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Checking the Anaesthesia Machine: Miller's Anesthesia (10th Edition)

Overview

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The 15 ASA Items for Pre-Anaesthesia Checkout

Item 1: Verify Auxiliary Oxygen Cylinder and Self-Inflating Manual Ventilation Device Are Available and Functioning

• A self-inflating manual ventilation device (e.g., Ambu bag) in every anaesthetic location
• A separate oxygen cylinder independent of the wall supply and machine

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Item 2: Verify Patient Suction Is Adequate to Clear the Airway

• Adequate negative pressure (typically ≥ −80 mmHg)
• Suction tubing and Yankauer catheter properly connected
• No obstruction in the system

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Item 3: Turn On Anaesthesia Delivery System and Confirm That AC Power Is Available

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Item 4: Verify Availability of Required Monitors and Check Alarms

• Pulse oximeter
• ECG
• Non-invasive blood pressure
• Capnograph (EtCO₂)
• Temperature monitor
• Gas analyser / agent monitor

• Confirm cables are plugged in and monitors are powered on
• Check and reset alarm thresholds — alarm settings may drift between cases due to provider manipulation
• Departmental default alarm settings can be programmed into the workstation and confirmed by technicians, but the provider bears final responsibility for alarm correctness for their specific patient

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Item 5: Verify That Pressure Is Adequate on the Spare Oxygen Cylinder Mounted on the Anaesthesia Machine

• Open the E-cylinder valve(s) on the back of the machine
• Read the cylinder gauge pressure
• Replace if pressure is inadequate
• Air and N₂O cylinders need checking only if those gases are required

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Item 6: Verify That Piped Gas Pressures Are ≥ 50 psig

• Confirm pipeline pressure gauges show ≥ 50 psig for oxygen (and other gases in use)
• Inspect supply hoses and connections
• Confirm oxygen content in the inspiratory limb is > 90%
• The oxygen supply failure alarm should be tested by disconnecting the wall oxygen supply and shutting off the tank — this is the only way to confirm the alarm functions correctly (though this manoeuvre is not mandated by the ASA Recommendations)

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Item 7: Verify That Vaporizers Are Adequately Filled and Filler Ports Are Tightly Closed

• Check agent level in the sight glass before every case using an inhaled anaesthetic
• Refill if needed (using the keyed filler system)
• Confirm the filler port is tightly closed after filling — an open or improperly sealed port is a source of vapour leak and occupational exposure
• Not all automated self-tests include a low-agent alarm — manual verification prevents intraoperative light anaesthesia and awareness

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Item 8: Verify That No Leaks Are Present in the Gas Supply Lines Between the Flowmeters and the Common Gas Outlet

• The check valve prevents breathing circuit pressure from being transmitted retrograde into the low-pressure section
• A negative-pressure leak test uses a bulb syringe:
  1. Turn all vaporisers to the "on" position and set flowmeters to zero
  2. Disconnect the common gas outlet from the breathing circuit
  3. Attach a suction bulb syringe to the common gas outlet
  4. Squeeze the bulb to create negative pressure
  5. A competent (leak-free) low-pressure section will hold the collapsed bulb in a deflated state; a leak will cause the bulb to re-inflate
  6. Repeat with each vaporiser open

• Close the APL valve, occlude the Y-piece
• Pressurise the circuit to ~30 cmH₂O using the O₂ flush
• Monitor for pressure decay
• A significant leak indicates a problem in either the breathing circuit or the low-pressure section

⚠️ Machines with an outlet check valve cannot use the positive-pressure method to detect low-pressure leaks. The check valve blocks retrograde flow.

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Item 9: Test Scavenging System Function

• Confirm the scavenging interface device is properly connected to the APL valve and ventilator relief valve
• Confirm adequate negative pressure (for active systems) or open reservoir bag (for passive systems)
• Assess flow within the scavenging circuit
• The scavenging system must be functional to protect both the patient and operating room staff from chronic waste gas exposure

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Item 10: Calibrate, or Verify Calibration of, the Oxygen Monitor and Check the Low-Oxygen Alarm

• The oxygen monitor sensor must be calibrated daily
• Calibration typically involves exposing the sensor to room air (21% O₂) for low-end calibration and 100% oxygen for high-end calibration
• The low-oxygen alarm must be confirmed functional
• The alarm threshold is typically set at 18–25% to warn of a hypoxic mixture
• Since this check verifies the integrity of the oxygen concentration monitoring, it is one of the most critical daily steps

