Direct Oral Anticoagulants (DOACs) Pharmacology

Quick Answer

Direct Oral Anticoagulants (DOACs), also termed Non-vitamin K Antagonist Oral Anticoagulants (NOACs), represent a pharmacological revolution in anticoagulation therapy, offering predictable pharmacokinetics, fixed dosing, and reduced monitoring requirements compared to warfarin. DOACs are classified into two mechanistic categories: Direct Thrombin Inhibitors (dabigatran) and Factor Xa Inhibitors (rivaroxaban, apixaban, edoxaban). Dabigatran etexilate is a prodrug with 6.5% oral bioavailability, predominantly renal elimination (80%), and specific reversal with idarucizumab. Rivaroxaban demonstrates the highest oral bioavailability among DOACs (80-100% when taken with food), undergoes dual CYP3A4/renal elimination, and has a half-life of 5-13 hours. Apixaban exhibits the lowest renal elimination (27%), making it preferable in renal impairment, with a half-life of approximately 12 hours. Edoxaban is administered once daily with 62% bioavailability and 50% renal elimination. All DOACs are substrates for P-glycoprotein (P-gp), creating important drug interactions with P-gp inhibitors (ketoconazole, amiodarone) and inducers (rifampicin). Perioperative management requires cessation based on half-life, renal function, and bleeding risk—typically 24-48 hours for minor procedures and 48-96 hours for major surgery or neuraxial procedures. Reversal agents include idarucizumab (humanized monoclonal antibody fragment for dabigatran), andexanet alfa (recombinant modified Factor Xa for Xa inhibitors), and prothrombin complex concentrates (PCC) as non-specific options. Regional anaesthesia timing follows ASRA/ANZCA guidelines with minimum 72-hour cessation for dabigatran and 72 hours for Factor Xa inhibitors before neuraxial blockade. DOACs have transformed management of atrial fibrillation and venous thromboembolism through landmark trials including RE-LY, ROCKET-AF, ARISTOTLE, and ENGAGE AF-TIMI 48. [1-10]

Classification and Overview

Pharmacological Classification

Direct Oral Anticoagulants target specific coagulation factors within the common pathway, in contrast to vitamin K antagonists that broadly inhibit multiple vitamin K-dependent clotting factors (II, VII, IX, X). This targeted mechanism provides more predictable anticoagulation with wider therapeutic indices.

Coagulation Cascade Context

Understanding DOAC targets requires appreciation of the coagulation cascade:

Extrinsic Pathway: Tissue Factor + Factor VII → Factor VIIa → activates Factor X Intrinsic Pathway: Contact activation → Factor XII → XI → IX → Factor X activation Common Pathway: Factor Xa (with Factor Va as prothrombinase complex) converts prothrombin to thrombin → Thrombin converts fibrinogen to fibrin

Factor Xa occupies a pivotal position where intrinsic and extrinsic pathways converge, amplifying the coagulation signal approximately 1000-fold. One molecule of Factor Xa generates approximately 1000 molecules of thrombin, explaining the potency of Xa inhibition. Thrombin (Factor IIa) is the final serine protease that converts fibrinogen to fibrin, activates platelets, and provides positive feedback by activating Factors V, VIII, and XI. [11-15]

Historical Development

The development of DOACs addressed major limitations of warfarin therapy:

Dabigatran received FDA approval in 2010, followed by rivaroxaban (2011), apixaban (2012), and edoxaban (2015), based on phase III non-inferiority trials demonstrating comparable or superior efficacy to warfarin with improved safety profiles. [16-18]

Individual Drug Profiles

Dabigatran Etexilate

Chemistry and Structure

Dabigatran etexilate is administered as a double prodrug requiring enzymatic hydrolysis to the active moiety, dabigatran. The etexilate ester and hexyloxycarbonyl groups are cleaved by carboxylesterases in the liver and plasma to form the active compound. Dabigatran is a benzimidazole derivative with a molecular weight of 627.7 Da (etexilate) and 471.5 Da (active form). The drug is formulated in capsules containing tartaric acid-coated pellets to create an acidic microenvironment essential for absorption—this formulation must not be crushed or opened. [19-21]

Mechanism of Action

Dabigatran is a direct, competitive, reversible inhibitor of thrombin (Factor IIa). Key mechanistic features include:

