Viscoelastic Measures of Coagulation

Initially developed in the 1940s, viscoelastic measures of coagulation have undergone a resurgence in popularity. The unique aspect of viscoelastic monitors lies in their ability to measure the entire spectrum of clot formation from early fibrin strand generation through clot retraction and eventual fibrinolysis. The thromboelastograph (TEG) (Haemoscope, Niles, IL), developed by Hartert in 1948,^[840] uses a small 0.35-mL blood sample placed into a disposable cuvette within the instrument. The cuvette is maintained at a temperature of 37°C and continuously rotates around an axis of approximately 5 degrees. A metal piston attached by a torsion wire to an electronic recorder is lowered into the blood within the cuvette. As clot formation occurs, the piston becomes enmeshed within the clot, and rotation of the cuvette is transferred to the piston and electronic recorder.

Although variables derived from the TEG tracing do not coincide directly with laboratory-based tests of coagulation, the TEG is capable of detecting characteristic abnormalities in clot formation and fibrinolysis.^[841] Various parameters that describe the characteristics of clot formation and lysis are inscribed by the TEG recorder. The R value (reaction time) measures the time to initial clot formation (normal, 7.5 to 15 minutes). It is considered to be comparable to the whole blood clotting time and may be accelerated by adding celite to the TEG sample cuvette. The R value is prolonged by a deficiency of one or more plasma coagulation factors. Maximum amplitude (MA) provides a measure of clot strength and may be decreased by either qualitative or quantitative platelet dysfunction or decreased fibrinogen concentration. Normal MA is 50 to 60 mm. The alpha angle and K (BiKoatugulierung or coagulation) values measure the rate of clot formation and may be prolonged by any factor slowing clot generation, such as plasma coagulation factor deficiency or heparin anticoagulation ( Fig. 32-53 and Fig. 32-54 ).

The Sonoclot (Sienco, Inc., Wheat Ridge, CO) provides an alternative viscoelastic measure of coagulation. When compared with TEG, the Sonoclot immerses a rapidly vibrating probe into a 0.4-mL sample of blood. As clot formation occurs, impedance to probe movement through the blood increases and generates an altered electrical signal and characteristic clot "signature." The Sonoclot may be used to derive the ACT, as well as provide information regarding clot strength and clot lysis.^[842]

Both the TEG and Sonoclot generate characteristic diagrams by translating the mechanical resistance encountered by the sensor as it moves through the clotting blood sample. Measurements derived from these diagrams

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Figure 32-53 The functional components of the thromboelastograph consist of a rotating cuvette, a suspended piston attached to a torsion wire, and a recorder. As the blood sample clots, rotation of the cuvette is transferred to the torsion wire and translated by the recorder as a characteristic tracing. Quantitative parameters derived from the thromboelastograph trace are the reaction time (R), the BiKoatugulierung value or coagulation time (K), the alpha angle (α), and maximum amplitude (MA). See the text for greater detail. (Redrawn from Tuman K, Speiss B, McCarthy R, et al: Effects of progressive blood loss on coagulation as measured by thromboelastography. Anesth Analg 66:856, 1987.)

Figure 32-54 Characteristic thromboelastograph tracings.