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Item 11: Verify Carbon Dioxide Absorbent Is Not Exhausted

• Assess the CO₂ absorbent granules for colour change (e.g., ethyl violet indicator turns purple when exhausted)
• However, colour change is not as reliable as capnographic evidence of exhausted absorbent — a visually normal-appearing absorbent may still be nonfunctional
• Capnometry must be used with every anaesthetic using the circle system
• The provider should remain vigilant for inspired CO₂ > 0 on the capnogram during the case
• It is no longer recommended for providers to breathe into the circuit manually to test absorbent function

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Item 12: Breathing System Pressure and Leak Testing

1. Positive pressure can be generated and sustained in the breathing circuit
2. The APL (adjustable pressure-limiting) valve properly relieves excess pressure

1. Set ventilator to "bag" / manual mode
2. Set all gas flows to zero
3. Close the APL valve completely
4. Occlude the patient Y-piece (finger or test cap)
5. Pressurise the circuit to approximately 30 cmH₂O using the O₂ flush button
6. The circuit passes if pressure holds for at least 10 seconds without decay
7. Any pressure drop → inspect all connections, tubing, absorber canister (a common leak site, especially after absorbent changes)

• After pressurising, open the APL valve fully and confirm pressure drops rapidly to zero
• To test pressure-limiting function: set APL to 30 cmH₂O, occlude Y-piece, set fresh gas flow to ~5 L/min — confirm pressure stabilises near the APL set point

Automated circuit leak tests are a feature of most modern workstations. They also calculate breathing system compliance to guide accurate tidal volume delivery. The automated test should be performed with the circuit that will actually be used. If circuit size or configuration changes between patients, a fresh compliance calibration may be required.

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Item 13: Verify That Gas Flows Properly Through the Breathing Circuit During Inspiration and Expiration

1. Attach a test lung (or spare breathing bag) to the patient Y-piece
2. In manual mode, ventilate the test lung via the breathing bag
3. Then actively "exhale" (squeeze) the test lung back — mimicking a patient's exhalation
4. Repeat in a to-and-fro motion while watching:
   • The inspiratory unidirectional valve (should open on inspiration, close on exhalation)
   • The expiratory unidirectional valve (should open on exhalation, close on inspiration)
5. Proper valve function ensures CO₂ is directed through the absorber and not rebreathed

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Item 14: Document Completion of Checkout Procedures

• Provides a contemporaneous record for medicolegal purposes
• Supports quality improvement and auditing
• Establishes accountability

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Item 15: Confirm Ventilator Settings and Evaluate Readiness to Deliver Anesthesia Care

• Set ventilator parameters appropriate for the upcoming patient (tidal volume, rate, PEEP, I:E ratio)
• Confirm APL valve is open (for manual/spontaneous breathing mode at start)
• Confirm fresh gas flow is appropriate
• Verify the workstation is in manual/spontaneous mode (not ventilator mode) before induction

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Automated Machine Checkout Procedures

1. No single automated checkout checks all required items — manual steps remain mandatory
2. Automated checks differ between manufacturers and models
3. Many providers do not fully understand what their machine's automated procedure actually tests
4. Providers frequently make false assumptions about what automated checks cover
5. Some automated compliance calculations are only performed in the full daily checkout, not before every case — meaning compliance calibration may not be repeated if you swap circuits mid-day

Miller's emphasises: "When developing a local PAC procedure, providers should gain familiarity with their machine's automated checkout procedure through the user manual. If an important item is not specifically part of the described self-check, it should not be assumed that it can be neglected."

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The MS MAIDS Mnemonic (Anesthesia Time-Out)

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Developing a Local PAC Checklist

• Ergonomically ordered — minimise redundant movements and wasted time
• As short as possible, yet detailed enough that critical items are not omitted
• Quality control tool — standardised across all providers
• Assign responsibilities clearly — distinguish what is provider-only vs. may be delegated to a technician
• Reflect the specific equipment in use — a generic checklist cannot account for machine-specific design differences (e.g., presence or absence of an outlet check valve)

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Summary Table: Frequency and Responsibility

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Posterior Fossa Surgery: A Comprehensive Anaesthetic Guide

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1. Anatomy and Surgical Significance

