Anaesthesia for Aortic Arch Surgery

Quick Answer

Aortic arch surgery requires deep hypothermic circulatory arrest (DHCA) with or without selective cerebral perfusion (SCP). Indications: Aortic aneurysm, acute dissection (Type A), atherosclerotic disease. Core principles: Brain protection (18-28°C nasopharyngeal temperature, SCP if arrest >30-40 minutes), myocardial protection, organ preservation. Monitoring: Arterial lines (radial + femoral, may be phase shift), TEE, EEG/NIRS (cerebral oximetry), jugular bulb catheter (rare). Neuroprotection: Temperature (hypothermia reduces metabolic demand 50% per 7°C), SCP (10-15 mL/kg/min to brachiocephalic vessels, antegrade or retrograde), pharmacological (thiopental, propofol, methylprednisolone, mannitol). Postoperative: Bleeding management, spinal cord monitoring ( delayed ischemia), stroke surveillance, rewarming complications. [1-15]

Pathophysiology

Aortic Arch Anatomy and Pathology

Anatomy:

• Ascending aorta: From aortic valve to brachiocephalic (innominate) artery origin
• Arch: Brachiocephalic, left common carotid, left subclavian arteries
• Descending aorta: From left subclavian to diaphragm
• Isthmus: Junction of arch and descending (ligamentum arteriosum remnant)

Pathologies Requiring Surgery:

1. Aortic Aneurysm:

• Degenerative: Atherosclerotic, cystic medial necrosis (Marfan, Ehlers-Danlos)
• Indications: >5.5 cm diameter, growth >0.5 cm/year, symptomatic, pseudoaneurysm
• Risk of rupture: Increases exponentially with diameter (>6 cm: 10-15%/year)

2. Acute Aortic Dissection (Type A):

• Mechanism: Intimal tear → blood enters media → false lumen
• DeBakey I: Involves ascending and descending
• DeBakey II: Ascending only
• Stanford A: Involves ascending (surgical emergency)
• Mortality: 1-2% per hour initially (without surgery)
• Complications: Malperfusion (coronaries, arch vessels, viscera, limbs), tamponade, rupture

3. Atherosclerotic Arch Disease:

• Penetrating aortic ulcer: Atherosclerotic plaque ulceration through intima
• Intramural hematoma: Hemorrhage into media without intimal tear
• Calcification: May require endarterectomy or replacement

Classification of Arch Extent:

• Zone 0: Ascending to innominate
• Zone 1: Innominate to left carotid
• Zone 2: Left carotid to left subclavian
• Zone 3: Distal to left subclavian (thoracic, not arch)

Deep Hypothermic Circulatory Arrest (DHCA)

Rationale:

• Metabolic reduction: Cerebral metabolic rate for oxygen (CMRO₂) decreases 6-7% per 1°C cooling
• Q10 effect: Metabolic rate halves for every 10°C decrease (Q10 ≈ 2.0-2.5)
• Safe duration: Depends on temperature
  • 18°C: ~30-40 minutes (acceptable neurological outcomes)
  • 25°C: ~20-30 minutes
  • 28°C: ~15-20 minutes
• Beyond safe duration: Increased stroke risk, cognitive dysfunction

Temperature Measurements:

• Nasopharyngeal: Correlates with brain temperature (target 18-28°C)
• Tympanic: Similar to nasopharyngeal
• Bladder: Core temperature (lags brain by 5-10°C during cooling)
• Oesophageal: Cardiac/visceral temperature
• Pulmonary artery: Mixed venous (slowest to cool)

Cooling Strategy:

1. Pre-cooling: Before cross-clamp (reduces rewarming during ischemia)
2. Active cooling: On bypass with heat exchanger
3. Cooling rate: 1-2°C every 3-5 minutes
4. Uniform cooling: Ensure no gradients (risk of neurological injury)

Rewarming:

• Gradual: 1°C every 3-5 minutes (max gradient 10°C between blood and patient)
• Target: 36-37°C (avoid hyperthermia - cerebral injury)
• Post-bypass: Continue active warming to prevent afterdrop

