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Pressure Ulcers: Causes

Pressure ulcers tend to develop on bony prominences and on areas of the body that have little
body fat to cover them. This includes the hips, buttocks, heels, shoulder blades and the small of the back.

Sitting in a chair or wheelchair for long periods of time, without pressure being relieved, is even more likely to cause pressure ulcer development.

Pressure ulceration occurs when the skin and underlying tissues are
compressed for a period of time, between the bone and the surface, on which the patient is sitting or lying.

Blood cannot circulate causing a lack of oxygen and nutrients to the tissue cells. Furthermore, the lymphatic system cannot function properly to remove waste products.

If the pressure continues, the cells die and the area of dead tissue that results is called pressure damage.

The amount of time this takes will vary, but may develop
in as little as one hour in patients at greatest risk.

Aetiology of Pressure Sores

The factors causing pressure ulcers are divided into 2 Groups

INTRINSIC - Disease, medication, malnourishment, age, dehydration/fluid status, lack of mobility, incontinence, skin condition, weight.

EXTRINSIC – External influences which cause skin distortion –
Pressure, Shearing Forces, Friction, Moisture.


The blood pressure at the arterial end of the capillaries is approximately 32 mmHg, while at the venous end this drops to10 mmHg.

The average mean capillary pressure equals about 17 mm Hg and any external pressures exceeding this will cause capillary obstruction.

Tissues that are dependent on these capillaries are deprived of their blood supply. Eventually the ischaemic tissues will die.

Shearing Forces

Shearing forces will only exist if pressure, usually caused by the persons own body weight, is also present. Shear forces occur when a part of the body tries to move but the surface of the skin remains fixed. (See Waterlow Manual for a better understanding of this phenomenon).


Friction forces occur when the shearing force increases sufficiently to overcome the bodies resistance to movement. The movement has an abrasive action.(See Waterlow Manual for a better understanding of this action).


Skin should not be left wet as moist skin sticks to material (e.g. bathing, perspiration, incontinence (as urine and faeces are acidic), amniotic fluid) as it can become macerated making it more susceptible to shear and friction.

Certain areas of the body are more vulnerable to pressure ulcer formation than others.

Bony prominences in particular : heels, sacrum, buttocks, hips , elbows etc.

Causes - Symptoms - Treatment - Prevention

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