• Brainstem (midbrain, pons, medulla oblongata)
• Cerebellum and its vermis
• Cranial nerve nuclei (especially V–XII)
• Cardiorespiratory centers in the medulla
• Reticular activating system
• Ascending and descending sensorimotor pathways
• Crucial protective reflex networks: gag, swallowing, cough, eyeblink

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2. Indications for Posterior Fossa Surgery

Tumors

Vascular

• Cerebellar arteriovenous malformations (AVMs)
• Posterior circulation aneurysms (basilar tip, PICA, AICA, SCA)
• Cavernous malformations (cavernomas)

Cranial Nerve Decompression (Jannetta Procedures)

• Microvascular decompression (MVD) of CN V → Trigeminal neuralgia (tic douloureux)
• MVD of CN VII → Hemifacial spasm
• MVD of CN IX → Glossopharyngeal neuralgia

Structural / Obstructive

• Chiari malformation → Foramen magnum decompression
• Arachnoid cysts
• Hydrocephalus (fourth ventricle obstruction) → ETV or shunt procedures

Other

• Brainstem biopsy / resection
• Epidermoid / dermoid cysts
• Balloon compression of the trigeminal ganglion (Meckel's cave) for trigeminal neuralgia

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3. Patient Positions

A. Sitting Position (Semi-Fowler / "Park Bench" Variant)

• The patient is placed in a modified recumbent position, not truly upright
• Legs elevated with pillows under knees to maximise venous return
• Head secured in a Mayfield/Sugita pin head holder attached to the back of the table (not under thighs or legs) — this allows rapid lowering of the head and CPR without removing the head holder
• Blood pressure transducers must be zeroed at the level of the external auditory canal (not the arm) to measure true cerebral perfusion pressure
• If a manual BP cuff is used on the arm, correct for the hydrostatic difference between arm and operative field

• Patent foramen ovale (PFO) — risk of paradoxical air embolism
• Significant cerebrovascular disease / stenosis
• Symptomatic cerebral ischaemia when awake
• Poorly controlled hypertension
• Severe cardiomyopathy (poorly tolerates the sudden increase in SVR)
• Cervical spinal stenosis (risk of spinal cord ischaemia with neck flexion)

B. Prone Position

• Used for midline posterior fossa, cervical-occipital junction, upper cervical spine procedures
• Requires particular attention to:
  • Retinal ischaemia (orbital compression every 15 minutes)
  • Posterior ischaemic optic neuropathy (associated with Wilson frame, large blood loss, prolonged surgery, large fluid volumes)
  • Macroglossia from neck flexion compressing the tongue base
  • Brachial plexus injury (maintain "90-90" arm position)
  • Pressure necrosis of forehead, maxilla, chin
  • IVC compression (diverts blood to epidural plexus → increased surgical bleeding)
  • Neck flexion must leave ≥ 2–3 fingerbreadths of submandibular space

C. Lateral / Semilateral (Park Bench) Position

• Alternative for CPA tumours (schwannomas, MVD procedures)
• Lower VAE incidence than sitting
• Used when sitting position is contraindicated

D. Lateral (True) Position

• Used for some CPA and upper cervical spine procedures

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4. Cardiovascular Effects of the Sitting Position

• Pre-positioning IV hydration
• Compression stockings on legs
• Slow, incremental positioning
• Vasopressors (phenylephrine, norepinephrine) as required

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5. Monitoring

Standard Monitors

• ECG (5-lead preferred for arrhythmia detection)
• SpO₂
• EtCO₂ (capnography)
• Temperature
• Urinary catheter

Invasive Monitoring (Indications — Box 53.7 from Miller's)

• Elevated ICP management
• Neural ischaemia risk
• Sitting position (with external auditory canal reference)
• Brainstem manipulation / cranial nerve surgery
• Cardiovascular instability
• Anticipated light anaesthesia without paralysis
• Beat-by-beat BP serves as depth-of-anaesthesia monitor — most of the brain is insensate; sudden arousal from CN traction manifests as sudden hypertension

• All patients in the sitting position for posterior fossa surgery must have a right heart catheter for air retrieval
• Catheter tip position: multiorifice catheter → 2 cm below SVC-atrial junction; single-orifice → 3 cm above SVC-atrial junction
• Confirmed by: (1) CXR, (2) intravascular ECG (biphasic P-wave indicates mid-right atrial position), or (3) TEE
• For nonsitting positions, the right heart catheter may be omitted after discussion with the surgeon (e.g., MVD of CN V/VII where head-up posture is minimal)

