Coagulation Cascade and Thromboelastography

Postgraduate Anaesthesia Examination Answer (30 Marks)

1. Introduction (2 marks)

2. The Coagulation Cascade (12 marks)

2.1 Classical (Waterfall) Model

Fig. 1 - Coagulation cascade: laboratory (intrinsic/extrinsic pathways) vs. in vivo (tissue factor-driven). Red = inactive factors, Blue = active factors, Green = cofactors. (Robbins Pathologic Basis of Disease)

• Tissue factor (TF), a subendothelial transmembrane glycoprotein constitutively expressed on fibroblasts and vascular smooth muscle cells, is exposed by vascular injury
• TF binds plasma FVII, activating it to FVIIa (the TF:FVIIa extrinsic tenase complex)
• TF:FVIIa activates FX → FXa and also FIX → FIXa
• The PT assay assesses factors VII, X, V, II (prothrombin), and fibrinogen

• Initiated in vitro by negatively charged surfaces activating FXII → FXIIa (Hageman factor)
• FXIIa activates FXI → FXIa, which activates FIX → FIXa
• FIXa + FVIIIa (intrinsic tenase complex on platelet surface) → activates FX
• The aPTT assesses factors XII, XI, IX, VIII, X, V, II, and fibrinogen

• FXa + FVa (prothrombinase complex, assembled on negatively charged platelet membrane in the presence of calcium) converts prothrombin (FII) → thrombin (FIIa)
• Thrombin cleaves fibrinogen → fibrin monomers → polymerised insoluble fibrin
• Thrombin activates FXIII → FXIIIa, a transglutaminase that covalently cross-links fibrin strands, stabilising the clot
• Thrombin also activates FV, FVIII, FXI (positive feedback amplification loops), and platelet activation via protease-activated receptor-1 (PAR-1)

2.2 Cell-Based Model of Coagulation (Current Paradigm)

• Cleave fibrinogen → fibrin monomers
• Activate FXIII → cross-linked fibrin
• Activate TAFI (thrombin activatable fibrinolysis inhibitor)
• Sustain further platelet and factor activation (Schwartz's Principles of Surgery 11e; Rockwood & Green's Fractures in Adults 10e)

2.3 Anticoagulant Mechanisms (4 marks)

2.4 Fibrinolysis

3. Thromboelastography (TEG) and Rotational Thromboelastometry (ROTEM) (12 marks)

3.1 Principle and Technology

3.2 TEG Trace Parameters and Clinical Interpretation

Fig. 2 - Standard TEG trace. The enzymatic phase (R time) is followed by fibrinogen-mediated clot formation (K time, alpha angle), platelet-dependent clot strength (MA), and fibrinolysis (LY30, EPL). (Morgan & Mikhail's Clinical Anesthesiology 7e)

3.3 TEG/ROTEM-Guided Transfusion Algorithms

1. Major trauma and haemorrhage - identifying trauma-induced coagulopathy (TIC), which is characterised by early hyperfibrinolysis, dilution, hypothermia, and acidosis (the "lethal triad")
2. Cardiac surgery - distinguishing surgical from coagulopathic bleeding
3. Liver transplantation - monitoring complex coagulopathy including hyperfibrinolysis
4. Obstetric haemorrhage - real-time guidance for clotting factor replacement
5. Neurosurgery - detecting hypercoagulability and guiding anticoagulation

3.4 Advantages over Standard Coagulation Tests

3.5 Limitations of TEG/ROTEM

• Temperature sensitivity: samples must be maintained at 37°C
• Inter-operator variability; time-sensitive sample processing required
• Do not assess endothelial function or vascular component of haemostasis
• Weak sensitivity for antiplatelet drugs (aspirin, ADP receptor inhibitors) - specific platelet mapping reagents needed
• ROTEM and TEG values are not interchangeable and have different reference ranges
• Evidence base predominantly from cardiac surgery; evidence in trauma and obstetrics is growing but certainty remains variable
• Cost and availability in lower-resource settings