Selective Cerebral Perfusion (SCP)

Indication:

• Prolonged procedures (>30-40 minutes circulatory arrest anticipated)
• Older patients (reduced ischemic tolerance)
• Previous stroke/cerebrovascular disease
• Complex arch reconstruction (elephant trunk, fenestrated grafts)

Techniques:

1. Antegrade Cerebral Perfusion (ACP):

• Cannulation: Direct cannulation of brachiocephalic vessels
  • Innominate artery (single cannula, perfuses all three via collaterals)
  • Individual cannulation of all three vessels
  • Y-graft from arterial line
• Flow: 10-15 mL/kg/min (or 500-1000 mL/min total)
• Pressure: 50-70 mmHg in right radial artery (monitor)
• Temperature: 15-20°C (mild cooling maintains protection)
• Advantages: Physiological flow pattern, better brainstem perfusion
• Complications: Dissection, embolization, catheter malposition

2. Retrograde Cerebral Perfusion (RCP):

• Cannulation: Superior vena cava (SVC) via internal jugular or direct
• Flow: 200-500 mL/min (low pressure)
• Pressure: 20-25 mmHg in SVC (higher causes edema)
• Mechanism: Venous → arterial via capillary networks
• Limitations: Incomplete perfusion (cortical > deep), edema risk, controversial benefit
• Current status: Largely superseded by ACP, occasional use as adjunct

3. Regional Low-Flow Perfusion:

• Partial bypass: Low flow to whole body (50-60 mL/kg/min)
• Used in: Elephant trunk procedures where some flow maintained

Neuroprotection Strategies

Temperature-Based:

• Hypothermia: Primary protection method (DHCA)
• Target: 18-28°C depending on anticipated arrest time
• Cooling rate: Gradual, uniform
• Avoid: Rewarming too rapidly, hyperthermia

Pharmacological:

1. Barbiturates (Thiopental):

• Dose: 5-10 mg/kg before circulatory arrest
• Mechanism: CMR reduction, free radical scavenging, membrane stabilization
• Evidence: Controversial benefit in humans (animal models show protection)
• Side effects: Hypotension, prolonged awakening, myocardial depression
• Current use: Declining (replaced by propofol or omitted)

2. Propofol:

• Infusion: 100-200 μg/kg/min during cooling/arrest
• Mechanism: CMR reduction, antioxidant properties
• Advantages: Short-acting, titratable
• Limitation: Hypotension, propofol infusion syndrome (rare at these doses)

3. Corticosteroids (Methylprednisolone):

• Dose: 1 g IV (surgeon preference)
• Mechanism: Anti-inflammatory, membrane stabilization
• Evidence: Variable (may reduce SIRS)

4. Mannitol:

• Dose: 0.5-1 g/kg
• Mechanism: Osmotic diuresis, free radical scavenger, rheological improvement
• Timing: Before arrest

5. Lidocaine:

• Dose: 1-2 mg/kg
• Mechanism: Membrane stabilization, reduced excitotoxicity

6. Magnesium:

• Dose: 2-4 g IV
• Mechanism: NMDA receptor blockade, vasodilation
• Benefits: Neuroprotective, antiarrhythmic

Physiological:

• Glucose control: Avoid hyperglycemia (>10 mmol/L worsens ischemic injury)
• Hematocrit: Maintain 25-30% (balance between viscosity and oxygen carrying)
• pH management: Alpha-stat (temperature-uncorrected) preferred for brain protection

Spinal Cord Protection

Risk Factors for Paraplegia:

• Extent of aortic replacement (descending involvement)
• Cross-clamp time >30 minutes
• Previous abdominal aortic surgery (intercostal sacrifice)
• Hypotension (MAP <60-70 mmHg)
• Hypothermia (core <34°C without protection)

Protective Strategies:

1. Distal perfusion: Maintained during arch work if possible
2. Intercostal reimplantation: Key vessels (T8-L1) reimplanted to graft
3. Cerebrospinal fluid drainage: Lumbar drain (reduces CSF pressure, improves perfusion pressure gradient)
   • Target CSF pressure <10 mmHg
   • Volume drained: 10-20 mL/hour
4. Mild hypothermia: 32-34°C core (if no DHCA)
5. Pharmacological: Steroids, naloxone, magnesium (variable evidence)
6. Neurogenic monitoring: SSEP, MEP (rarely used in arch surgery)

Monitoring for Ischemia:

• Postoperative: Lower limb motor/sensory examination
• Delayed onset: Can occur 24-72 hours postoperatively (reperfusion, hypotension)
• Immediate intervention: Raise MAP (>90 mmHg), CSF drainage, re-exploration if graft-related

Clinical Presentation

Preoperative Assessment

History:

• Symptoms: Chest/back pain (dissection, expansion), dysphagia (compression), hoarseness (recurrent laryngeal), syncope (tamponade), limb ischemia (malperfusion)
• Aetiology: Hypertension, Marfan/Ehlers-Danlos (connective tissue), bicuspid aortic valve, previous cardiac surgery, aortitis (Takayasu, giant cell), trauma
• Comorbidities: CAD, CVD, renal dysfunction, COPD, diabetes
• Medications: Antihypertensives, anticoagulants, statins

Physical Examination:

• Cardiovascular: Hypertension (most common), pulse deficits (dissection), new neurological deficits (embolic), JVP elevation (tamponade), diastolic murmur (aortic regurgitation from root dilation/dissection)
• Respiratory: Baseline status
• Neurological: Baseline cognitive function, focal deficits
• Peripheral pulses: All four limbs (comparison, documentation)

Investigations:

Imaging:

• CT angiography: Gold standard for diagnosis, extent, branch involvement
• TEE: Aortic valve function, intimal tear location, cardiac tamponade, dissection flap mobility
• MRI: Alternative (slower, not for acute)
• Angiography: Preoperative planning (arch vessel origins, anomalies)

Cardiac:

• ECG: Ischemia (coronary involvement in dissection), LVH
• Echocardiography: LV function, aortic valve competence (regurgitation common in root dilation/dissection), pericardial effusion (tamponade)
• Coronary angiography: If stable and time permits (identifies CAD requiring CABG)

Laboratory:

• FBC, coagulation: Baseline, cross-match (6-10 units PRBC)
• Creatinine, electrolytes: Renal function (contrast risk, bypass-related injury)
• LFTs: Hepatic function
• ABG: Baseline oxygenation, lactate (malperfusion)
• Type and screen/cross: Massive transfusion likely
• Fibrinogen, TEG/ROTEM: If time permits (coagulation assessment)

Specific Risk Stratification:

• EuroSCORE II: Predicts mortality (arch surgery >10% typically)
• Stroke risk: 5-15% (higher with DHCA duration, age, previous CVD)
• Spinal cord ischemia: 2-5% (if descending aorta involved)
• Renal failure: 10-20% requiring dialysis

Emergency vs. Elective Surgery

Acute Type A Dissection (Emergency):

• Mortality: 50% at 48 hours without surgery
• Preparation: Minimal (life-saving, cannot delay for optimization)
• Priorities: Establish bypass quickly, prioritize cerebral protection
• Blood products: Have massive transfusion protocol active
• Anticoagulation: Heparin must be given (bypass) despite bleeding risk

Elective Arch Aneurysm:

• Optimization time: Days to weeks
• Goals: BP control, renal function optimization, nutritional status
• Second opinion: May seek other options (TEVAR - thoracic endovascular aortic repair)
• Decision: Open surgery vs. hybrid vs. endovascular (factors: age, comorbidities, anatomy)

Management

Anaesthetic Technique

Monitoring:

Standard Plus:

• Arterial lines: Bilateral (right radial + femoral or left radial)
  • Phase shift between sites (ascending vs. descending aorta pressure differences)
  • Right radial: Innominate artery (monitors cerebral perfusion pressure during SCP)
  • Femoral: True systemic pressure (bypass target)
• Central venous catheter: CVP, drug administration
• PA catheter: Optional (complex cases, poor LV function)
• TEE: Essential (LV function, aortic valve, de-airing, guide cannulation)
• EEG: Cerebral activity during cooling (target burst suppression at <20°C)
• Cerebral oximetry (NIRS): Regional saturation monitoring (frontal lobes)
• Jugular bulb catheter: Rare (jugular venous oxygen saturation, research use)
• Temperature: Multiple sites (nasopharyngeal, bladder, oesophageal, PA)
• Urinary catheter: Hourly output (renal perfusion)
• CSF drain: If spinal protection planned (lumbar catheter)

Induction:

• Strategy: Hemodynamic stability (hypertension risks aortic rupture/dissection progression, hypotension risks malperfusion)
• Pre-oxygenation: 100% O₂
• Agents: Etomidate 0.2-0.3 mg/kg (stable) or ketamine 1-2 mg/kg (sympathomimetic)
• Opioid: High-dose (fentanyl 10-20 μg/kg or remifentanil infusion)
• Muscle relaxant: Rocuronium 0.6-1 mg/kg (sugammadex available)
• Line insertion: After induction (unless emergency)

Maintenance:

• TIVA preferred: Propofol 100-200 μg/kg/min + remifentanil 0.1-0.3 μg/kg/min
  • Neuroprotective (propofol reduces CMR)
  • Early wake-up for neurological assessment
• Volatile alternative: Isoflurane or sevoflurane (1 MAC) + remifentanil
  • Preconditioning effects debated
• Neuromuscular blockade: Continuous infusion (atracurium/cisatracurium) or intermittent boluses

Cardiopulmonary Bypass Strategy

Cannulation:

• Arterial: Femoral (most common), axillary (right side, for ACP), direct aortic (ascending, if no dissection)
• Venous: Right atrial bicaval (standard), femoral (if redo/median sternotomy risk)
• Y-graft: For ACP (branches to arch vessels)
• Vent: LV via right superior pulmonary vein

Initiation:

• ACT: >480 seconds before arterial flow
• Flow: 2.0-2.4 L/min/m²
• MAP: 50-70 mmHg
• Temperature: Begin active cooling immediately

Cooling Phase:

• Target: 18-28°C nasopharyngeal
• Rate: Gradual (1°C per 3-5 minutes)
• Neuroprotection: Administer thiopental/propofol, methylprednisolone, mannitol
• Glucose: Monitor hourly, treat if >10 mmol/L (insulin)
• pH strategy: Alpha-stat (temperature-uncorrected, vasodilation improves cooling)

Circulatory Arrest:

• Sequence:
  1. Achieve target temperature
  2. Confirm EEG burst suppression (if used)
  3. Stop bypass pump
  4. Remove arterial cannula (or clamp)
  5. Position head down (Durant position - prevents air embolism)
  6. Begin surgical repair
• Duration monitoring: Track cumulative arrest time
• Neuroprotection: Pack head in ice (topical cooling)

Selective Cerebral Perfusion (if used):

• Initiation: At start of circulatory arrest
• Flow: 10-15 mL/kg/min via innominate or separate cannulae
• Monitoring: Right radial pressure 50-70 mmHg
• Temperature: 15-20°C (blood temperature)
• Blood gas: Intermittent sampling (maintain normocapnia)

Rewarming:

• Trigger: After arch repair complete, de-airing finished
• Restart bypass: Gradual resumption of flow
• Rewarming rate: Max 10°C gradient between blood and patient
  • Faster rewarming → gas embolism risk
  • Slower rewarming → coagulopathy, prolonged bypass
• Target: 36-37°C
• Continuation: Active warming post-bypass (afterdrop prevention)

Specific Surgical Considerations

Open Distal Anastomosis (Hemiarch/Total Arch):

• Technique: Aorta transected distal to arch, graft anastomosis under DHCA
• DHCA duration: 20-40 minutes typically
• Extension: Elephant trunk (graft left in descending for future TEVAR)