• Placed parasternally, 2nd–3rd or 3rd–4th intercostal space (left or right)
• Gold-standard bedside monitor for VAE detection
• When good heart tones are confirmed, additional confirmation manoeuvres are unnecessary

• More sensitive than precordial Doppler for VAE
• Also detects right-to-left shunting across PFO (paradoxical air embolism)
• Safety with prolonged use and pronounced neck flexion is not firmly established

Neurophysiologic Monitoring

• Essential for vestibular schwannoma resection and MVD procedures
• Monitors cochlear nerve (CN VIII) integrity
• Cerebellar retraction → stretches CN VII/VIII → prolonged I–V interpeak latency → ultimately loss of all waves beyond Wave I
• Timely retractor release upon BAEP change prevents permanent hearing loss
• BAEPs significantly improve hearing preservation rates

• Monitors CN VII integrity during CPA surgery (schwannomas, MVD for hemifacial spasm)
• Lateral spread response (LSR) monitoring: in hemifacial spasm, stimulating one branch of CN VII produces EMG in a different branch — this LSR disappears with successful decompression and predicts postoperative relief
• Requires no neuromuscular blockade (or consistent partial paralysis)

• Used in brainstem tumours, posterior circulation aneurysms, spinal cord surgery
• Impose constraints on anaesthetic choices (avoid volatile agents at high concentrations; TIVA preferred)
• For posterior circulation aneurysms: significant false-negative monitoring risk — many at-risk structures cannot be directly monitored

• Monitors risk of injury to airway-protective cranial nerves during floor-of-fourth-ventricle dissection

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6. Venous Air Embolism (VAE)

Incidence

Sources of VAE

• Major cerebral venous sinuses — transverse, sigmoid, posterior half of sagittal sinus (noncollapsible due to dural attachments)
• Emissary veins (suboccipital musculature)
• Diploic space of the skull (violated by craniotomy and pin fixation)
• Cervical epidural veins
• Air under pressure in ventricles / subdural space (anecdotal)
• Pin sites — pin head holders must be removed after the patient leaves head-up posturing

Why Sitting Position Increases VAE Risk

• Surgical field above the heart → subatmospheric venous pressure at wound → air entrainment whenever a vein is opened
• Spontaneous ventilation worsens risk (intermittent negative intrathoracic pressure)

Detection: Sensitivity Hierarchy (Fig. 53.11, Miller's)

1. TEE (most sensitive; also detects PAE)
2. Precordial Doppler (practical standard; high sensitivity when well-positioned)
3. Expired N₂ (theoretically attractive; instrumentation limits sensitivity for small emboli)
4. EtCO₂ ↓ (reliable for moderate-large events)
5. PAP ↑
6. CVP ↑
7. Arterial BP ↓
8. ECG changes (late, with large emboli)
9. Oesophageal stethoscope (least sensitive)

Paradoxical Air Embolism (PAE)

• Air crosses the interatrial septum via a patent foramen ovale (PFO), present in ~25% of adults
• The minimum pressure gradient required to open a probe PFO is approximately 5 mmHg
• When VAE occurs, RAP rises abruptly > LAP → any pre-existing PFO becomes haemodynamically relevant
• Consequences: cerebral and coronary arterial air embolism
• Some centres perform pre-operative bubble study (echo or TCD) to identify PFO and use alternative positions
• PEEP and Valsalva release both increase PAE risk (raise RAP > LAP)

Transpulmonary Air Passage

• Large volumes of air can traverse the pulmonary vascular bed to reach systemic circulation
• Pulmonary vasodilators (including volatile anaesthetics) may lower this threshold
• N₂O should be eliminated after any VAE event — air that reaches the left circulation (via PFO or transpulmonary passage) can be expanded by N₂O

Management of Acute VAE (Box 53.6, Miller's)

• Notify surgeon immediately
• Surgeon floods or packs the surgical field
• Apply jugular venous compression bilaterally (raises cerebral venous pressure)
• Lower the patient's head (if possible)

• Aspirate the right heart catheter (direct air removal from right atrium)
• Discontinue N₂O immediately
• Increase FiO₂ to 1.0
• Vasopressors and inotropes for haemodynamic support
• Chest compressions if cardiac arrest (air lock)

⚠️ PEEP is not recommended — it increases PAE risk and impairs venous return when cardiovascular function is already compromised. Jugular compression is superior for raising cerebral venous pressure.

