Transfusion Medicine

The transfusion threshold for a low hematocrit depends on the stability of the patient. If the hematocrit is below 30% and the patient is bleeding or hemodynamically unstable, one should transfuse packed red cells. Stable patients can tolerate lower hematocrits, and an aggressive transfusion policy may even be detrimental.^2,49 If the patient is bleeding, has florid DIC, or has received platelet aggregation inhibitors, then keeping the platelet count above 50 × 10³/µL is reasonable. There are data regarding massive transfusion showing that keeping the platelet count above 50 × 10³/µL resulted in less microvascular bleeding.⁵⁰ The conventional dose of platelets is 6 to 8 platelet concentrates or 1 plateletpheresis unit.

While in the past fibrinogen targets of 50 to 100 mg/dL were recommended, recent data indicate that a target of 150 mg/dL or higher may be more appropriate.^51–53 Severe fibrinolysis may occur in certain clinical situations such as brain injuries, hepatic trauma, or ischemic limb reperfusion, and the use of large amounts of cryoprecipitate can be anticipated. In patients with an INR greater than 2 and an abnormal aPTT, one can give 2 to 4 units of FFP. For an aPTT greater than 1.5 times normal, 2 to 4 units of plasma should be given. Elevation of the aPTT above 1.8 times normal control is associated with microvascular bleeding in trauma patients.⁵⁴ Patients with marked abnormalities (eg, anaPTT more than 2 times normal) may require aggressive therapy with at least 15 to 30 mL/kg (4–8 units for an average adult) of plasma.⁵⁵

Recently there has been increasing interest in the use of thromboelastography (TEG) in massive transfusion.⁵⁶ This is a point-of-care assay performed on fresh whole blood that can assess multiple facets of hemostasis, including coagulation, platelet function, and fibrinolysis.^57,58 TEG is performed by placing a 0.35-mL sample of whole blood into an oscillating container with a sensor pin that measures the force of thrombus formation. TEG measures 5 parameters:

• r time: time from starting TEG until clot formation
• K time: time needed for tracing to go from 2 mm to 20 mm
• alpha angle: slope of tracing between r and K time
• MA: greatest amplitude of TEG tracing
• Whole blood lysis index: amplitude of tracing 60 minutes after MA.

Several centers have incorporated TEG into resuscitation protocols that include standardized strategies for responding to abnormalities. Data suggest that use of TEG may decrease the use of blood products, especially in cardiac surgery, but this has not been prospectively studied in massive transfusions.^56,59

COMPLICATIONS OF MASSIVE TRANSFUSIONS

Electrolyte abnormalities are unusual even in patients who receive massive transfusions.⁶⁰ Platelet concentrates and plasma contain citrate that can chelate calcium. However, the citrate is rapidly metabolized, and it is rare to see clinically significant hypocalcemia. Although empiric calcium replacement is often recommended, one study suggests that this is associated with a worse outcome and should not be done.⁶¹ If hypocalcemia is a clinical concern, then levels should be drawn to guide therapy. Stored blood is acidic, with a pH of 6.5 to 6.9. However, acidosis attributed solely to transfused blood is rare and most often is a reflection of the patient’s stability. Empirical bicarbonate replacement has been associated with severe alkalosis and is not recom mended.^62,63 Although potassium leaks out of stored red cells, even older units of blood contain only 8 mEq/L of potassium, so hyperkalemia is usually not a concern.