• Inhibits free thrombin: Prevents conversion of fibrinogen to fibrin
• Inhibits clot-bound thrombin: Unlike heparins, dabigatran accesses and inhibits thrombin within formed clots
• Inhibits thrombin-induced platelet aggregation: Reduces thrombin-mediated platelet activation
• No antithrombin III requirement: Direct inhibition independent of cofactors

The Ki (inhibition constant) for dabigatran against human thrombin is approximately 4.5 nM, indicating high-affinity binding. Inhibition is concentration-dependent and rapidly reversible upon drug elimination. [22-24]

Pharmacokinetics

Absorption: The low bioavailability results from poor intestinal absorption of the prodrug. Tartaric acid in the formulation lowers gastric pH to enhance dissolution. Proton pump inhibitors may reduce absorption by 20-30%, though this is not considered clinically significant. Food does not significantly affect bioavailability. [25-27]

Distribution: Dabigatran distributes into extravascular compartments with moderate tissue binding. Unlike warfarin (99% protein bound), the 35% protein binding allows effective removal by haemodialysis in overdose situations.

Elimination: Predominantly renal (80% unchanged drug), making it highly sensitive to renal impairment. The remaining 20% undergoes conjugation to acyl glucuronides (which retain pharmacological activity) with subsequent renal excretion.

Renal Dosing Adjustments

ANZCA Primary Relevance: Dabigatran's high renal dependence is frequently examined. Candidates should understand that half-life doubles with moderate renal impairment and may exceed 24 hours with severe impairment, significantly affecting perioperative timing. [28-30]

Rivaroxaban

Chemistry and Structure

Rivaroxaban is an oxazolidinone derivative with a molecular weight of 435.9 Da. Unlike dabigatran, rivaroxaban is administered as the active drug, not a prodrug. The molecule contains a chlorothiophene ring system that provides selectivity for Factor Xa. It is available as film-coated tablets in 2.5 mg, 10 mg, 15 mg, and 20 mg strengths. [31-32]

Mechanism of Action

Rivaroxaban directly and selectively inhibits Factor Xa through competitive, reversible binding to the active site. Key features include:

• Inhibits free Factor Xa: Prevents prothrombinase complex formation
• Inhibits prothrombinase-bound Xa: Accesses Xa within the Factor Va/Xa complex
• Inhibits clot-associated Xa: Penetrates into thrombi
• No effect on platelet aggregation: Unlike thrombin inhibitors

The Ki for human Factor Xa is approximately 0.4 nM, indicating extremely high affinity. The selectivity for Factor Xa over thrombin is greater than 10,000-fold. [33-35]

Pharmacokinetics

Absorption: Rivaroxaban demonstrates the highest bioavailability among DOACs when taken with food. The 15 mg and 20 mg doses show significant food-dependent absorption—bioavailability increases from 66% to nearly 100% when taken with food. This is attributed to enhanced solubility of the lipophilic molecule in the presence of dietary fat. Lower doses (10 mg) show food-independent absorption. [36-38]

Clinical Pearl: Always counsel patients to take rivaroxaban 15 mg or 20 mg doses with their largest meal of the day. This is critical for ensuring therapeutic drug levels.

Distribution: High protein binding (92-95%) to albumin means rivaroxaban is not effectively removed by dialysis. Distribution is primarily extracellular with limited CNS penetration.

Metabolism and Elimination: Rivaroxaban undergoes dual elimination—approximately one-third is excreted unchanged by the kidneys (active tubular secretion via P-gp), and two-thirds is metabolized hepatically via CYP3A4/CYP2J2 with subsequent renal and faecal excretion of inactive metabolites. This dual pathway provides some redundancy in patients with moderate renal impairment. [39-41]

Renal Dosing Adjustments

Apixaban

Chemistry and Structure

Apixaban is a pyrazole-based Factor Xa inhibitor with a molecular weight of 459.5 Da. It is administered as the active drug in tablet form (2.5 mg and 5 mg strengths). The molecular structure provides high selectivity for Factor Xa with a Ki of approximately 0.08 nM—the highest affinity among oral Factor Xa inhibitors. [42-44]

Mechanism of Action

Similar to rivaroxaban, apixaban directly inhibits Factor Xa through reversible, competitive binding. It inhibits free Factor Xa, prothrombinase-bound Xa, and clot-associated Xa. The high binding affinity translates to potent anticoagulation at relatively low plasma concentrations.