Note: Lateral positioning (right side up) has been advocated to prevent right ventricular air lock, but this is nearly impossible with a patient in a pin head holder, and its haemodynamic benefit has not been confirmed even in animal studies.

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7. Brainstem Stimulation — Cardiovascular Responses

• Lower pons and upper medulla (floor of fourth ventricle surgery)
• Extra-axial CN V (surgery at CPA — acoustic neuromas, MVD)

• Bradycardia + hypotension
• Tachycardia + hypertension
• Bradycardia + hypertension
• Ventricular dysrhythmias

• Requires meticulous continuous ECG and direct arterial pressure monitoring
• Any abrupt cardiovascular change must be immediately communicated to the surgeon
• If CN VII EMG or MEP monitoring is in use, neuromuscular blockade is omitted → these patients are particularly vulnerable to sudden arousal
• Pharmacologic treatment of dysrhythmias should be cautious — the dysrhythmia may be the only warning of impending cranial nerve nucleus or respiratory centre injury

Balloon Compression of the Trigeminal Ganglion (Meckel's Cave)

• Rapid balloon inflation within Meckel's cave causes profound, transient bradycardia — this is actually sought as confirmation of adequate nerve compression
• Requires general anaesthesia (both needle insertion and balloon inflation are intensely stimulating)
• External pacemaker pads are sometimes advocated but generally unnecessary for the brief bradycardia

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8. Pneumocephalus

• Air entering the intracranial space during or after surgery
• Particularly common with the sitting position — gravity facilitates CSF drainage and replacement by air
• Can develop de novo postoperatively in patients with residual dural defects communicating with nasal sinuses
• Presents as delayed awakening, headache, confusion, or focal neurological deficits postoperatively
• N₂O must be avoided or discontinued before dural closure — N₂O diffuses into the pneumocephalus, dramatically expanding intracranial air volume ("tension pneumocephalus")
• Sitting position + N₂O use throughout the case = high risk of clinically significant pneumocephalus

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9. Macroglossia

• Mechanism: Neck flexion (required for surgical access in both sitting and prone positions) reduces the AP dimension of the oropharynx → compression ischaemia of the tongue base and soft palate → reperfusion oedema after positioning is reversed → rapidly developing airway obstruction post-extubation
• Foreign bodies in the oropharynx (ETT, oesophageal stethoscope, oral airway) exacerbate compression

• Avoid unnecessary apparatus in the oral cavity
• Do NOT omit a bite block entirely — use a rolled gauze bite block (prevents tongue entrapment between teeth during progressive swelling without adding bulk)
• Minimum 2–3 fingerbreadths of submandibular clearance between chin and chest
• Avoid over-flexion of the neck

• If suspected after extubation: re-intubate immediately (swelling is rapid and can completely obstruct the airway)
• Involves ICU care for definitive management

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10. Quadriplegia

• Mechanism: Excessive neck flexion in the sitting position → cervical spinal cord ischaemia, particularly in patients with:
  • Pre-existing cervical stenosis
  • Degenerative cervical disc disease
  • Compromised posterior circulation
• Hypotension at the level of the cervical cord compounds ischaemia
• Must lower the safe MAP threshold in elderly, hypertensive, or cervical stenosis patients

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11. Anaesthetic Management

Pre-operative Assessment

• Assess for raised ICP (headache, vomiting, papilloedema, altered consciousness)
• Evaluate cranial nerve deficits (particularly airway-protective reflexes — gag, cough, swallow)
• Neuroimaging (MRI) to define tumour size, location, vascular anatomy, degree of brainstem involvement
• Echocardiography / bubble study to screen for PFO if sitting position is planned
• Cervical spine MRI if sitting position contemplated
• Cardiovascular assessment — SVR tolerance, cardiomyopathy, valvular disease
• Baseline neurological examination

Pre-medication

• Avoid respiratory depressants in patients with raised ICP or compromised brainstem
• Dexamethasone (if tumour oedema present, typically started preoperatively)
• Antiepileptics if seizures are a risk (supratentorial extension)
• Anti-sialagogue (glycopyrrolate) in prone position to prevent ET tube tape loosening

