Epidural Opioids - MD Anaesthesia Exam Notes

1. Mechanism of Action

1. Spinal (primary): Diffusion into CSF → dorsal horn receptor binding - this is the desired effect
2. Supraspinal (secondary): Systemic vascular absorption → supraspinal opioid receptors

Key advantage: Epidural opioids produce analgesia without sympatholytic effect or motor blockade - unlike epidural local anaesthetics. (Barash Clinical Anesthesia, 9e)

2. Lipophilicity - The Cornerstone Concept

Lipophilic opioids (fentanyl, sufentanil) are readily absorbed into systemic circulation - several studies suggest this is their principal analgesic mechanism when given epidurally. (Miller's Anesthesia, 10e)

Important caveat: Bolus epidural fentanyl appears segmental; continuous infusion appears non-segmental (systemic). (Barash, 9e)

3. Individual Drugs - Doses and Key Features

Morphine (prototype hydrophilic)

• Epidural bolus: 1-5 mg (optimal: 2.5-3.75 mg to balance analgesia vs side effects)
• Continuous infusion: 0.1-0.4 mg/h
• Onset: 30-60 minutes
• Duration: Up to 24 hours
• DepoDur (extended-release liposomal morphine): Single-shot lumbar epidural - 10-15 mg for lower abdominal surgery, 15 mg for major lower limb orthopaedic surgery; provides up to 48 hours of analgesia
• Risk of delayed respiratory depression due to rostral spread in CSF

Fentanyl (lipophilic)

• Epidural: 50-100 mcg bolus; continuous 25-100 mcg/h
• Infusion concentration: 2-5 mcg/mL (combined with local anaesthetic)
• Onset: 5-15 minutes
• Duration: 2-4 hours (short - requires continuous infusion)
• Primarily systemic absorption when infused

Sufentanil (most lipophilic)

• Epidural: 10-50 mcg bolus
• Infusion concentration: 0.2-0.5 mcg/mL
• Drug of choice for opioid-tolerant patients taking >250 mg/day oral morphine due to high intrinsic activity (Barash, 9e)

Hydromorphone (intermediate - excellent meningeal permeability)

• Epidural: 0.5-1 mg bolus; continuous 0.1-0.2 mg/h
• Infusion concentration: 0.02 mg/mL
• Recommended over morphine+bupivacaine by some authorities - combines advantages of hydrophilic opioid with excellent meningeal permeability (Barash, 9e)

Meperidine (unique - has intrinsic local anaesthetic properties)

• Only opioid with local anaesthetic properties
• Can produce motor block and sympathectomy - unlike other opioids

4. Dosing Table for Labor Analgesia (Morgan & Mikhail, 7e)

5. Side Effects

Respiratory Depression - EXAM FAVOURITE

• Early respiratory depression (within 2 hours): Due to rapid systemic absorption - more with lipophilic opioids (fentanyl)
• Delayed respiratory depression (2-12 hours or later): Due to rostral spread in CSF - classic with morphine
• Treat with IV naloxone (0.1-0.4 mg increments) - titrate to avoid reversing analgesia
• Risk factors: High dose, thoracic placement, concomitant systemic opioids, sedatives, respiratory disease, obesity

Never administer opioids by multiple simultaneous routes - additive respiratory depression risk. Epidural opioid + PCA opioid combination should be avoided. (Barash, 9e)

6. Synergism with Local Anaesthetics

• Opioid + local anaesthetic combination = synergistic analgesia
• Local anaesthetics block afferent input (stress response); opioids act on dorsal horn receptors
• Combination allows dose reduction of each drug - minimises side effects of both
• Most common: Bupivacaine + fentanyl
• Epidural opioids do NOT prolong motor block unlike increasing local anaesthetic concentration (Miller's, 10e)
• There appears to be a ceiling effect - above a certain dose, only side effects increase without additional analgesia

7. Catheter Placement - Congruence Principle (Barash, 9e)

• Catheter tip should be congruent (at same dermatomal level) with surgical incision
• When catheter is incongruent with incision level:
  • Lipophilic opioids (fentanyl) lose their segmental effect
  • Hydrophilic opioids (morphine, hydromorphone) are required - they spread rostrally in CSF to reach the surgical dermatomal level

If lumbar catheter is placed for mid-to-high thoracic surgery (incongruent), hydrophilic opioids are required. (Barash Table 55-21)

8. Adjuvants to Epidural Opioids

• Epidural clonidine (2 mcg/mL, 5-20 mcg/h): Alpha-2 agonist - reduces both epidural LA and opioid requirements; side effects: hypotension, bradycardia, sedation
• Epidural dexmedetomidine: Reduces intraoperative anaesthetic requirements, improves postoperative analgesia
• Epidural neostigmine: Provides labor analgesia without respiratory depression, hypotension, or motor impairment