Pharmacokinetics

Key Feature—Lowest Renal Elimination: Apixaban's 27% renal elimination makes it the preferred DOAC in patients with renal impairment. Approximately 73% undergoes hepatic metabolism and biliary excretion. This provides a safety advantage in chronic kidney disease, though dose adjustment is still recommended. [45-47]

Absorption: Unlike rivaroxaban, apixaban absorption is not significantly affected by food. Bioavailability is approximately 50% with linear pharmacokinetics up to 10 mg doses.

Distribution: The smaller volume of distribution (21 L) compared to other DOACs reflects more limited tissue distribution. Protein binding to albumin is 87%.

Metabolism: Primarily hepatic via CYP3A4 with minor contributions from CYP1A2 and CYP2J2. Apixaban is a substrate (not inhibitor) of P-glycoprotein, affecting drug interactions. [48-50]

Renal Dosing Adjustments

Dose Reduction Criteria (any 2 of 3):

• Age ≥80 years
• Weight ≤60 kg
• Serum creatinine ≥133 μmol/L

This patient-specific dosing algorithm distinguishes apixaban from other DOACs and reflects the ARISTOTLE trial design. [51-53]

Edoxaban

Chemistry and Structure

Edoxaban is a synthetic small molecule Factor Xa inhibitor with a molecular weight of 548.1 Da. It is available as 15 mg, 30 mg, and 60 mg tablets. The structure contains a bicyclic amine that provides selectivity for Factor Xa.

Pharmacokinetics

Unique Consideration—Paradoxical High CrCl Effect: In the ENGAGE AF-TIMI 48 trial, patients with CrCl >95 mL/min showed reduced efficacy compared to warfarin. This is attributed to enhanced renal clearance lowering drug exposure. Consequently, edoxaban is contraindicated or used with caution in patients with CrCl >95 mL/min in some jurisdictions. [54-56]

Renal Dosing Adjustments

Comparative Pharmacology Summary

Clinical Indications

Atrial Fibrillation

DOACs are first-line therapy for stroke prevention in non-valvular atrial fibrillation (NVAF), demonstrating non-inferiority or superiority to warfarin in landmark trials:

Key findings across trials include consistent reduction in intracranial haemorrhage (the most feared warfarin complication) and comparable or improved efficacy for stroke prevention. [57-62]

Contraindications in AF:

• Mechanical heart valves (increased thromboembolism demonstrated with dabigatran)
• Moderate-severe mitral stenosis (rheumatic valvular disease)

Venous Thromboembolism

DOACs are approved for acute treatment and secondary prevention of deep vein thrombosis (DVT) and pulmonary embolism (PE):

Treatment Approach:

• Rivaroxaban/Apixaban: Single-drug approach (no initial parenteral anticoagulation)
  • Rivaroxaban: 15 mg BD × 3 weeks → 20 mg OD
  • Apixaban: 10 mg BD × 7 days → 5 mg BD
• Dabigatran/Edoxaban: Require 5-10 days initial parenteral anticoagulation before switching

Extended Treatment: All DOACs have reduced-intensity dosing options for extended secondary prevention (apixaban 2.5 mg BD, rivaroxaban 10 mg OD). [63-65]

VTE Prophylaxis

Orthopaedic Surgery:

• Rivaroxaban 10 mg OD (THR: 35 days; TKR: 14 days)
• Apixaban 2.5 mg BD (THR: 32-38 days; TKR: 10-14 days)
• Dabigatran 220 mg OD (or 150 mg if CrCl 30-50 mL/min)

Medical Patients: Rivaroxaban and betrixaban are approved in some jurisdictions for medically ill patients at high VTE risk.

Perioperative Management

Principles of DOAC Cessation

Perioperative management requires balancing thrombotic risk (from drug cessation) against bleeding risk (from residual anticoagulation). Key determinants include:

1. Drug half-life (varies by agent and renal function)
2. Renal function (affects elimination, especially dabigatran)
3. Bleeding risk of procedure (low vs high)
4. Thrombotic risk (recent VTE, high CHA₂DS₂-VASc)

Pre-Procedural Cessation Timing

Low Bleeding Risk Procedures (endoscopy without biopsy, dental extractions, minor skin surgery):

High Bleeding Risk Procedures (major surgery, neuraxial anaesthesia, cardiac surgery):

