2024.2 Day 1 VIVA 5

Pain Medicine

You have a 68-year-old male for a total knee replacement (TKR) on your list in Scenarioville.

He has a history of chronic pain secondary to widespread osteoarthritis (OA), wellcontrolled hypertension and hypercholesterolaemia. His medications are:

  • perindopril 10mg

  • rosuvastatin 40mg

  • buprenorphine transdermal patch (20mcg per hour, 7-day)

  • paracetamol slow release (SR) 665mg x 2 tds.

He has had a colonoscopy and a laparoscopic cholecystectomy in the past with no anaesthetic issues.

On examination:

  • Observations all within normal range

  • Body Mass Index (BMI) is 30 (92kg, 175cm)

  • Mallampati 2 with a good range of movement of the neck and thyromental distance of >7cm

  • Cardio-respiratory exam is unremarkable, and ECG shows normal sinus rhythm

  • He has significantly reduced movement of his lower back due to long-standing OA in the lumbar spine

Please outline your anaesthetic plan for this gentleman’s total knee replacement.

  1. Preoperative Optimization:

    • Chronic Pain Management:

      • Continue buprenorphine patch (20 mcg/hr) to prevent withdrawal. Rationale: Abrupt cessation risks withdrawal and hyperalgesia.

      • Add paracetamol 1 g IV pre-op (synergistic with opioids).

    • Cardiovascular: Perindopril withheld on morning of surgery (risk of hypotension under neuraxial anaesthesia).

  2. Anaesthetic Choice:

    • Primary Plan: Spinal anaesthesia (hyperbaric bupivacaine 12–15 mg + fentanyl 25 mcg). Rationale: Gold standard for TKR; reduces opioid use, PONV, and delirium risk.

    • Backup Plan: GA with endotracheal intubation (if spinal fails). *Rationale: Mallampati 2 + adequate neck ROM suggests low airway risk.*

  3. Multimodal Analgesia Plan:

    • Intra-op Adjuncts:

      • Ketamine 0.5 mg/kg bolus + 0.1 mg/kg/hr infusion. Rationale: NMDA antagonism reduces opioid tolerance and central sensitization.

      • Dexamethasone 8 mg IV (prolongs analgesia, antiemetic).

    • Regional Technique: Adductor canal block (ACB) with ropivacaine 0.5%. Rationale: Spares quadriceps strength for early mobilization.

What challenges does his chronic pain present?

  • Opioid Tolerance:

    • Buprenorphine's partial agonism → blunts response to full µ-agonists

    • Requires 30-50% higher opioid doses post-op

  • Hyperalgesia Risk:

    • Chronic opioid use sensitizes NMDA receptors

    • Mandates ketamine/magnesium prophylaxis

  • Withdrawal Prevention:

    • Continue buprenorphine perioperatively

    • Abrupt cessation → pain exacerbation + autonomic instability

How would spinal OA affect your plan?

    • Technical adjustments:

      • Paramedian approach often needed

      • Consider sitting position despite obesity

    • Pharmacologic changes:

      • Reduce bupivacaine to 10-12mg hyperbaric

      • Add fentanyl 25mcg for synergistic effect

    • Monitoring: anticipate slower block onset

  • Scenarioville Context:

    • May lack CSE equipment → have GA drugs prepared

What equipment would you prepare for potential airway management?

  • Standard: Mac 3 blade, size 3 LMA

  • Backup: Videolaryngoscope (if available)

  • Emergency: Front-of-neck access kit

  • Scenarioville constraint: May lack difficult airway cart

How would you assess his cardiovascular risk for this surgery?

  • RCRI criteria:

    • Age >65 (+1)

    • Hypertension (+1)

    • Estimated risk <5% (no further testing needed)

  • Preop ECG: Already shows normal sinus rhythm

  • Functional capacity: Document any exertional limitations

Outline your intraoperative management plan.

  1. Monitoring:

    • Standard monitoring + invasive BP if prolonged surgery.

    • Sedation: Propofol TCI (target 0.5–1 mcg/mL) or midazolam 1–2 mg.