9. High-Yield One-Liners for Exam

1. Hydrophilic opioids = slow onset, long duration, broad band, risk of delayed respiratory depression (rostral spread), high CSF concentration
2. Lipophilic opioids = fast onset, short duration, narrow/segmental band, mostly systemic mechanism, low CSF concentration (partition into fat)
3. Pruritus = most common side effect; mediated by mu receptors (not histamine); reversed by naloxone/nalbuphine
4. Morphine = gold standard epidural opioid; dose 1-5 mg epidural; lasts 12-24 hrs
5. Sufentanil = drug of choice in opioid-tolerant patients
6. DepoDur = extended-release morphine; single-shot; lasts 48 hours
7. Meperidine = only opioid with intrinsic local anaesthetic properties
8. Bolus fentanyl = segmental; infusion fentanyl = non-segmental (systemic)
9. Epidural opioids alone = NO motor block, NO sympatholytic effect (unlike LA)
10. Do NOT combine epidural opioid with IV PCA opioid simultaneously (additive respiratory depression)

Epidural Opioids

Model Answer for 10-Mark Exam Question

Introduction (0.5 marks)

Mechanism of Action (1 mark)

1. Spinal mechanism (primary): Drug diffuses across dura into CSF → binds to pre- and post-synaptic opioid receptors in dorsal horn → inhibits release of substance P → blocks pain transmission
2. Supraspinal mechanism (secondary): Systemic vascular absorption → supraspinal centres → descending inhibitory pathways

Classification Based on Lipophilicity (2 marks)

Key point: Lipophilic opioids like fentanyl are so rapidly absorbed into the bloodstream that when given as a continuous epidural infusion, their principal mechanism of analgesia is systemic (non-segmental), not spinal. Bolus epidural fentanyl, however, does show segmental effects. (Miller's Anesthesia, 10e)

Individual Drugs and Dosing (2 marks)

• Epidural bolus: 1-5 mg (optimal 2.5-3.75 mg)
• Continuous infusion: 0.1-0.4 mg/h (concentration: 0.1 mg/mL)
• Onset: 30-60 min | Duration: 12-24 hours
• Risk: Delayed respiratory depression due to rostral spread
• DepoDur (liposomal morphine): Single-shot epidural; 10-15 mg; lasts 48 hours - avoids need for catheter

• Epidural bolus: 50-100 mcg | Infusion: 25-100 mcg/h
• Infusion concentration: 2-5 mcg/mL with local anaesthetic
• Onset: 5-15 min | Duration: 2-4 hours (requires continuous infusion)
• Primarily systemic mechanism when infused continuously

• Infusion concentration: 0.2-0.5 mcg/mL
• Drug of choice in opioid-tolerant patients (>250 mg/day oral morphine) due to highest intrinsic activity (Barash, 9e)

• Concentration: 0.02 mg/mL; combines advantages of hydrophilic opioid with good CSF penetration
• Ideal when morphine + bupivacaine combination is required

• Only epidural opioid with intrinsic local anaesthetic properties
• Can produce motor block and sympathectomy unlike all other neuraxial opioids

Synergy with Local Anaesthetics (1 mark)

• Combination of epidural opioid + local anaesthetic is synergistic (not just additive)
• Allows dose reduction of each drug, minimising side effects of both
• Local anaesthetic blocks afferent input and stress response; opioid acts on dorsal horn receptors
• Opioids do not prolong motor block when added to local anaesthetic
• Most common combination: Bupivacaine + fentanyl
• There is a therapeutic ceiling effect - beyond a certain dose, only side effects increase without additional analgesia (Miller's, 10e)

Side Effects and Management (2.5 marks)

• High dose, increasing age, concurrent systemic opioids or sedatives
• Hydrophilic opioids (morphine) - delayed onset up to 12 hours post-injection
• Obstructive sleep apnoea, major surgery
• Principle: Never administer epidural opioid + IV PCA opioid simultaneously (additive respiratory depression) (Barash, 9e)

Catheter Placement - Congruence Principle (0.5 marks)

• Congruent placement: Either lipophilic or hydrophilic opioid can be used
• Incongruent placement (e.g., lumbar catheter for thoracic surgery): Hydrophilic opioids (morphine, hydromorphone) are mandatory - they spread rostrally in CSF to reach the surgical level; lipophilic opioids cannot do this (Barash Table 55-21; Miller's, 10e)

Adjuvants (0.5 marks)

Summary Points (for marks pickup)