Frozen Elephant Trunk:

• Hybrid procedure: Open arch repair + endovascular stent into descending
• Advantage: Single-stage treatment of arch and proximal descending
• Anaesthetic: Same as open arch, may be shorter DHCA

Elephant Trunk Procedure:

• Two-stage: Open arch today, TEVAR later
• Advantage: Reduces second-stage open thoracotomy
• Positioning: Head down during DHCA (prevents air embolism to brain)

Aortic Dissection Repair:

• Principles: Resect intimal tear, obliterate false lumen, restore true lumen flow
• Challenges: Fragile tissue, ongoing malperfusion, valve resuspension/replacement
• DHCA: Often shorter than aneurysm (focal repair)

Concomitant Procedures:

• Aortic root replacement: Bentall procedure (valved conduit + coronary reimplantation)
• CABG: If coronary ostial involvement or pre-existing CAD
• Arch vessel debranching: Revascularization before TEVAR (hybrid)

Post-Bypass Management

Weaning:

• Criteria: Rewarmed, de-aired, rhythm stable, bleeding controlled
• LV function: May require inotropes (dobutamine, milrinone, adrenaline)
• RV function: Rarely affected (unless pulmonary artery involvement)
• Pacing: Atrial or AV sequential if bradycardia/heart block

Hemodynamics:

• Target MAP: 70-80 mmHg (adequate spinal cord perfusion)
• Avoid: Hypertension (bleeding), hypotension (spinal ischemia)
• Vasodilators: GTN, hydralazine, clevidipine (for BP control)
• Vasopressors: Noradrenaline, vasopressin (if vasoplegic)

Bleeding Management:

• Common: Coagulopathy from hypothermia, hemodilution, bypass
• Strategy:
  1. Surgical hemostasis (suture lines, anastomoses)
  2. Protamine (1 mg per 100 IU heparin)
  3. Tranexamic acid (if not given, or additional dose)
  4. Fibrinogen concentrate/cryoprecipitate (target >2 g/L)
  5. FFP (if INR >1.5)
  6. Platelets (if <100 × 10⁹/L)
  7. Recombinant Factor VIIa (last resort)
• Re-exploration: If >300-500 mL/hour or tamponade

Reperfusion Issues:

• Systemic inflammatory response: From bypass, ischemia-reperfusion
• Treatment: Supportive, steroids (if severe), vasopressors
• Myocardial stunning: From cardioplegia, cross-clamp (inotropes)

Postoperative Care

Immediate (ICU):

• Ventilation: Protective strategy (6-8 mL/kg, PEEP 5-8 cm H₂O)
• Neurological assessment: Hourly (GCS, pupils, movement)
  • Delayed awakening: CT head (stroke, hypoxic injury)
  • Focal deficit: CT head, possible intervention
• Spinal cord monitoring: Lower limb motor/sensory (4-6 hourly)
  • Deterioration: Raise MAP >90 mmHg, CT angiography (graft compromise)
• Renal function: Hourly urine output, creatinine daily
• Temperature: Prevent hypothermia (rewarming blankets)

Delayed Complications:

• Stroke: 5-15% (embolic, hypoperfusion, air)
  • Risk factors: DHCA duration >40 minutes, age >70, previous stroke
• Spinal cord ischemia: 2-5% (if descending involved)
  • Early detection and MAP augmentation critical
• Renal failure: 10-20% (bypass, hypoperfusion, contrast)
• Respiratory failure: Prolonged ventilation common
• Coagulopathy: DIC, massive transfusion related

Extubation:

• Delayed typically: 24-48 hours (hemodynamic stability, bleeding controlled)
• Criteria: Awake, following commands, minimal inotropes, acceptable ABG
• Failed extubation: Re-intubation, tracheostomy if prolonged

Indigenous Health Considerations

Aboriginal and Torres Strait Islander Patients

Health Disparities:

• Cardiovascular disease: Higher rates of hypertension, IHD, affecting aortic pathology
• Rheumatic heart disease: Valvular involvement may coexist with aortic disease
• Diabetes: Higher prevalence, affects wound healing, infection risk
• Renal disease: Higher rates of chronic kidney disease (contrast nephropathy risk)

Access Barriers:

• Geographic: Remote communities require transfer to major centres (Sydney, Melbourne, Brisbane)
• Delayed presentation: May present with complications (rupture, dissection) rather than elective repair
• Emergency nature: Less opportunity for preoperative optimization

Cultural Considerations:

• Family involvement: Extended family may need to travel for support
• Communication: Complex surgery requires clear explanation in accessible language
• Informed consent: Ensure understanding of high-risk nature, complications (stroke, paraplegia)
• Aboriginal Liaison Officers: Essential for cultural support, communication
• Discharge planning: Remote follow-up challenging (telemedicine, local primary care liaison)

Postoperative Challenges:

• Rehabilitation: Cardiac rehabilitation services may not exist in remote areas
• Anticoagulation monitoring: Warfarin management difficult without local pathology
• Secondary prevention: Lifestyle modification support needed

Māori Health Considerations

Health Inequities:

• Cardiovascular mortality: Disproportionately high
• Aortic disease: Presentation patterns similar to Aboriginal populations
• Comorbidities: Higher rates of diabetes, renal disease

Cultural Safety:

• Whānau involvement: Family conferences for treatment decisions
• Karakia and tikanga: Respect for cultural practices if requested
• Communication: Risk explanation in culturally appropriate manner
• Māori Health Workers: Liaison support
• Postoperative care: Coordination with primary care for ongoing management

Geographic Considerations:

• New Zealand context: Main centres (Auckland, Wellington, Christchurch) for aortic surgery
• Inter-island transfer: May be required from South Island to Auckland for complex cases
• Rural Māori communities: Similar access challenges to remote Australia

Shared Decision-Making:

• Alternative treatments: Discuss TEVAR, hybrid options (may be preferred for high-risk patients)
• Palliative approach: For elderly/frail patients, conservative management may be culturally appropriate choice
• End-of-life: Advance care planning if high operative risk

ANZCA Final Exam Focus

SAQ Patterns

Common Questions:

• "Describe the principles of cerebral protection during aortic arch surgery."
• "What are the indications for selective cerebral perfusion during DHCA?"
• "How would you manage a patient who has been under DHCA for 45 minutes?"
• "Discuss the complications of deep hypothermic circulatory arrest."
• "Compare antegrade vs retrograde cerebral perfusion."

Marking Scheme Priorities:

• Temperature targets (18-28°C nasopharyngeal) and rationale
• SCP techniques (ACP flow rates, pressures, cannulation)
• Neuropharmacology (barbiturates, propofol, mannitol)
• Monitoring (bilateral arterial lines, EEG/NIRS, temperature)
• Safe DHCA duration by temperature
• Postoperative complications (stroke, spinal cord ischemia, bleeding)

Viva Scenarios

Scenario 1: Prolonged DHCA

• Patient on DHCA for 50 minutes (unexpected surgical difficulty)
• Discuss options: Continue vs. reperfusion vs. SCP conversion
• Emphasis: Cumulative ischemic time, neurological risk

Scenario 2: Postoperative Stroke

• New right hemiparesis in ICU
• Differential: Embolic vs. hypoperfusion vs. air
• Management: CT head, BP optimization, neurosurgical consultation

Scenario 3: Spinal Cord Ischemia

• New paraplegia 12 hours post-surgery
• Urgent management: MAP augmentation, CSF drainage if catheter in situ
• Imaging: CT angiography to exclude graft malperfusion

Scenario 4: Temperature Management

• Cooling phase, temperature gradient between sites
• Alpha-stat vs. pH-stat discussion
• Uniform cooling importance