  2. Analgesia Adjuncts:

    • Ketamine: 0.1 mg/kg/hr infusion (reduces opioid-induced hyperalgesia).

    • Magnesium: 2 g IV over 20 min (NMDA antagonism). *Rationale: Scenarioville may lack ketamine infusion pumps → Mg2+ is alternative.*

  3. Fluid Management:

    • Restricted crystalloids (1–2 L max) + vasopressors (phenylephrine 50 mcg boluses) for spinal-induced hypotension.

How would you manage a high spinal block?

  • Immediate actions:

    • Declare emergency

    • Call for help

    • 100% O₂, vasopressors (phenylephrine boluses)

    • Head-up position if conscious

  • Airway:

    • Prepare for intubation if respiratory compromise

  • Monitoring:

    • Check block level q5min

    • ECG for bradycardia

What would you do if the surgeon reports excessive bleeding?

  • Medical management:

    • TXA 1g IV if not already given

    • Maintain normothermia

  • Communication:

    • Discuss transfusion triggers (Hb <70-80g/L)

  • Scenarioville limitation:

    • No blood on site → early retrieval planning

How would you modify analgesia if ketamine isn't available?

  • Alternative adjuvants:

    • Magnesium 2g IV over 20min

    • Lidocaine 1mg/kg bolus → 1mg/kg/hr infusion

  • Opioid strategy:

    • Higher PCA basal rate (e.g., 0.5mg/hr morphine)

    • More frequent bolus dosing

The patient becomes hypertensive (BP 190/100) - causes?

  • Pain (inadequate analgesia)

  • Tourniquet pain (after 60-90min)

  • Light anesthesia (if sedated)

  • Bladder distension

  • Preexisting hypertension rebound

Outline your postoperative analgesia and chronic pain prevention strategy.

  1. Immediate Post-op:

    • PCA morphine: 1.5 mg bolus, 5-min lockout (higher due to tolerance).

    • Scheduled Analgesics:

      • Paracetamol 1 g Q6H.

      • Celecoxib 200 mg BD (unless CKD).

      • Gabapentin 300 mg nocte (neuropathic prophylaxis).

  2. Regional Follow-Up:

    • ACB catheter (if available): Ropivacaine 0.2% @ 8 mL/hr.

    • Scenarioville Fallback: Single-shot ACB + PRN opioids.

  3. CPSP Prevention:

    • Ketamine: Continue infusion for 48 hrs (0.1 mg/kg/hr).

    • Mobilization: Day 1 physio (limited in Scenarioville → family-assisted).

How would you manage his buprenorphine postoperatively?

  • Continue current dose (20mcg/hr patch)

  • Supplemental opioids:

    • Expect higher than normal requirements

    • Monitor for sedation/respiratory depression

  • Conversion options:

    • 20mcg/hr buprenorphine ≈ 30mg oral morphine/day

What signs would suggest opioid toxicity?

  • Early: Sedation, miosis, nausea

  • Late: Respiratory depression (RR<8), hypoxia

  • Severe: Coma, aspiration risk

  • Scenarioville challenge:

    • Limited ICU beds → early naloxone for respiratory depression

How would you modify care if he can't use a PCA?

  • Alternative options:

    • Nurse-controlled analgesia (NCA) with 2mg morphine q10min

    • Scheduled oxycodone IR 5mg q4h

  • Monitoring:

    • More frequent obs (hourly sedation/respiratory rate)

  • Non-pharmacologic:

    • Cryotherapy, elevation

What factors increase his risk of chronic post-surgical pain?

  • Pre-existing:

    • Chronic pain history

    • Opioid tolerance

  • Surgical:

    • Extensive tissue trauma

  • Post-op:

    • Poor acute pain control

  • Prevention strategies:

    • Continue ketamine infusion 48h

    • Early gabapentin if neuropathic features

Critical Phrases Examiners Expect:

  • "Multimodal approach to address opioid tolerance"

  • "Dose reduction for neuraxial in spinal pathology"

  • "Proactive rather than reactive management"

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