1. Morphine = gold standard; 1-5 mg epidural; 12-24 hr duration; risk of delayed respiratory depression
2. Fentanyl = fast onset, short duration; primarily systemic mechanism during infusion
3. Sufentanil = choice for opioid-tolerant patients
4. Pruritus = most common side effect; not histamine-mediated; treat with naloxone/nalbuphine
5. Delayed respiratory depression (>6 hrs) = hallmark of hydrophilic opioids (morphine); due to rostral CSF spread
6. Epidural opioids = no motor block, no sympatholytic effect (advantage over LA)
7. LA + opioid combination = synergistic; reduces doses and side effects of each

Anaesthetic Management of Post-Burn Contracture Release and Split Skin Grafting (SSG)

Model Answer - 10 Mark MD Anaesthesia Exam Question

Introduction (0.5 marks)

• Healed burns (chronic phase) - not acute burn physiology
• Contractures affecting airway, joints, and vascular access
• Scarred, difficult anatomy throughout
• Altered pharmacology persisting from the burn injury

Preoperative Assessment (2 marks)

1. Airway Evaluation (most critical)

2. Respiratory Assessment

• Restrictive pattern common with chest wall/axillary contractures
• Spirometry/PFTs if significant chest involvement
• Baseline SpO₂; history of inhalation injury may leave residual pulmonary damage

3. Cardiovascular Assessment

• Chronic anaemia is common (nutritional deficiency, repeated procedures)
• Poor nutritional status → reduced plasma proteins → altered drug binding
• Check Hb, electrolytes, serum albumin, coagulation profile

4. Vascular Access Assessment

• Scarred veins make IV access extremely difficult
• Plan for central venous access if peripheral access unavailable
• Identify unscarred areas: feet, neck veins, subclavian, femoral
• Intraosseous access as last resort, especially in children

5. Site and Extent of Surgery

• Location of contracture (neck > axilla > elbow > hand > lower limb > perineum)
• Anticipated blood loss (can be significant especially with multiple SSG donor sites)
• Duration of surgery (heat/fluid losses proportional to exposed raw area)
• Patient positioning required

6. Previous Anaesthetic Records

• Prior intubation details, any difficulties
• Previous drug reactions

7. Special Investigations

• CBC (Hb, platelets), BMP, LFTs (albumin), coagulation, cross-match and reserve blood
• ECG if prolonged procedure planned or cardiovascular disease
• X-ray cervical spine if neck contracture

Preoperative Preparation (0.5 marks)

• Informed consent including difficult airway plan and possible awake intubation
• Premedication: Anxiolytic (midazolam 0.5-1 mg IV cautiously) if airway not severely compromised. In severe neck contracture - NO heavy sedation before securing airway
• Antacid prophylaxis: Ranitidine + metoclopramide (aspiration risk if difficult airway)
• Blood cross-match and availability of packed RBCs
• Warming preparation: warm OT (>26°C), warm IV fluids, forced-air warming blanket
• Difficult airway cart at bedside: videolaryngoscope, FOB, LMAs (various sizes), surgical airway kit

Airway Management - The Core Challenge (2 marks)

Decision Framework:

1. Topical airway anaesthesia: nebulised lignocaine 4%, transtracheal injection of lidocaine 2-4 mL 2%, superior laryngeal nerve blocks
2. Sedation if needed: dexmedetomidine infusion (0.5-1 mcg/kg/h) or small-dose ketamine (0.5 mg/kg) - maintains airway tone and spontaneous respiration
3. Perform nasal or oral fibreoptic bronchoscopy-guided intubation
4. Confirm ETT position by capnography and bronchoscopy before inducting GA

• Standard RSI or modified RSI may be appropriate
• Videolaryngoscopy (McGrath, C-MAC) as first choice over direct laryngoscopy given scar distortion
• Always have surgical airway (cricothyrotomy) kit immediately available

• Neck contracture release itself improves airway anatomy after the first surgical step
• Reinforced (armoured/spiral) ETT preferred - resists kinking in prone or twisted positions
• Nasal intubation preferred for chin/perioral contracture release (keeps oral field clear)

Induction of Anaesthesia (1 mark)

• Propofol (1.5-2 mg/kg) + fentanyl (1-2 mcg/kg) - standard if haemodynamically stable
• Ketamine (1-2 mg/kg IV) preferred if:
  • Haemodynamic instability/anaemia
  • Difficult IV access (can give IM 4-6 mg/kg)
  • Pediatric patients (commonest group)
  • Maintains airway reflexes, bronchodilator

SUCCINYLCHOLINE IS CONTRAINDICATED from 5 days post-burn until 1-2 years after healing (or until full re-epithelialisation)

