12.3 Viscoelastic Hemostatic Assays (TEG/ROTEM) & Tranexamic Acid (TXA)

12.3 Viscoelastic Hemostatic Assays (TEG/ROTEM) & Tranexamic Acid (TXA)

Trauma-Induced Coagulopathy (TIC) occurs in up to 35% of severely injured trauma patients upon arrival and is driven by tissue trauma, hypoperfusion, systemic anti-coagulation, consumption of clotting factors, and hyperfibrinolysis. Traditional laboratory coagulation tests (Prothrombin Time [PT], International Normalized Ratio [INR], activated Partial Thromboplastin Time [aPTT], and static plasma fibrinogen levels) are inadequate in acute resuscitation because they are performed on cell-free plasma at 37 degrees C, take 45 to 60 minutes to return, and only evaluate initial fibrin formation. Viscoelastic Hemostatic Assays (VHA)—specifically Thromboelastography (TEG) and Rotational Thromboelastometry (ROTEM)—provide rapid, real-time, point-of-care evaluation of whole blood coagulation mechanics from initial clot formation through clot strength and fibrinolysis, guiding targeted hemostatic therapy.

Mechanics of Viscoelastic Testing (TEG vs. ROTEM)

Viscoelastic assays measure the viscoelastic properties of whole blood as it clots under shear stress:

• TEG Technology: A pin is suspended into an oscillating cylindrical cup containing a whole blood sample. As fibrin-platelet strands form between the cup wall and the pin, torque is transmitted to the pin and converted into a characteristic graphic tracing.
• ROTEM Technology: The cup remains stationary while an optical pin oscillates within the blood sample. ROTEM utilizes specific automated channels (EXTEM, INTEM, FIBTEM, APTEM) to isolate extrinsic pathways, intrinsic pathways, fibrinogen contribution, and hyperfibrinolysis.

TEG Parameters, Clinical Interpretations, and Targeted Hemostatic Therapies

Viscoelastic testing allows the trauma resuscitation team to transition from empiric 1:1:1 transfusion to goal-directed component therapy based on specific numeric parameters:

1. Reaction Time (R-Time) / ROTEM Clotting Time (CT)

• Definition: Time (in minutes) from test initiation until initial fibrin formation occurs (amplitude reaches 2 mm).
• Normal TEG Value: 5.0 to 10.0 minutes.
• Clinical Interpretation: Reflects the activity and concentration of soluble coagulation cascade factors (intrinsic and extrinsic pathways).
• Targeted Treatment: A prolonged R-time (> 10 minutes) indicates clotting factor deficiency. The indicated intervention is administration of Fresh Frozen Plasma (FFP, typically 2 to 4 units) or Prothrombin Complex Concentrate (PCC).

2. K-Time and Alpha Angle (α-Angle) / ROTEM Clot Formation Time (CFT) and Alpha Angle

• Definition: K-time is the time from R-time until clot amplitude reaches 20 mm. Alpha angle measures the slope of the tracing tangent to the expanding clot curve.
• Normal TEG Values: K-time = 1.0 to 3.0 minutes; Alpha Angle = 53 to 72 degrees.
• Clinical Interpretation: Evaluates the rate of clot kinetics and propagation, primarily dependent on fibrinogen availability and cleavage into fibrin strands.
• Targeted Treatment: A prolonged K-time (> 3 minutes) or decreased Alpha Angle (< 53 degrees) signifies low functional fibrinogen. The indicated intervention is administration of Cryoprecipitate (typically 10 to 20 units or 1 to 2 donor pools) or Fibrinogen Concentrate (Riastap/Fibryga).

3. Maximum Amplitude (MA) / ROTEM Maximum Clot Firmness (MCF)

• Definition: The maximum width/height (in millimeters) of the TEG tracing.
• Normal TEG Value: 50 to 70 mm.
• Clinical Interpretation: Represents ultimate structural clot strength and rigidity. Clot strength is determined 80% by platelet count and function (platelet-fibrinogen cross-linking via GPIIb/IIIa receptors) and 20% by fibrinogen.
• Targeted Treatment: A decreased MA (< 50 mm) indicates severe platelet deficiency or dysfunction. The indicated intervention is administration of Platelets (1 apheresis unit). If the patient is on antiplatelet agents (e.g., aspirin, clopidogrel), Desmopressin (DDAVP 0.3 mcg/kg IV) may be co-administered to stimulate von Willebrand factor release.

4. Lysis at 30 Minutes (LY30) / ROTEM Maximum Lysis (ML)

• Definition: The percentage decrease in clot amplitude 30 minutes after MA is achieved.
• Normal TEG Value: 0.0% to 3.0%.
• Clinical Interpretation: Measures the rate of clot breakdown (fibrinolysis). Elevated LY30 (> 3.0%) indicates pathologically accelerated clot dissolution, known as hyperfibrinolysis.
• Targeted Treatment: An elevated LY30 indicates hyperfibrinolysis driven by tissue plasminogen activator (tPA) surge. The immediate indicated intervention is administration of an antifibrinolytic agent, specifically Tranexamic Acid (TXA) or Aminocaproic Acid.

ROTEM FIBTEM Utility

ROTEM includes a specialized assay (FIBTEM) that uses cytochalasin D to irreversibly inhibit platelets. The resulting clot firmness (FIBTEM MCF) reflects purely functional fibrinogen concentration, allowing rapid differentiation between fibrinogen deficiency (FIBTEM MCF < 10 mm) and platelet deficiency without waiting for full MA results.

Tranexamic Acid (TXA): Pharmacodynamics and Clinical Evidence

Tranexamic Acid (TXA) is a synthetic derivative of the amino acid lysine that acts as a potent antifibrinolytic agent.

Mechanism of Action

TXA competitively binds to the lysine-binding sites on plasminogen molecules. By blocking these sites, TXA prevents plasminogen from binding to fibrin surface strands, thereby inhibiting the activation of plasminogen into plasmin. Without plasmin, enzymatic degradation of stable fibrin clots (fibrinolysis) is halted, preserving early hemostatic plugs at injury sites.

Landmark Clinical Trial Evidence: CRASH-2 and CRASH-3

• CRASH-2 Trial: Evaluated over 20,000 adult trauma patients with significant hemorrhage or risk of hemorrhage. Demonstrated that early administration of TXA significantly reduced all-cause mortality and bleeding-related mortality without increasing thromboembolic events (DVT, PE, stroke, MI). Crucially, sub-analysis proved that TXA administered within 1 hour of injury yielded maximum benefit, and administration between 1 and 3 hours post-injury remained beneficial. However, TXA administered more than 3 hours after injury resulted in increased mortality and heightened risk of fatal thrombosis.
• CRASH-3 Trial: Investigated TXA in traumatic brain injury (TBI). Demonstrated that early TXA administration (< 3 hours) reduced 28-day head injury-related mortality in mild-to-moderate TBI (GCS 9-14), confirming intracranial antifibrinolytic benefit.

Standard Trauma Dosing Protocol

• Initial Bolus: 1 gram of Tranexamic Acid diluted in 100 mL of 0.9% Normal Saline or Lactated Ringer's, administered IV piggyback over 10 minutes (infusing faster than 10 minutes may cause transient hypotension). Must be initiated within 3 hours of injury.
• Maintenance Infusion: 1 gram of TXA infused IV over the subsequent 8 hours.