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Normal physiologic hemostasis is achieved through the interplay of pro- and anti-coagulant properties of the vascular endothelium, platelets, and the coagulation cascade. Conventional laboratory tests look at this system in a piecemeal fashion and give a limited indication of in vivo functionality. These tests take significant time for results, further limiting real-time usefulness.
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To address these limitations, anesthesiologists have adopted the use of thromboelastography (TEG). TEG provides a rapid point-of-care assessment of the coagulation process, with initial results available within 5 minutes. TEG measures viscoelastic changes of clot formation through clot lysis, evaluating the integrity of the coagulation system, platelet function, fibrin polymerization, clot strength, and fibrinolysis. Results of TEG permit targeted transfusion. TEG is used in liver transplant, cardiac bypass, and, increasingly, trauma and general surgery.
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TECHNIQUE AND INTERPRETATION
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The TEG analyzer consists of an electronic recorder attached to a torsion wire that suspends a plastic pin into a plastic cup or cuvette. A 0.36-mL whole blood sample is mixed with an activator, such as kaolin or celite, to trigger clot formation. The sample is pipetted into the cup, and the cup begins to oscillate slowly through an angle of 4° at a temperature of 37°C. Initially, the movement of the cup does not affect the pin, but as the blood thickens, the pin becomes entangled in the developing clot, coupling the motion of the cup to the pin. The movement of the pin is transferred through the torsion wire to the electronic recorder, and the analyzer produces a tracing (Figure 7-1).
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Several parameters are presented on the tracing:
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Reaction time—The time in minutes elapsed from the start of the test until the clot moves the pin enough to produce a 2-mm amplitude on the tracing is defined as the reaction time (R). R reflects the activity of the coagulation cascade; a coagulation factor deficiency produces a prolonged R and hypercoagulability yields a shortened R time. The normal values for R depend on the type of clotting activator used.
Alpha angle—The alpha angle (α) is a measure in degrees of the speed of clot formation. It is defined as the angle between the horizontal axis of the tracing and the tangent to the tracing at 20-mm amplitude. Decreased angles indicate a slower rate of clot strengthening, as seen with low fibrinogen levels. Normal values are in the range of 45°–55°.
Coagulation time—The coagulation time (K) is measured in minutes from the end of R to when the tracing amplitude reaches 20 mm. Like the alpha angle, K is determined by the rate at which the clot ...