• Use rocuronium (0.6-1.2 mg/kg) for intubation - if CICV, reversal with sugammadex (16 mg/kg) is life-saving
• Post-burn patients also show resistance to non-depolarising muscle relaxants (vecuronium, rocuronium, atracurium) - require 2-3x normal doses, likely due to receptor upregulation
• Use neuromuscular monitoring (TOF) to guide dosing

Maintenance of Anaesthesia (1 mark)

• Volatile anaesthetic (isoflurane/sevoflurane in O₂:air) + opioid infusion (fentanyl/morphine)
• Or TIVA with propofol + remifentanil infusion - especially for prone/unusual positions, or where inhalational delivery is difficult
• Ketamine infusion (0.2-0.5 mg/kg/h) excellent for burn patients - provides analgesia, maintains haemodynamics, bronchodilation

• Post-burn patients often have opioid tolerance from repeated procedures and chronic pain
• Require higher than expected opioid doses for adequate analgesia
• Multimodal analgesia: paracetamol + NSAIDs (if not contraindicated) + ketamine + regional blocks

• Axillary contracture: infraclavicular/axillary brachial plexus block
• Elbow/forearm contracture: forearm blocks
• Lower limb: femoral nerve block, sciatic block, spinal/epidural (if anatomy permits)
• Regional analgesia reduces opioid requirement and provides excellent postop pain control
• Note: Neuraxial blocks may be technically difficult due to back/lumbar scars

Intraoperative Monitoring and Specific Concerns (1 mark)

Temperature Management (Critical)

• Burns patients have severely impaired thermoregulation - lose heat rapidly through raw wound surfaces
• SSG donor sites add to heat/fluid loss proportional to area
• OT temperature maintained >26-28°C (especially for children)
• Warm IV fluids (fluid warmer)
• Forced-air warming blankets on non-operative surfaces
• Humidified anaesthetic gases
• Temperature monitoring: oesophageal/rectal probe mandatory

Blood Loss Management

• SSG harvest and contracture release can cause significant blood loss
• Subcutaneous adrenaline infiltration (1:200,000 to 1:400,000) into donor and recipient sites by surgeon to reduce blood loss - anaesthetist must be aware (tachycardia, hypertension)
• Tumescent technique reduces blood loss at donor site
• Maintain Hb >8-9 g/dL for adequate oxygen delivery to grafts
• Tourniquet use for extremity procedures reduces blood loss

Fluid Management

• Raw surfaces = fluid losses (similar to "third spacing")
• Replace: maintenance + losses from raw area (estimate 0.5-1 mL/kg/hr per %TBSA exposed)
• Monitor urine output (0.5-1 mL/kg/hr as minimum)
• Colloids/albumin if hypoalbuminaemia (common in chronic burns)

Positioning

• Often requires unusual positions (prone, lateral, arms abducted)
• Ensure all pressure points padded
• Eye protection critical (periorbital scarring may prevent lid closure)
• ETT secured extremely carefully (tape may not stick to scarred/grafted face - use ties)

Monitoring

• Standard ASA monitoring (ECG, SpO₂, NIBP, EtCO₂, temp)
• NIBP cuff placement may be impossible over contracture sites - use unscarred limb; arterial line if no suitable site
• SpO₂ probe on unscarred digit/earlobe

Postoperative Management (1 mark)

Extubation

• Extubate only when fully awake with return of adequate airway reflexes
• A patient who was difficult to intubate preoperatively may be EASIER after neck contracture release (improved extension) - but this is not guaranteed
• If any doubt about airway patency post-contracture release (oedema, residual restriction), keep intubated and plan early morning extubation in ICU
• Have re-intubation equipment at bedside during extubation

Pain Management

• Post-burn patients need aggressive multimodal analgesia
• Paracetamol 15 mg/kg 6-hourly (IV in immediate postop)
• NSAIDs (if no contraindication)
• Low-dose ketamine infusion (0.1-0.2 mg/kg/h) - reduces opioid requirements and opioid-induced hyperalgesia
• Opioid PCA (higher doses than standard due to tolerance)
• Regional blocks wherever feasible

SSG-Specific Concerns

• Immobilisation of grafted area - avoid pressure and shearing on graft (risk of graft displacement)
• Pressure dressings applied in OT - do not disturb
• Donor site dressings - also painful
• Nurse with grafted area elevated

Temperature and Fluids

• Continue active warming in recovery room
• Maintain urine output, monitor electrolytes

Paediatric Considerations (0.5 marks)

• Higher metabolic rate → greater heat and fluid loss per kg
• Ketamine is ideal induction/maintenance agent (IM if no IV)
• Succinylcholine contraindication is absolute
• Airway anatomy changes with growth after contracture release
• Parental separation anxiety - appropriate premedication (oral midazolam 0.5 mg/kg)
• Smaller blood volume → transfusion trigger reached sooner

Summary of Key Exam Points