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Anatomy of Cerebral Circulation — MD Anesthesia Exam

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1. Overview

• Internal carotid arteries (ICA) → anterior circulation
• Vertebral arteries → posterior circulation (unite to form the basilar artery)

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2. Anterior Circulation — Internal Carotid Artery

Segments of the ICA (mnemonic: C P C S — Cervical, Petrous, Cavernous, Supraclinoid)

Branches of supraclinoid ICA — mnemonic OPAAM

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3. Posterior Circulation — Vertebrobasilar System

• Vertebral arteries arise from subclavian arteries → ascend through foramina transversaria of C1–C6 → enter foramen magnum → unite to form basilar artery

Key branches of vertebral arteries:

• PICA (posterior inferior cerebellar artery) — lateral medulla + inferior cerebellum (lesion = Wallenberg syndrome)
• Anterior spinal artery

Key branches of basilar artery:

• AICA (anterior inferior cerebellar artery) — CN VII/VIII territory
• SCA (superior cerebellar artery) — superior cerebellum
• PCA (posterior cerebral artery) — terminal branches → occipital and inferomedial temporal lobes

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4. Circle of Willis

Components (anterior → posterior):

ACA ←—— AComm ——→ ACA
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ICA               ICA
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PComm             PComm
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PCA ←—— Basilar ——→ PCA

Key exam point: A complete circle is present in only ~34% of individuals. Incomplete variants are common and have implications for collateral flow during carotid endarterectomy and cross-clamping.

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5. Vascular Territories of the Three Main Cerebral Arteries

ACA branches:

• Pericallosal artery, callosomarginal artery
• Recurrent artery of Heubner (off A1) → head of caudate, anterior putamen, anterior internal capsule

MCA segments:

• M1 (sphenoid) → gives off lenticulostriate arteries
• M2 (insular) → superior and inferior divisions within Sylvian fissure
• Superior division → frontal/parietal cortex above Sylvian fissure
• Inferior division → temporal lobe below Sylvian fissure

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6. Deep Perforating Arteries — High-Yield for Anesthesia

Anesthesia relevance: Lacunar strokes of the posterior limb of the internal capsule (lenticulostriate or anterior choroidal territory) produce pure motor hemiplegia — a key postoperative neurological complication.

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7. Venous Drainage

Three systems:

Anesthesia relevance: Air embolism via open dural sinuses (e.g., posterior fossa surgery in sitting position) is a life-threatening complication. The cavernous sinus surrounds the intracavernous ICA — relevant to carotid-cavernous fistulas and pituitary surgery.

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8. Cerebral Blood Flow — Quantitative Data

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9. Exam Mnemonics Summary

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10. Anesthesia-Specific High-Yield Points

1. Carotid endarterectomy: Patchy or incomplete circle of Willis means cross-clamping the ICA may not allow adequate collateral — monitor with TCD or EEG; use shunt if needed.
2. Sitting/beach-chair position: Open dural sinuses → venous air embolism risk.
3. Posterior fossa surgery: PICA/AICA/SCA territory; cranial nerve monitoring essential.
4. Pituitary surgery (transsphenoidal): Cavernous sinus with ICA immediately lateral; risk of ICA injury.
5. Cerebral perfusion pressure = MAP − ICP; normal ICP < 15 mmHg.
6. Autoregulation: Maintained between MAP 50–150 mmHg; abolished by hypoxia, hypercarbia, volatile agents.

