the major preventative measure.
But is it truly so?
Many wheelchair users spend hours in seated position in which the pressure is continuously applied beyond 32mmHg, but not all users develop pressure ulcers.
In addition, not all pressure ulcer preventive items maintain pressure
under 32mmHg, though to an extent they contribute to pressure
There seems to be another measure other than pressure dispersion to prevent pressure ulcers. In short, there may be other factors besides the pressure which is highly related to the cause of pressure ulcers.
"Pressure" here means the vertical force.
32mmHg is a small amount of pressure which can be comapred to the pressure when a 350ml size pet bottle is placed upside down on the fist.
In order to maintain the pressure on sacrum of a person who is lying or sitting below 32mmHg, you would have to use pressure ulcer preventive products that is very soft and thick.
What would happen if such products are used as an easy solution for pressure ulcer prevention?
It may make it difficult for patients to move for repositioning or for caregivers to assist them.
Of course there are cases in which pressure redistribution needs to be the highest priority in the treatment of pressure ulcers, but it is important to select an item which would best fit the patient's physical capability and nursing situation among many pressure ulcer preventive products.
Past studies revealed that shear force, along with pressure, was also an important factor which affected pressure ulcers.
Shear force is a term of mechanics.
There are various cases in which pressure ulcers did not develop under great pressure or developed even under the low pressure level.
Hence, the theory "pressure is the cause of pressure ulcers" cannot be applied for all cases.
Shear force seem to hold a key to solve this mystery.
Let's look at the shear force further in details.
Shear force and friction are different though they seem similar.
Gently rub the back of your hand with a finger.
Friction occurs on the skin when it is rubbed without pressure.
Of course, this would not block the blood flow under the skin which causes pressure ulcers.(Illustration A)
Now, try to rub your skin while pressing down the finger.
When pressure is added as the finger glides, it also makes the skin underneath slide.
This is the example of shear force. (Illustration B)
How does the shear force behave under the skin, and what is the effect of it
to the occurance of pressure ulcers?
Blood capillaries run under the skin.
When the capillary is crushed, the blood flow becomes poor and causes pressure ulcers.
This mechanism is demonstrated in the below illustration.
TAMAGODOFU-very soft TOFU made with eggs is used as inner part of skin, and a hole run through it as a blood capillary.
The result of this experiment leads to the following hypotheses.
The experiment using the TAMAGODOFU was concluded with two hypotheses.
How about it in the human body?
We made another experiment using a special device which can take photo images of blood flow in the capillary while under pressure.
So, what happened?
When 40mmHg is applied to a capillary, no change was observed, and it maintained a good blood flow.
Pressure level continued to increase, but it was not until 80mmHg when the blood flow finally stagnated.
Moreover, even under 80mmHg of pressure, the capillary vessels were not occluded. Thus, it seemed that the pressure itself would not be the only cause of pressure ulcers.
Next, we experimented the same way with a rat this time.
After all, the blood flow did not stop even under the pressure of 80mmHg. However, with a slight shear, it caused a blood stagnation.
Under a microenvironment, type of capillary network is geometrically equivalent among giant elephant, rat and human. Therefore, the results of this experiment with rat is applicable to human.
In all three experiments-with TAMAGODOFU, special device and rat, the following hypotheses were verified:
Based on these findings, we learned that "shear force treatment" is as important as pressure.
For the future of pressure ulcer prevention, dispersion of shear force must be considered just as much as pressure redistribution.