Direct pressure on a wound is usually ineffective in
controlling hemorrhage. Pressure is applied for the purpose of
minimizing the size of the vessel opening by compression,
temporarily or for an extended period, thereby lessening the
amount and the velocity of the escaping blood and aiding clot
formation. Firm pressure may be applied to a wound if there is no
broken bone in or near the wound. Hemorrhage does not always stop
immediately. At times, firm pressure on the dressing over the
wound may be required for many minutes until a clot has formed
with sufficient strength to hold with only the help of dressing
ties. If a clot does not form quickly, a tourniquet must be
considered (if feasible).
The dressing should be of absorbent material that spreads
and slows the blood it absorbs. This spreading and slowing action
exposes a relatively large and thin surface of the outflowing
blood to the air and thereby aids clot formation. Accordingly,
one dressing partially filled with the victim's blood is more
effective in controlling hemorrhage than is a series of others
because clot formation is in progress within the bloody dressing.
Clot formation tends to spread back toward and into the wound
until diminished air exposure, coupled with an adequate
circulating speed, brings the bleeding to a halt.