Induction

• Goals: haemodynamic stability, avoid ICP spikes, smooth laryngoscopy
• Propofol-based TIVA preferred, especially when neurophysiological monitoring (MEPs, SSEPs) is planned
• Fentanyl or remifentanil infusion for blunting laryngoscopy response
• Avoid ketamine (raises CBF and ICP) unless ICP is not a concern
• Rocuronium for intubation — but must plan for NO further relaxant if CN EMG or MEP monitoring is used
• Lidocaine IV pre-laryngoscopy to attenuate ICP spike

Airway Management

• Oral RAE or reinforced (armoured) ETT preferred in prone position
• Secure the tube carefully — tape + benzoin adhesive in prone position
• Avoid oropharyngeal airways or esophageal stethoscopes in prolonged prone/sitting neck-flexion cases (macroglossia risk)

Maintenance

• TIVA (propofol + remifentanil) is strongly preferred when MEP or facial EMG monitoring is in use — volatile agents suppress MEPs in a dose-dependent manner
• If volatile agent is used, keep ≤ 0.5 MAC when monitoring SSEPs/MEPs
• N₂O considerations:
  • N₂O does not increase the incidence of VAE
  • N₂O does not worsen the haemodynamic response to VAE — provided it is discontinued immediately when VAE occurs
  • N₂O is absolutely contraindicated once pneumocephalus is present or suspected
  • Many neuroanesthesiologists prefer to avoid N₂O entirely for posterior fossa sitting cases
• Maintain normocapnia to mild hypocapnia (PaCO₂ 35–40 mmHg) for ICP control
• Avoid hyperglycaemia (worsens ischaemic neurological injury)
• Temperature management: normothermia; mild hypothermia is no longer routinely recommended (multiple trials negative)
• Maintain CPP ≥ 60 mmHg at head level (external auditory canal zero reference for arterial transducer)

Fluid Management

• Isotonic crystalloids preferred (0.9% NaCl, Plasma-Lyte)
• Avoid hypotonic solutions (lower plasma osmolality → cerebral oedema)
• Avoid glucose-containing fluids (hyperglycaemia worsens ischaemic injury)
• Blood products as needed to maintain haemoglobin adequate for O₂ delivery
• In sitting position: avoid excessive preloading that worsens venous engorgement

Extubation

Miller's is explicit: "There should be an interaction between the anaesthesiologist and the surgeon in making decisions about whether extubation is appropriate and where postoperative observation should occur (ICU vs. non-ICU)."

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12. Neurophysiological Monitoring Summary

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13. Specific Procedures

Vestibular Nerve Schwannoma (Acoustic Neuroma)

• Most common CPA tumour
• BAEPs monitor cochlear nerve; changes in I–V interpeak latency warn of retraction-induced stretch injury
• Facial EMG monitors CN VII — both free-running and stimulated
• Lateral spread response monitoring for hemifacial spasm MVD
• Size and preoperative auditory function are best predictors of postoperative hearing preservation

Microvascular Decompression (MVD)

• Semilateral / lateral position usually sufficient
• Right heart catheter often omitted for MVD of CN V/VII (low VAE risk with minimal head-up posture and small retromastoid craniectomy)
• However: if any head-up posture is used, or if the procedure is near the transverse-sigmoid sinus junction (Jannetta procedure), VAE risk may still be substantial
• BAEP monitoring for CN VIII preservation

Trigeminal Ganglion Balloon Compression

• Intense brainstem stimulation → profound transient bradycardia (desired as confirmation)
• Requires GA; general anaesthesia is necessary for both needle insertion and balloon inflation
• External pacemaker pads generally unnecessary

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14. Postoperative Considerations

Immediate Post-operative

• Posterior fossa has minimal compensatory volume — small amounts of oedema or haematoma cause rapid decompensation
• Monitor for:
  • Delayed awakening (pneumocephalus, opioid, haematoma)
  • New cranial nerve deficits (loss of gag/swallow/cough)
  • Cerebellar signs (ataxia, nystagmus)
  • Respiratory irregularity or apnoea (medullary injury)
  • Haemodynamic instability (brainstem injury)

Respiratory Management

• CN IX, X, XII dysfunction → loss of upper airway patency and swallowing
• Brainstem swelling → impaired respiratory drive
• Low threshold for delayed extubation or re-intubation

Macroglossia Watch

• Can manifest rapidly after extubation (minutes to hours)
• Complete airway obstruction may occur
• Emergency re-intubation is the definitive treatment

