1. Therapeutic anticoagulation preoperatively and postoperatively
is becoming increasingly more common. The patient’s risk
of intraoperative and postoperative bleeding should guide the need
for reversal of anticoagulation therapy preoperatively and the timing
of its reinstatement postoperatively.
2. The need for massive transfusion should be anticipated and guidelines
should be in place to provide the simultaneous administration of
blood, plasma, and platelets.
3. The acute coagulopathy of trauma results from a combination of
activation of protein C and fibrinolysis. It is distinct from disseminated
intravascular coagulation, is present on arrival to the emergency department,
and is associated with an increase in mortality.
Hemostasis is a complex process whose function is to limit blood loss
from an injured vessel. Four major physiologic events participate
in the hemostatic process: vascular constriction, platelet plug formation,
fibrin formation, and fibrinolysis. Although each tends to be activated
in order, the four processes are interrelated so that there is a
continuum and multiple reinforcements. The process is shown schematically
in Fig. 4-1.
Biology of hemostasis. The four physiologic processes that
interrelate to limit blood loss from an injured vessel are illustrated
and include vascular constriction, platelet plug formation, fibrin
clot formation, and fibrinolysis.
Vascular constriction is the initial response to vessel injury.
It is more pronounced in vessels with medial smooth muscles and
is dependent on local contraction of smooth muscle. Vasoconstriction is
subsequently linked to platelet plug formation. Thromboxane A2 (TXA2)
is produced locally at the site of injury via the release of arachidonic
acid from platelet membranes and is a potent constrictor of smooth
muscle. Similarly, endothelin
synthesized by injured endothelium and serotonin (5-hydroxytryptamine)
released during platelet aggregation are potent vasoconstrictors. Lastly,
bradykinin and fibrinopeptides, which are involved in the coagulation
scheme, also are capable of contracting vascular smooth muscle.
The extent of vasoconstriction varies with the degree of vessel
injury. A small artery with a lateral incision may remain open due
to physical forces, whereas a similarly sized vessel that is completely
transected may contract to the extent that bleeding ceases spontaneously.
Platelets are anucleate fragments of megakaryocytes. The normal circulating
number of platelets ranges between 150,000 and 400,000/μL.
Up to 30% of circulating platelets may be sequestered in
the spleen. If not consumed in a clotting reaction, platelets are normally
removed by the spleen and have an average life span of 7 to 10 days.
Platelets play an integral role in hemostasis by forming a hemostatic
plug and by contributing to thrombin formation (Fig.
4-2). Platelets do not normally adhere to each other or to
the vessel wall but can form a plug that aids in cessation of bleeding
when vascular disruption occurs. Injury to the ...