4. Current Evidence (4 marks)

5. Anaesthetic Relevance - Practical Summary (marks incorporated above)

• Understand the cell-based model rather than the classical cascade alone, to interpret the clinical significance of factor deficiencies and guide replacement
• Recognise the limitations of PT/aPTT: normal values do not exclude coagulopathy, and they do not detect fibrinolysis or platelet dysfunction
• Use TEG/ROTEM as decision support tools within a goal-directed algorithm, not as isolated tests
• Apply tranexamic acid early in high-risk haemorrhage (CRASH-2 / WOMAN trial evidence), as TEG-confirmed hyperfibrinolysis (LY30 >7.5%) indicates further benefit
• Account for the confounders of anaesthesia on haemostasis: hypothermia impairs enzymatic reactions and platelet function; acidosis disrupts the coagulation factor environment; haemodilution reduces factor and platelet concentrations - all components of the lethal triad in major haemorrhage

Summary Table

• Navy header and footer on every page with page numbers
• Colour-coded section banners for each of the 5 sections
• Both textbook diagrams embedded (coagulation cascade figure + annotated TEG trace)
• All formatted tables: TEG/ROTEM parameters, ROTEM equivalents, anticoagulant mechanisms, goal-directed algorithm, advantages vs. standard tests, and summary table
• Cell-based model displayed as a highlighted three-phase table
• Full reference list with Cochrane 2026 citation (PMID: 42145275)
• Optimised for A4 printing

1. Print on A4 paper, landscape orientation, at 100% scale
2. Cut along the centre line (marked with a thin grey rule)
3. Laminate each half - you get two A5-landscape cards

• Annotated TEG trace diagram (R · K · α · MA · LY30 labelled)
• Full TEG parameters table with normal ranges and immediate actions
• TEG ↔ ROTEM equivalents table
• Colour-coded ROTEM reagent key (EXTEM · INTEM · FIBTEM · APTEM)

• Goal-directed intervention table: VHA finding → likely defect → TEG parameter → ROTEM parameter → what to give (colour-coded rows: green=FFP, amber=cryo, red=TXA, purple=mixed deficit)
• Drug and condition effects on VHA (heparin, warfarin, DOACs, aspirin, P2Y12 inhibitors, TXA, hypothermia, DIC)
• Lethal triad summary with clinical context priority list
• Evidence footnote from Cochrane 2026 (PMID: 42145275)

Ultrasound in Anaesthesia

Postgraduate Anaesthesia Examination Answer (30 Marks)

1. Introduction (2 marks)

2. Physics of Ultrasound (4 marks)

2.1 Basic Principles

Wave speed (c) = Frequency (f) × Wavelength (λ)

• High frequency (10-15 MHz): Superior spatial resolution, poor tissue penetration (shallow structures - nerve blocks, vascular access)
• Low frequency (2-5 MHz): Poor resolution, deep penetration (cardiac, abdominal, deep vessels)

2.2 Piezoelectric Effect

2.3 Image Generation

• Amplitude (brightness - creating the B-mode greyscale image)
• Time of arrival (used to calculate depth: depth = c × time/2)

2.4 Imaging Modes

2.5 Artefacts

3. Needle Guidance Techniques (2 marks)

• Needle inserted parallel to the transducer's long axis
• Entire needle shaft and tip visualised as a hyperechoic line
• Preferred for nerve blocks (full needle course visible, safer)

• Needle inserted perpendicular to the probe
• Only needle tip (or shaft cross-section) seen as a bright dot
• Risk of mistaking shaft for tip - "tip tracking" techniques required
• Preferred for central venous cannulation (short access distance)

4. Vascular Access (5 marks)

4.1 Central Venous Cannulation

• Probe placed transversely at the level of the thyroid cartilage
• IJV: compressible, oval, lateral to the common carotid artery
• Carotid artery: non-compressible, round, pulsatile, with colour Doppler confirming arterial flow
• The out-of-plane approach is most commonly used; the in-plane approach visualises the entire needle course