Key Points for Examination Success

1. Temperature: 18-28°C nasopharyngeal for DHCA; Q10 effect (metabolism halves per 10°C)
2. Safe DHCA duration: 18°C = 30-40 min, 25°C = 20-30 min (cumulative risk beyond)
3. SCP flow: ACP 10-15 mL/kg/min, right radial pressure 50-70 mmHg
4. Neuroprotection: Propofol/thiopental, methylprednisolone, mannitol, avoid hyperglycemia
5. Monitoring: Bilateral arterial lines (phase shift), EEG burst suppression at <20°C
6. Spinal protection: CSF drainage (if descending involved), MAP >70-80 mmHg postoperatively
7. Rewarming: Max 10°C gradient, target 36-37°C, prevent afterdrop
8. Complications: Stroke 5-15%, spinal ischemia 2-5%, renal failure 10-20%, bleeding common
9. pH management: Alpha-stat preferred for neuroprotection
10. Positioning: Head down (Durant) during arrest to prevent air embolism

Assessment Content

SAQ 1: DHCA and Neuroprotection (20 marks)

Question: A 62-year-old man is undergoing elective total aortic arch replacement for a 6.2 cm degenerative aneurysm. The surgeon anticipates 35-40 minutes of circulatory arrest time.

a) Outline the principles of cerebral protection during this procedure. (8 marks) b) What are the advantages and disadvantages of antegrade cerebral perfusion compared to simple DHCA? (6 marks) c) Describe the anaesthetic neuropharmacological adjuncts used during DHCA and their rationale. (6 marks)

Model Answer:

a) Principles of cerebral protection (8 marks):

• Temperature reduction: Deep hypothermia 18-28°C nasopharyngeal (2 marks)
  • Q10 effect: CMRO₂ decreases 6-7% per 1°C, halves per 10°C cooling
  • Safe duration at 18°C: 30-40 minutes; at 25°C: 20-30 minutes
• Selective cerebral perfusion (SCP): Antegrade preferred for >30 min arrest (2 marks)
  • Flow 10-15 mL/kg/min to brachiocephalic vessels
  • Right radial pressure monitoring (target 50-70 mmHg)
• Uniform cooling: Gradual (1°C per 3-5 min), avoid gradients (2 marks)
  • Multiple temperature sites (nasopharyngeal, bladder, oesophageal)
  • Alpha-stat pH management (vasodilation improves cooling)
• Pharmacological: Barbiturate/propofol burst suppression, steroids, mannitol (1 mark)
• Glucose control: Avoid hyperglycemia >10 mmol/L (worsens ischemic injury) (1 mark)

b) Antegrade cerebral perfusion (ACP) advantages/disadvantages (6 marks):

Advantages:

• Extends safe arrest time beyond 30-40 minutes (1 mark)
• Physiological anterograde flow pattern (brainstem perfusion) (1 mark)
• Better neurological outcomes in prolonged procedures (1 mark)
• Allows complex reconstructions (elephant trunk, fenestrated grafts)

Disadvantages:

• Additional cannulation complexity (surgeon skill required) (1 mark)
• Risk of arterial dissection, embolization (1 mark)
• Potential malperfusion if catheter malposition (1 mark)

c) Neuropharmacological adjuncts (6 marks):

• Thiopental/propofol: 5-10 mg/kg thiopental or propofol infusion (1.5 marks)
  • CMR reduction, free radical scavenging
  • Target EEG burst suppression at <20°C
• Methylprednisolone: 1 g IV (1 mark)
  • Anti-inflammatory, membrane stabilization
  • May reduce systemic inflammatory response
• Mannitol: 0.5-1 g/kg (1.5 marks)
  • Osmotic diuresis, free radical scavenger, rheological improvement
• Lidocaine: 1-2 mg/kg (1 mark)
  • Membrane stabilization, reduces excitotoxicity
• Magnesium: 2-4 g IV (1 mark)
  • NMDA receptor blockade, neuroprotective

SAQ 2: Postoperative Spinal Cord Ischemia (20 marks)

Question: A 58-year-old woman undergoes total arch replacement with elephant trunk procedure. On postoperative day 2, she develops new onset paraplegia (loss of lower limb motor function and sensation).