Urgent/Emergency Surgery

When surgery cannot be delayed for adequate drug clearance:

Immediate Assessment:

1. Identify DOAC and time of last dose
2. Assess renal function (affects residual drug levels)
3. Quantify DOAC level if available (anti-Xa assay, dilute thrombin time)
4. Estimate bleeding risk of procedure

Management Algorithm:

Last dose <12-24 hours → Consider reversal agent
                       → Idarucizumab for dabigatran
                       → Andexanet alfa or PCC for Xa inhibitors

Last dose >24-48 hours → Proceed with caution
                       → PCC available if unexpected bleeding

Last dose >48-72 hours → Usually safe to proceed
                       → Normal renal function assumed

Non-Specific Measures:

• Activated charcoal if ingestion <2-4 hours
• Tranexamic acid (adjunct)
• Red cell transfusion for anaemia
• Maintain normothermia, avoid acidosis
• Surgical/endoscopic haemostasis

Post-Procedural Resumption

Low Bleeding Risk: Resume 6-8 hours post-procedure (VTE prophylaxis dose) or 24-48 hours (full therapeutic dose)

High Bleeding Risk: Resume 48-72 hours post-procedure once haemostasis assured

Bridging: Generally NOT recommended for DOACs due to rapid onset. If anticoagulation required earlier than safe DOAC resumption, consider prophylactic LMWH. [66-70]

Monitoring

Routine Monitoring—Generally Not Required

A major advantage of DOACs is predictable pharmacokinetics eliminating routine monitoring. However, understanding their effects on coagulation tests remains important:

When Monitoring May Be Useful

1. Bleeding/overdose: Confirm drug presence and estimate level
2. Urgent surgery: Assess residual anticoagulation
3. Extreme body weight: Verify adequate anticoagulation (>120 kg) or exclude accumulation (<50 kg)
4. Renal impairment progression: Evaluate for drug accumulation
5. Drug interaction concerns: Assess impact
6. Suspected non-compliance: Confirm drug presence
7. Recurrent thromboembolism on therapy: Assess drug levels

Interpretation Caveats

• PT/aPTT are not reliable for quantifying DOAC effect
• Normal PT/aPTT does not exclude clinically significant DOAC levels
• Anti-Xa assays must be calibrated to the specific DOAC
• Drug levels vary with timing of last dose (peak vs trough)

Reversal Agents

Idarucizumab (Praxbind)

Mechanism and Pharmacology

Idarucizumab is a humanized monoclonal antibody fragment (Fab) that specifically binds dabigatran with approximately 350 times greater affinity than dabigatran has for thrombin. This high-affinity binding rapidly neutralizes free and thrombin-bound dabigatran, restoring haemostasis within minutes.

RE-VERSE AD Trial Evidence

The RE-VERSE AD study (n=503) demonstrated:

• Complete reversal of dabigatran effect in 98% within 4 hours
• Median time to cessation of bleeding: 2.5 hours
• Thrombotic events: 4.8% at 30 days (consistent with underlying thrombotic risk)
• No pro-coagulant signal beyond expected baseline risk

Indications

• Life-threatening or uncontrolled bleeding on dabigatran
• Emergency surgery/urgent invasive procedure requiring normal haemostasis

Administration

• 5 g total dose (2 × 2.5 g vials, 50 mL each)
• Administer as two consecutive infusions over 5-10 minutes each, or as bolus injection
• No dose adjustment for renal impairment
• Can repeat dose if bleeding recurs or emergent surgery required

Important: Idarucizumab only reverses dabigatran—it has no effect on Factor Xa inhibitors. [71-74]

Andexanet Alfa (Ondexxya/Andexxa)

Mechanism and Pharmacology

Andexanet alfa is a recombinant modified Factor Xa molecule that acts as a "decoy receptor" for Factor Xa inhibitors. The molecule lacks catalytic activity (cannot cleave prothrombin) but retains high-affinity binding for Xa inhibitors.