• Reduces arterial puncture risk from ~6.3% (landmark) to <1%
• Reduces haematoma, pneumothorax, and overall complication rate
• Particularly beneficial in: obesity, coagulopathy, abnormal anatomy, previously attempted sites
• Subclavian approach - linear probe in infraclavicular fossa; ultrasound guidance reduces pneumothorax risk but is technically more demanding
• Femoral approach - useful in coagulopathic patients; ultrasound identifies femoral vein medial to femoral artery

4.2 Arterial Cannulation

4.3 Peripheral Venous Access

5. Ultrasound-Guided Regional Anaesthesia (6 marks)

5.1 Historical Context and Evidence

5.2 Advantages over Nerve Stimulator Technique

5.3 Nerve Sonoanatomy

5.4 Common Blocks in Anaesthetic Practice

• Interscalene brachial plexus block - roots C5/C6/C7 visualised between anterior and middle scalene muscles at C6 level. Used for shoulder surgery. Risk: phrenic nerve paralysis (ipsilateral hemidiaphragm palsy in 100% - avoid bilateral or in severe respiratory compromise).
• Supraclavicular block - trunks/divisions of brachial plexus at the first rib, around the subclavian artery. "Cluster of grapes" appearance. Dense forearm and hand block.
• Infraclavicular block - cords around the axillary artery in the infraclavicular fossa. Lateral, posterior, medial cords identified at 9, 6, and 3 o'clock positions respectively.
• Axillary block - terminal branches (radial, ulnar, median, musculocutaneous) identified around the axillary artery. Multiple injections required.

• Femoral nerve block - nerve lies lateral to femoral artery, deep to fascia iliaca. Used for hip fracture analgesia, anterior knee surgery.
• Adductor canal block (ACB) - saphenous nerve in the adductor canal, mid-thigh. Motor-sparing alternative to femoral nerve block for knee arthroplasty analgesia - preserves quadriceps strength.
• Popliteal sciatic nerve block - sciatic nerve bifurcation above the popliteal crease. Used for foot/ankle surgery.

• Transversus abdominis plane (TAP) block - local anaesthetic deposited between internal oblique and transversus abdominis muscles. Provides somatic (not visceral) analgesia for abdominal wall; T7-L1 coverage depending on approach.
• Erector spinae plane (ESP) block - injection into the plane deep to erector spinae, over transverse process. Covers both somatic and visceral pain via paravertebral spread. Used for thoracic, abdominal, and rib fracture analgesia.
• Serratus anterior plane (SAP) block - between serratus anterior and either the latissimus dorsi (superficial) or the ribs (deep). T2-T9 coverage - used for thoracic surgery, rib fractures, mastectomy.
• Rectus sheath block - between rectus abdominis and posterior rectus sheath. Bilateral blocks provide midline abdominal wall analgesia.

5.5 Local Anaesthetic Systemic Toxicity (LAST) Prevention

6. Point-of-Care Cardiac Ultrasound (POCUS) (5 marks)

6.1 Focused Cardiac Ultrasound (FoCUS)

6.2 Clinical Applications of FoCUS in Anaesthesia

• Rapid assessment of ventricular function before emergency/urgent surgery
• Detection of pericardial effusion or tamponade
• Preoperative cardiac risk stratification (hip fracture patients - delay for echocardiography associated with increased mortality)
• ROSE protocol (Rapid Obstetric Screening Echocardiography) for haemodynamically unstable obstetric patients

• Feasible in >90% of surgical cases (Miller's Anesthesia 10e)
• Impacts management in 22-66% of intraoperative uses
• Differentiation of shock subtypes (hypovolaemic, cardiogenic, distributive, obstructive)
• Detection of acute right heart strain (pulmonary embolism - D-shaped interventricular septum)
• Monitoring response to fluid resuscitation