a) What is the differential diagnosis for this presentation? (4 marks) b) Outline the immediate management priorities. (8 marks) c) What strategies could have been employed intraoperatively to prevent this complication? (8 marks)

Model Answer:

a) Differential diagnosis (4 marks):

• Spinal cord ischemia from intercostal artery sacrifice (2 marks)
• Graft malperfusion (kinking, thrombosis) compromising spinal supply (1 mark)
• Epidural hematoma (if lumbar drain inserted) (1 mark)
• Consider conversion disorder (rare, but functional)

b) Immediate management (8 marks):

• Urgent MAP augmentation: Increase to >90 mmHg (optimally 100-110 mmHg) (2 marks)
  • Vasopressors (noradrenaline, phenylephrine)
  • Avoid hypotension at all costs
• CSF drainage: If lumbar catheter in situ, drain to <10 mmHg (1.5 marks)
  • Improves spinal perfusion pressure gradient
  • Volume: 10-20 mL/hour
• Imaging: Emergency CT angiography of aorta (1.5 marks)
  • Exclude graft malperfusion, kinking, thrombosis
  • May need re-exploration if graft-related
• Neurological consultation: Document deficits, prognosis discussion (1 mark)
• Oxygenation: Ensure SpO₂ >95%, avoid anemia (Hb >100 g/L) (1 mark)
• Temperature: Maintain normothermia (avoid hypothermia) (1 mark)

c) Preventive strategies (8 marks):

• CSF drainage: Prophylactic lumbar drain (2 marks)
  • Target pressure <10 mmHg, drain 10-20 mL/hour
  • Reduces CSF pressure, improves perfusion gradient
• Distal perfusion: Maintain lower body perfusion during arch work if possible (2 marks)
  • Partial bypass or selective visceral perfusion
• Intercostal reimplantation: Reimplant key intercostals (T8-L1) (1.5 marks)
  • Identified by neurophysiological monitoring (MEP/SSEP)
• Mild hypothermia: 32-34°C core if no DHCA (1 mark)
• Pharmacological: Methylprednisolone, naloxone, magnesium (variable evidence) (1 mark)
• Blood pressure management: Postoperative MAP >70-80 mmHg (0.5 marks)

Viva Scenario: DHCA Management

Examiner: "You are anaesthetising a 55-year-old man for hemiarch replacement. The nasopharyngeal temperature is 20°C, and the surgeon is ready to commence circulatory arrest."

Candidate: "I would confirm we've achieved adequate cooling. At 20°C nasopharyngeal, we have approximately 20-30 minutes of safe arrest time based on the Q10 metabolic reduction. I would check that EEG shows burst suppression, confirming cerebral metabolic suppression. I would ensure neuroprotective medications have been administered - propofol or thiopental for burst suppression, methylprednisolone, and mannitol."

Examiner: "How do you manage the arrest phase?"

Candidate: "I would position the patient head down to prevent air embolism. I would stop the bypass pump, ensuring the ACT remains adequate. The head should be packed in ice for topical cooling. I would track the cumulative arrest time closely and communicate this to the surgeon every 5 minutes. If we approach 30 minutes, I would alert the surgeon to the increasing neurological risk."

Examiner: "What if the repair takes longer than anticipated and you're at 40 minutes?"

Candidate: "At 40 minutes with simple DHCA, we're beyond the generally accepted safe duration. I would discuss with the surgeon the possibility of brief reperfusion - restoring circulation for 5-10 minutes to reoxygenate the brain - then completing the repair. Alternatively, if ACP was not initiated but available, we could consider converting to selective perfusion. The risk of stroke increases significantly beyond 40 minutes."

Examiner: "How do you manage rewarming?"

Candidate: "Rewarming must be gradual - maximum 10°C gradient between the blood temperature and patient temperature. Faster rewarming risks gas embolism from changes in gas solubility. We target 36-37°C, avoiding hyperthermia which can worsen neurological injury. We continue active warming post-bypass to prevent afterdrop - the continued cooling as cold peripheral blood returns to the core."

References

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