Dosing

ANNEXA-4 Trial Evidence

The ANNEXA-4 study (n=352) demonstrated:

• Anti-Xa activity reduction: 92% (rivaroxaban), 92% (apixaban)
• Excellent/good haemostatic efficacy: 82% at 12 hours
• Thrombotic events: 10% at 30 days (higher than underlying risk)

Limitations

• Short half-life requires infusion
• High cost
• Limited availability
• Associated with increased thrombotic events
• Does not reverse LMWH or fondaparinux effectively

[75-77]

Prothrombin Complex Concentrate (PCC)

Mechanism

4-Factor PCC (Beriplex, Kcentra, Octaplex) contains concentrated vitamin K-dependent clotting factors (II, VII, IX, X) plus Protein C and S. For DOAC reversal, PCC "overwhelms" Factor Xa inhibition by providing supraphysiological factor concentrations, restoring thrombin generation.

Evidence

• No randomized trials specifically for DOAC reversal
• Observational data suggest haemostatic efficacy in 65-80%
• Lower cost than andexanet alfa
• Wider availability

Dosing for DOAC Reversal

Considerations

• Pro-thrombotic potential (DIC, thromboembolism)
• Monitor for thrombotic complications
• Not approved for DOAC reversal (off-label use)
• aPCC (FEIBA) may be more effective for dabigatran than standard PCC

[78-80]

Comparison of Reversal Strategies

Drug Interactions

P-Glycoprotein Interactions

All DOACs are substrates for P-glycoprotein (P-gp), an efflux transporter in the intestine and kidney that limits drug absorption and enhances renal secretion.

P-gp Inhibitors (increase DOAC levels):

• Ketoconazole, itraconazole (strong)
• Verapamil, dronedarone
• Amiodarone
• Clarithromycin
• Cyclosporine, tacrolimus

P-gp Inducers (decrease DOAC levels):

• Rifampicin (strong)—contraindicated with all DOACs
• Phenytoin, carbamazepine
• St John's Wort

CYP3A4 Interactions (Rivaroxaban, Apixaban)

Clinical Recommendations

Dabigatran-specific: Administer dabigatran ≥2 hours before P-gp inhibitors to minimize absorption interaction.

[81-83]

Regional Anaesthesia and Neuraxial Blockade

Risk of Spinal Haematoma

Neuraxial haematoma is a rare but devastating complication. Risk is increased with:

• Residual anticoagulation at time of needle placement
• Traumatic/bloody tap
• Indwelling epidural catheter
• Anticoagulation during catheter dwell or at removal
• Anatomical abnormalities (spinal stenosis, coagulopathy)

ASRA/ANZCA Guidelines

The 4th Edition ASRA Evidence-Based Guidelines (2018) provide the following recommendations for DOACs and neuraxial procedures:

Pre-Procedural Cessation

Post-Procedural Resumption

Epidural Catheter Removal

• Remove catheter at trough DOAC concentration
• Timing: Before next dose or ≥12-24 hours after last dose
• Resume DOAC ≥6 hours after catheter removal

ANZCA Professional Document PS29

ANZCA guidelines align with ASRA recommendations, emphasizing:

• Shared decision-making with patients
• Individual risk-benefit assessment
• Documentation of timing and rationale
• Vigilant neurological monitoring post-procedure

ANZCA Viva Point: Candidates should be prepared to discuss the risk-benefit analysis of neuraxial blockade in patients on DOACs, including options for regional anaesthetic techniques (peripheral nerve blocks may be acceptable with shorter DOAC cessation) and alternative analgesic strategies.

[84-87]

Indigenous Health Considerations

Indigenous Australians (Aboriginal and Torres Strait Islander peoples) and New Zealand Māori experience significant disparities in cardiovascular disease burden, atrial fibrillation prevalence, and anticoagulation outcomes that are directly relevant to perioperative DOAC management.

Epidemiological Context: Indigenous Australians develop atrial fibrillation 10-20 years earlier than non-Indigenous Australians and are hospitalized for AF at 2-6 times the rate depending on age group. This younger age of onset has profound implications for lifetime anticoagulation requirements and perioperative encounters. The higher burden of comorbidities—including rheumatic heart disease (which may preclude DOAC use), chronic kidney disease, and type 2 diabetes—complicates anticoagulation management.

Rheumatic Heart Disease Consideration: Rheumatic heart disease remains endemic in remote Indigenous communities, causing valvular heart disease that represents a contraindication to DOAC therapy. These patients require warfarin for stroke prevention, and anaesthetists must be aware that not all Indigenous patients with AF can be managed with DOACs. Careful cardiac assessment is essential.

Access and Adherence Challenges: Geographic isolation, health service accessibility, medication cost (prior to PBS changes), and health literacy barriers affect DOAC adherence in remote communities. The "fly-in-fly-out" model of healthcare delivery may complicate perioperative planning, as consistent communication with the patient's usual healthcare providers may be difficult.