• IVC diameter and collapsibility: IVC <1.5 cm with >50% inspiratory collapse = hypovolaemia; IVC >2.5 cm with <25% collapse = volume overload / raised CVP
• LV end-diastolic area (LVEDA): Reduced (<10 cm²) = hypovolaemia ("kissing ventricles" in severe hypovolaemia); elevated = volume overload or poor function
• Visually estimated LVEF: Reduced (<30%) = severe cardiogenic dysfunction

6.3 Transoesophageal Echocardiography (TOE)

• Intraoperative cardiac surgical monitoring
• Assessment of aortic disease, valve pathology
• Detection of intracardiac thrombus/air
• Monitoring after cardiopulmonary bypass (assessment of de-airing)

7. Lung Ultrasound (3 marks)

7.1 Principles

7.2 Pneumothorax Detection

7.3 eFAST (Extended Focused Assessment with Sonography for Trauma)

• Pericardial view (subcostal): pericardial effusion, tamponade
• Right upper quadrant (Morrison's pouch): hepatorenal free fluid
• Left upper quadrant (splenorenal): free fluid
• Pelvic view (bladder): pelvic free fluid
• Bilateral thoracic views: haemothorax (anechoic fluid above diaphragm), pneumothorax (absent lung sliding)

8. Airway Assessment (2 marks)

8.1 Gastric Ultrasound

Gastric volume (mL) = 27.0 + 14.6 × right-lateral CSA − 1.28 × age

8.2 Tracheal/Laryngeal Ultrasound

• Identifies tracheal midline for confirmation of endotracheal tube position (bilateral lung sliding confirms bilateral ventilation)
• Detects supraglottic structures for awake intubation planning
• Measures subglottic diameter to guide endotracheal tube selection (particularly in paediatric anaesthesia)
• Identifies cricothyroid membrane for emergency front-of-neck access (FONA) - the CTM is located between the thyroid and cricoid cartilages as a hypoechoic gap between two hyperechoic cartilaginous structures

9. Limitations, Training and Governance (2 marks)

9.1 Limitations

• Operator dependency: Image quality and interpretation are directly proportional to skill
• Acoustic windows: Obesity, subcutaneous emphysema, and dressings impair imaging
• Learning curve: Requires dedicated structured training; misinterpretation can cause harm (e.g., mistaking a thrombus for patent vessel, intraneural injection not prevented)
• Equipment: High-quality machines are expensive; portable devices have limitations
• POCUS does not replace formal echocardiography - a negative focused assessment does not exclude all cardiac pathology
• Anaesthetic interference: General anaesthesia, positive pressure ventilation, and the sterile surgical field complicate intraoperative use
• No standardisation: TEE values and POCUS reference ranges vary between institutions

9.2 Training and Competency

• Level 1: Basic POCUS (cardiac, lung, vascular access) - expected of all anaesthetists
• Level 2: Advanced perioperative echocardiography
• Level 3: Expert / diagnostic TOE for complex cardiac surgery

10. Summary Table (marks incorporated above)

References

• Miller RD et al. Miller's Anesthesia, 10th ed. Elsevier, 2023 - Chapter 33 (Monitoring), Chapter 42 (Regional Anaesthesia)
• Barash PG et al. Clinical Anesthesia, 9th ed. Wolters Kluwer, 2022
• European Society of Anaesthesiology Guidelines for Central Venous Access (1B evidence for IJV ultrasound guidance)
• Agency for Healthcare Research and Quality. Making Health Care Safer - 11 practices including ultrasound for CVC
• Koscielniak-Nielsen ZJ. Ultrasound-guided peripheral nerve blocks: what are the benefits? Acta Anaesthesiol Scand. 2008
• Pietroski Dos Santos N et al. Systemic review with meta-analysis: long-acting vs mixed local anaesthetics for US-guided peripheral nerve blocks. Reg Anesth Pain Med. 2026 [PMID: 39779278]
• La Grange P, Foster PA, Pretorius LK. Application of the Doppler ultrasound blood-flow detector. Br J Anaesth. 1978