ICU Indications

• Dissection on floor of fourth ventricle
• Suspected cranial nerve injury
• Prolonged surgery
• Any cardiovascular instability intraoperatively
• High VAE burden, large haemodynamic events
• Not fully awake at end of procedure

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15. Summary Framework: MS MAIDS for Posterior Fossa Cases

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Anaesthetic Management in Neurological Disease

MD Anaesthesiology Exam Preparation Guide

Based on Miller's Anesthesia, 10th Edition

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TOPIC A: ANAESTHESIA IN PARKINSON'S DISEASE

Pathophysiology Relevant to Anaesthesia

Mnemonic: TRAP — Tremor, Rigidity, Akinesia, Postural instability

• Autonomic dysfunction: orthostatic hypotension, sialorrhea, constipation, bladder dysfunction
• Dysphagia → high aspiration risk
• Dementia and cognitive impairment (especially in advanced disease)
• Sleep disorders (REM sleep behaviour disorder)
• Depression and anxiety
• Respiratory muscle rigidity → restrictive lung disease

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Parkinson's Disease Medications — Critical Drug Interactions

⭐ Exam Highlight: Never stop anti-Parkinson medications perioperatively. Sudden withdrawal causes "Parkinson's hyperpyrexia syndrome" (resembles NMS) — fever, rigidity, rhabdomyolysis, altered consciousness, autonomic instability.

Drug Interactions to Know for Exams:

Safe antiemetics in PD: Ondansetron (5-HT3 antagonist), Domperidone (peripheral D2 antagonist, does not cross BBB)

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Preoperative Assessment

1. Severity assessment — Hoehn and Yahr Scale (see below)
2. Medication history — exact timing of last dose; plan for perioperative continuation
3. Swallowing assessment — aspiration risk; consider NGT if severe dysphagia
4. Pulmonary function — restrictive pattern common from chest wall rigidity
5. Autonomic status — orthostatic BP, cardiac dysrhythmias
6. Cognitive status — MMSE/MoCA; informed consent
7. DBS assessment — if patient has Deep Brain Stimulator, neurosurgery team must be involved

Hoehn and Yahr Scale (Exam Favourite)

Stages I–II: elective surgery relatively safe. Stage IV–V: high perioperative risk; ICU planning required.

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Intraoperative Management

Premedication

• Avoid benzodiazepines (worsen rigidity and cognitive function)
• Anti-sialagogue (glycopyrrolate preferred over atropine — less CNS penetration)
• Continue all PD medications orally up to the time of surgery with a small sip of water

Induction

• Propofol is the induction agent of choice — does not worsen PD and is actually useful for tremor suppression
• Fentanyl or remifentanil to blunt laryngoscopy response
• Avoid etomidate — can cause myoclonus
• Careful aspiration precautions — RSI if significant dysphagia
• Anticipate difficult bag-mask ventilation (rigidity)

Airway

• Rigidity of neck and chest muscles can make mask ventilation and laryngoscopy challenging
• High aspiration risk — use cuffed ETT
• In advanced disease, videolaryngoscopy preferred

Muscle Relaxants

• Succinylcholine is generally safe in PD (no acetylcholine receptor upregulation unlike GBS/spinal cord injury)
• Non-depolarising agents: normal response, but chest wall rigidity may already limit compliance

Maintenance

• Propofol-based TIVA is commonly preferred:
  • Does not aggravate PD symptoms
  • Smooth emergence
  • Reduces nausea (antiemetic properties)
• If volatile agents used: sevoflurane or desflurane acceptable
• Avoid N₂O — increases PONV, which mandates use of dopamine-antagonist antiemetics

Monitoring

• Invasive arterial monitoring if significant autonomic dysfunction
• Temperature monitoring — PD patients are susceptible to temperature dysregulation
• Careful positioning — pressure points, joint stiffness, fall risk

Vasopressors

• Levodopa sensitises the myocardium → avoid epinephrine/adrenaline
• Phenylephrine or norepinephrine preferred for hypotension
• Avoid dopamine (dopaminergic agonism may be unpredictable with existing medications)

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Deep Brain Stimulator (DBS) Considerations

Exam Pearl: A patient with Parkinson's who has a DBS implant requires special precautions

