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"