A crush injury occurs when a body part is subjected to a high degree of pressure after being squeezed between two heavy or immobile objects.

Damage caused by a crush injury can include: laceration, fracture, bleeding, bruising and/or crush injury syndrome.

pain at the site
tenderness
associated wound and blood loss
deformity

call ‘000' for an ambulance if required
reassure casualty
treat any wounds
treat any fractures

Crush Injury Syndrome

A compressing force which traps a casualty can cause crush injury syndrome. This force, if applied to a large muscle mass, eg the thigh, causes the body to produce large quantities of acid and complex electrolytes, especially potassium, around the affected muscles. On release of the compressing force, the liberated blood takes the concentrated chemicals to the heart, with often fatal results. In addition, there is often a sudden loss of blood on releasing the compressing force.

Crush injury syndrome does not occur every time a casualty is trapped, but the first aid provider should consider the risk.

As a general rule, the requirement for consideration is based on three criteria:

A large muscle mass is involved
Prolonged compression
Compromised blood circulation

For instance, entrapment of a hand is unlikely to initiate the syndrome (no major muscle mass).
The major problem that faces the first aid provider when dealing with suspected crush injury is dissuading helpful bystanders from attempting to remove the compressing force without assessing the situation first.

extended compression time
large muscle mass involved
absent pulse and capillary return in the lower part of the limb
pale, cool, clammy skin
weak, rapid pulse
usually absence of pain in the affected region
onset of shock

call ‘000' for an ambulance
relieve the crushing force as quickly and gently as possible, provided it is safe to do so
reassure casualty
treat any other injuries
be prepared to assist the medical support team