Culturally Safe Care: Perioperative discussions about DOAC management should incorporate culturally appropriate communication, involve Aboriginal and Torres Strait Islander Health Workers where available, and respect cultural practices around surgery and hospitalisation. Family involvement in decision-making is often important in Indigenous health contexts.

Renal Function Assessment: Given the higher prevalence of chronic kidney disease in Indigenous populations, careful renal function assessment is critical before perioperative DOAC management, as dose adjustments and extended cessation times may be required.

[88-91]

SAQ Practice Question

Question: A 72-year-old man taking apixaban 5 mg twice daily for non-valvular atrial fibrillation presents for elective total hip arthroplasty. His CrCl is 45 mL/min. Discuss the perioperative management of his anticoagulation.

(15 marks, 25 minutes)

Model Answer

Introduction and Risk Assessment (2 marks)

This patient requires careful perioperative anticoagulation management balancing thrombotic risk (CHA₂DS₂-VASc score, indication for anticoagulation) against bleeding risk (major orthopaedic surgery with neuraxial anaesthesia likely considered).

Apixaban is a direct Factor Xa inhibitor with:

• Half-life: ~12 hours
• Renal elimination: 27% (lowest among DOACs)
• CYP3A4 metabolism

His CrCl of 45 mL/min represents mild-moderate renal impairment, affecting drug clearance.

Pre-operative Management (4 marks)

1. Cessation timing: For high bleeding risk surgery:

   • Standard recommendation: 48-72 hours pre-operatively
   • With CrCl 30-50 mL/min: Extend to 72-96 hours (approximately 6 half-lives)
   • Recommend last dose 4 days (96 hours) before surgery

2. No bridging required: DOACs have rapid offset; bridging with heparin increases bleeding risk without reducing thromboembolism

3. Pre-operative assessment:

   • Confirm actual time of last dose
   • Repeat renal function if clinical change suspected
   • Consider anti-Xa level if uncertainty about cessation

Intra-operative Considerations (4 marks)

1. Neuraxial anaesthesia considerations:

   • If spinal/epidural planned: Ensure 72+ hours cessation per ASRA/ANZCA guidelines
   • Alternative: General anaesthesia with multimodal analgesia
   • Alternative: Peripheral nerve blocks (adductor canal, PENG block) may be acceptable with shorter cessation (24-48 hours)

2. Haemostasis preparation:

   • Tranexamic acid (topical and/or IV)
   • Cross-match available
   • Know availability of reversal agents (andexanet alfa, 4F-PCC) if unexpected bleeding

3. If unexpected residual anticoagulation:

   • Delay surgery if feasible
   • Point-of-care anti-Xa level if available
   • Proceed with caution if urgent; have reversal available

Post-operative Management (3 marks)

1. VTE prophylaxis timing:

   • Mechanical prophylaxis immediately
   • Chemical prophylaxis: Resume apixaban 2.5 mg BD at 24-48 hours post-operatively (once surgical haemostasis assured)

2. Return to full anticoagulation:

   • Apixaban 5 mg BD can resume at 48-72 hours if no bleeding complications
   • Consider extended VTE prophylaxis dosing (2.5 mg BD) for first 7 days, then therapeutic dose

3. If epidural catheter in situ:

   • Remove catheter before DOAC resumption
   • Wait ≥6 hours after removal before first apixaban dose
   • Neurological monitoring post-removal

Monitoring and Follow-up (2 marks)

• Clinical assessment for bleeding (wound, haemoglobin)
• Neurological monitoring if neuraxial block performed
• VTE surveillance (clinical assessment, consider Doppler if symptoms)
• Arrange follow-up with usual prescriber for long-term anticoagulation

Viva Scenario

Examiner Instructions

A 68-year-old woman on rivaroxaban 20 mg daily for atrial fibrillation requires emergency laparotomy for small bowel obstruction with suspected strangulation. She took her last dose this morning (approximately 8 hours ago). CrCl is 60 mL/min.

Expected Discussion Points:

1. Assessment of residual anticoagulation
2. Reversal options
3. Anaesthetic management
4. Post-operative anticoagulation strategy

---

Model Viva Dialogue

Examiner: "This patient took rivaroxaban 8 hours ago and requires emergency surgery. How would you assess her anticoagulation status?"

