Urgent transfusion refers to administration of Red Blood Cells (RBCs) before the completion of standard pretransfusion testing, when a delay in transfusion may imperil the patient. Implicit is the understanding that it is necessary to reestablish both oxygen-carrying capacity and intravascular volume. In hypovolemic shock, most authorities recommend immediate volume restoration with crystalloid or colloid solutions, however a warning has recently been issued regarding possible adverse effects of albumin so-
lutions in critically ill patients.1"3 If volume replacement leads to clinical stabilization, transfusion is less urgent and should await the completion of compatibility testing.
If transfusion is necessary before completion of compatibility testing, group O RBCs should be used. Whenever possible, D-negative RBCs should be used in females of childbearing potential and children to avoid the possibility of sensitization to the D antigen. The patient's physician must sign a statement indicating the nature of the emergency either before or after the uncross-matched blood is issued. If the patient's screen for unexpected red cell antibodies is negative, the transfusion of uncrossmatched but type-specific Whole Blood or of type-compatible RBCs carries a very low risk of being incompatible.3This safety margin is dependent, however, on correct identification of the patient, the pretransfusion blood sample, and the blood components to be infused. Guidelines for conditions under which it is safe to switch the patient to type-specific blood are available.7 In the near future, administration of artificial oxygen carriers may provide a therapeutic bridge to the use of fully compatible blood when emergency transfusion is required.
Massive transfusion is defined as the replacement of one or more blood volumes within 24 hours. A blood volume is estimated as 75 mL/kg or about 5000 mL (10 or more units of Whole Blood) in a 70-kg adult Patients requiring massive transfusion frequently develop multiple complications related to hypovolemia, tissue ischemia, and acid-base disturbances. Many of these metabolic, coagulation, respiratory, and other complications have been ascribed solely to the transfusion of stored blood, but are caused principally by tissue damage or hypoperfusion secondary to trauma or hemor-rhage.l2>14 Hypothermia may impair hemostasis and should be avoided by warming the patient, the crystalloid solutions administered, and if indicated, the blood.
Massive transfusion may be an indication for the use of Whole Blood. However, Whole Blood may not be available, and RBCs administered with crystalloid or colloid solutions are
equally effective in restoring blood volume and oxygen-carrying capacity.15 The patient's history, vital signs, clinical situation, and hematocrit determine the urgency of red cell support. Plasma and platelet support should be based on the presence or absence of microvascular (not surgical) bleeding and on the results of screening tests of hemostasis [prothrombin time (PT), partial thromboplastin time (PTT), fibrinogen, and platelet count].
Many patients with major hemorrhage who require massive transfusion develop a coagulopathy, but not all develop diffuse microvascular bleeding as a result. This coagulopathy may be characterized by thrombocytopenia, hypofibrinogenemia, and prolongation of the PT and the PTT. The etiology of this coagulopathy can be multifactorial, and may be caused by consumption of coagulation factors or hemodilution. Patients with severe tissue injury and prolonged hypotension are more likely to develop diffuse microvascular bleeding and require greater hemostatic support." Preestablished formulas to guide component replacement, such as giving 2 units of Fresh Frozen Plasma (FFP) or 6 units of Platelets with every 5 units of RBCs, are not efficacious." Strict reliance on these formulas may provide insufficient support to patients with consumptive coagu-lopathies and unnecessary transfusion of components to those who do not develop disseminated intravascular coagulation (DIG). Through careful monitoring of laboratory tests of hemostasis, the timely and judicious transfusion of Platelets, FFP, or Cryoprecipitated AHF can be reserved for patients with documented deficiencies.
Mild-to-moderate prolongation of the PT or the PTT does not accurately predict subhemostatic clotting factor levels. Marked prolongations of these tests Often reflect factor levels below 20-30% (see Overview of Hemostasis), however, and supplemental FFP or Cryoprecipitated AHF may then be indicated." In bleeding patients with thrombocytopenia, platelets should be administered to maintain a platelet count of 50,000/pL. Patients who continue to bleed despite adequate levels of platelets or coagulation factors should be thoroughly re-evaluated and considered for surgical exploration. Close communication between
clinicians and the transfusion service director is essential in these cases.