B.8. Explain the technical and physiologic principles behind rotational thromboelastometry.
Answer:
Rotational thromboelastometry measures the viscoelastic properties of blood. Citrated blood is introduced into a cuvette together with an activator, which is heated to 30 °C to 40 °C. A pin with a rotating axis is suspended into the cuvette during the test, and as the blood becomes more viscous, pin rotation is impeded. This impedance is detected by an optical detector system. The time course of changing viscoelasticity is plotted graphically and numerically. The parameters defining a typical assay correlate to the events of coagulation as described by the cell-based model of hemostasis as summarized in Table 12.7.
Table 12.7: Rotational Thromboelastometry Numerical Parameters
| Parameter | Definition | Physiologic Correlate |
|---|---|---|
| Clotting time (CT) | Time for trace to reach 2 mm amplitude | Clot initiation phase |
| Clot formation time (CFT) | Time for trace to increase from 2 to 20 mm | Clot amplification and propagation phase |
| Amplitude 10 min (A10) | Trace amplitude 10 min after CT | Fibrin-platelet interaction via glycoprotein IIb/IIIA receptors |
| Maximum clot firmness (MCF) | Maximum amplitude (MA) of the trace, when test is run for 20 min | |
| Lysis Index 30 (LI30) | Percentage reduction in amplitude 30 min after MA is reached | Plasmin, thrombin-activatable fibrinolysis inhibitor (TAFI) and plasminogen activator inhibitor (PAI) activity |
ROTEM delta (Werfen North America) and ROTEM sigma coagulation testing (Instrumentation Laboratories) are two commercially available devices that can perform up to four tests concurrently. ROTEM delta requires manual pipetting of blood samples and test reagents, whereas ROTEM sigma is a fully automated point-of-care system that uses a cartridge with integrated freeze-dried reagents. Depending on the activator reagents used, assays targeting specific components of the coagulation cascade can be performed. The INTEM reagent contains ellagic acid and phospholipid, which produces contact activation of the intrinsic pathway. EXTEM contains tissue factor and preferentially activates the extrinsic pathway. FIBTEM also activates the extrinsic pathway but contains cytochalasin D, which blocks platelet contribution to clot formation, thus isolating the fibrinogen contribution to clot formation. APTEM contains aprotinin, an antifibrinolytic, and will "correct" an EXTEM assay in the presence of hyperfibrinolysis.
References
- Dias JD, Haney EI, Mathew BA, et al. New-generation thromboelastography: comprehensive evaluation of citrated and heparinized blood sample storage effect on clot-forming variables. Arch Pathol Lab Med. 2017;141:569-577.
- Erdoes G, Koster A, Levy JH. Viscoelastic coagulation testing: use and current limitations in perioperative decision-making. Anesthesiology. 2021;135:342-349.
- Schenk B, Görlinger K, Treml B, et al. A comparison of the new ROTEM® sigma with its predecessor, the ROTEMdelta. Anaesthesia. 2019;74:348-356.
- Whiting D, DiNardo JA. TEG and ROTEM: technology and clinical applications. Am J Hematol. 2014;89:228-232.