Candidate: "Given rivaroxaban's Tmax of 2-4 hours and half-life of 5-13 hours (approximately 9-11 hours at age 68), she likely has significant residual anticoagulation at 8 hours post-dose. I would:

1. Note the exact time of last dose from the patient or medication chart
2. Request a calibrated anti-Xa assay for rivaroxaban if available and time permits—this provides quantitative assessment
3. Recognize that PT may be prolonged but is not reliable for quantification
4. A normal anti-Xa level would suggest safe to proceed; elevated levels indicate residual effect

However, with emergency surgery for suspected bowel strangulation, I would not delay surgery for laboratory results."

Examiner: "The anti-Xa level is elevated. What reversal options do you have?"

Candidate: "For rivaroxaban reversal, options include:

1. Andexanet alfa (if available):

• Specific reversal agent—recombinant modified Factor Xa decoy
• Dose: High-dose protocol (800 mg bolus + 8 mg/min × 2 hours) as dose >10 mg and <8 hours ago
• Provides rapid anti-Xa activity reduction
• Limitations: Very expensive, short half-life requiring infusion, thrombotic risk, limited availability

2. 4-Factor PCC (more available):

• Off-label use
• Dose: 25-50 units/kg or fixed dose 1500-2000 units
• Mechanism: Overwhelms Factor Xa inhibition with supraphysiological coagulation factors
• Onset: 15-30 minutes
• Thrombotic risk exists

3. Supportive measures:

• Tranexamic acid as adjunct
• Ensure normothermia, avoid acidosis
• Blood products as needed for dilutional coagulopathy

Given likely availability constraints, I would administer 4-Factor PCC 25-50 units/kg while preparing for surgery."

Examiner: "You've given PCC. What are your anaesthetic considerations?"

Candidate: "Anaesthetic management considerations include:

Airway: Rapid sequence induction indicated (bowel obstruction)

Haemodynamic preparation:

• Large-bore IV access, arterial line
• Cross-matched blood available
• Vasopressors prepared (likely sepsis/third-spacing)

Coagulation management:

• Cell salvage available
• Additional blood products on standby
• Repeat anti-Xa or viscoelastic testing (ROTEM/TEG) if significant bleeding

Regional anaesthesia: Neuraxial is contraindicated given residual anticoagulation

Post-operative planning:

• ICU admission likely
• Ongoing coagulation monitoring
• VTE prophylaxis timing will depend on surgical haemostasis and bleeding risk"

Examiner: "When would you restart anticoagulation post-operatively?"

Candidate: "The decision to restart anticoagulation requires balancing:

Thrombotic risk: She has AF—need to know CHA₂DS₂-VASc score; stroke risk without anticoagulation is approximately 4-6% per year for typical AF patient

Bleeding risk: Intestinal anastomosis, recent reversal agent use, sepsis-associated coagulopathy

Approach:

1. Mechanical VTE prophylaxis immediately post-operatively
2. Delay rivaroxaban until surgical team confirms haemostasis—typically 48-72 hours minimum after major abdominal surgery
3. Consider prophylactic LMWH at 24-48 hours if bleeding risk acceptable
4. Resume therapeutic rivaroxaban 20 mg OD with food once:
   • Adequate oral intake established
   • No evidence of post-operative bleeding
   • Surgeon approval obtained
5. Document rationale and plan clearly for post-operative team"

Examiner: "What would be different if she was on dabigatran?"

Candidate: "Key differences with dabigatran:

Reversal: Idarucizumab 5 g IV—specific, highly effective, preferred over PCC

• Rapid onset, complete reversal in 98%
• No infusion required
• Would be first choice if available

Renal dependence: Dabigatran is 80% renally eliminated versus 36% for rivaroxaban

• At CrCl 60 mL/min, dabigatran half-life would be approximately 15 hours
• Residual effect may be greater at 8 hours post-dose

Dialysis: Dabigatran is dialyzable (35% protein bound) if idarucizumab unavailable

• Rivaroxaban is 92% protein bound—not dialyzable

PCC effectiveness: aPCC (FEIBA) may be more effective than 4F-PCC for dabigatran if idarucizumab unavailable

Assessment: Dilute thrombin time or ecarin clotting time would be the appropriate quantitative assays for dabigatran, not anti-Xa."

References

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