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What's Inside (11 Slides)

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Anaesthesia Management of Cleft Lip and Palate

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1. Overview and Surgical Timing

Staged Surgical Repair

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2. Preoperative Assessment

A. Airway Evaluation (Most Critical Step)

• Retrognathia (common in Pierre Robin, Treacher Collins) → anticipated difficult airway
• Large or bilateral cleft: tongue may prolapse into cleft, obstructing the airway
• Laryngoscope blade may lodge in the cleft during direct laryngoscopy
• Assess mouth opening, neck mobility, Mallampati score (as applicable for age)
• Review any prior intubation records
• Syndromic associations must be identified — the syndrome often defines the airway difficulty more than the cleft itself

B. Systemic Assessment

C. URTI Policy

• Active URTI with fever, purulent secretions, wheeze → postpone 4–6 weeks
• Mild clear rhinorrhea alone: anaesthetist's discretion (often acceptable to proceed)
• Post-URTI airway hyperreactivity persists for up to 6 weeks

D. Premedication

• Oral midazolam 0.5 mg/kg (max 15 mg) — 20–30 minutes before induction; reduces separation anxiety
• Atropine 20 mcg/kg IV at induction (some centres give IM pre-op) — reduces secretions, prevents bradycardia during intubation
• Routine fasting: 6h solid food, 4h breast milk, 2h clear fluids (234 rule)

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3. Induction of Anaesthesia

Standard Approach (Uncomplicated Airway)

• Inhalational induction with sevoflurane in O₂/air — preferred in infants; avoids IV needle in awake child
• Establish IV access once adequately deep
• Muscle relaxant only after confirming facemask ventilation is possible
• Options: Suxamethonium 2 mg/kg or atracurium/rocuronium (non-depolarising)

Anticipated Difficult Airway (Retrognathia, Syndromic)

Never administer a muscle relaxant until you have confirmed the ability to ventilate by mask.

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4. Airway Device — The RAE Tube

Why RAE?

• Preformed bend exits over the lower lip and chin, pointing downward
• Taped centrally in the midline
• Keeps the tube out of the surgical field while allowing the surgeon full access to the lips and palate
• Fixed intra-oral length is both advantage (airway security) and disadvantage (risk of endobronchial intubation)

RAE Tube Practical Points

Throat Pack

• A moist throat pack may be placed to absorb blood and debris from the palate
• Must be documented and counted — retained throat pack is a serious never event
• "Throat pack in — throat pack out" checklist is mandatory

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5. Maintenance of Anaesthesia

Patient Positioning

• Supine, shoulder roll to extend neck → improves surgical access
• Head ring to stabilise position
• Recheck tube position after every positional change — neck extension can advance the tube tip

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6. Analgesia (Multimodal)

Cleft Lip Repair

Cleft Palate Repair

• Paracetamol + NSAIDs (regular)
• Greater palatine nerve block (by surgeon intraoperatively)
• Opioids used cautiously — palate repair patients at higher risk of postoperative airway obstruction
• Dexamethasone 0.15 mg/kg IV — reduces swelling and has antiemetic effect

Infraorbital nerve block is the most effective regional technique for cleft lip and is strongly recommended. — Miller's Anesthesia 10e, p. 10644

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7. Extubation

Timing: Awake Extubation is Standard for Palate Repair

• Miller's Anesthesia explicitly states: "The child should be extubated awake" after palate repair
• Full reversal of neuromuscular block (check TOF ratio ≥0.9)
• Suction oropharynx under direct visualisation — careful not to disrupt suture lines
• Never blindly suction after palatoplasty
• Remove throat pack (confirm count)
• Extubate in left lateral or semi-prone position to allow drainage of secretions
• Have airway adjuncts ready: nasopharyngeal airway, facemask, LMA, reintubation equipment

Deep vs Awake Extubation

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8. Postoperative Care

Immediate (Recovery Room)

• Lateral/semi-prone position until fully awake
• Supplemental oxygen until stable SpO₂
• Monitor closely for 12–24 hours after palate repair (risk period for obstruction and bleeding)
• Nasopharyngeal airway may be left in situ for up to 6 hours if obstruction occurs post-palatoplasty (does NOT damage the repair)

Complications to Watch For

Admission Policy

• Cleft lip: overnight stay, usually discharge next day
• Cleft palate: 1–3 days admission (higher risk of bleeding and obstruction)
• Syndromic patients with comorbidities → PICU level care

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9. Associated Syndromes — Anaesthetic Relevance

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10. High-Yield Exam Summary