• Turn OFF DBS before using electrosurgery (monopolar diathermy can cause permanent damage to DBS leads)
• Use bipolar diathermy only or ensure the return electrode is far from the DBS device
• MRI is conditionally safe only with specific DBS models — confirm with neurosurgery/device manufacturer
• Postoperatively: DBS should be reactivated by the neurology/neurosurgery team

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Postoperative Management

1. Restart PD medications as early as possible (IV levodopa not available in most countries — use NG/OG tube if oral route not possible)
2. Watch for Parkinson's Hyperpyrexia Syndrome (NMS-like) if medications delayed
3. Avoid dopamine-antagonist antiemetics — use ondansetron
4. Monitor for aspiration pneumonia
5. Early physiotherapy
6. Cognitive monitoring (PD patients are at high risk for postoperative cognitive dysfunction — POCD)

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🎯 Exam High-Yield Points for Parkinson's

1. Never stop levodopa perioperatively
2. Metoclopramide + droperidol are ABSOLUTELY CONTRAINDICATED
3. Meperidine + MAO-B inhibitors = Serotonin Syndrome
4. Propofol is the drug of choice for induction and maintenance
5. Glycopyrrolate preferred over atropine for antisialagogue
6. Safe antiemetic = Ondansetron
7. DBS → switch off before monopolar diathermy
8. Parkinson's hyperpyrexia = sudden drug withdrawal emergency

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TOPIC B: ANAESTHESIA IN GUILLAIN-BARRÉ SYNDROME (GBS)

Definition and Pathophysiology

• Rapidly progressive ascending symmetrical limb weakness (flaccid paralysis)
• Absent or reduced deep tendon reflexes
• With or without sensory disturbances

Classic CSF finding: Cytoalbuminous dissociation — high protein + normal cell count (albuminocytologic dissociation)

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Subtypes of GBS (Exam Table)

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Clinical Spectrum and Prognostic Scores

Brighton Collaboration Classification (GBS Diagnostic Certainty)

GBS Disability Scale

Scores 4–5 = high anaesthetic risk

Erasmus GBS Outcome Score (EGOS) — Predicts Independent Walking at 6 Months

IGOS (International GBS Outcome Study) — ongoing

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Criteria for Mechanical Ventilation in GBS

The 20-30-40 Rule (Lawn's Criteria)

• Bulbar palsy → aspiration risk
• Rapid deterioration
• Bilateral facial weakness
• Autonomic instability
• SaO₂ < 92% despite O₂

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Treatment

⭐ IVIG and plasmapheresis are NOT contraindicated in pregnancy

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Anaesthetic Management (Miller's Box 31.8)

Preoperative Assessment

1. Respiratory function — FVC, NIF/MIP, MEP; current ventilatory requirement
2. Bulbar function — gag reflex, swallowing, cough strength → aspiration risk
3. Autonomic function — HR variability, orthostatic hypotension, diaphoresis
4. Current neurological status — GBS disability scale
5. Treatment received — IVIG, plasmapheresis
6. Pain assessment — neuropathic pain is common in GBS

Key Perioperative Considerations (Box 31.8, Miller's)

1. Succinylcholine is ABSOLUTELY CONTRAINDICATED in GBS

2. Non-depolarising NMBAs are not contraindicated but use with caution

• May cause prolonged neuromuscular blockade due to underlying muscle weakness
• Must be guided by objective quantitative neuromuscular monitoring (TOF-Watch, acceleromyography)
• Use minimal effective dose; full reversal must be confirmed before extubation

3. Autonomic dysfunction — Haemodynamic instability

• Exaggerated hypotension at induction — loss of compensatory cardiovascular responses
• Exaggerated hypertension to laryngoscopy and noxious stimuli
• Instability is typically short-lived and self-limited
• Invasive arterial monitoring is strongly recommended
• Small doses of titratable vasopressors (phenylephrine, norepinephrine) pre-prepared
• Avoid large fluid boluses (may exacerbate autonomic instability)

4. Aspiration precautions

• Cranial nerve paralysis and bulbar dysfunction → increased aspiration risk
• Decompress the stomach (NG tube) before induction
• RSI if bulbar function is impaired

5. Regional anaesthesia is controversial

• Has been implicated in exacerbation of neurological symptoms in GBS patients
• Case reports of GBS worsening after neuraxial blockade
• General anaesthesia is well tolerated and preferred
• If regional must be used: document baseline deficits; use lowest effective dose; discuss risk with patient

6. Post-operative respiratory monitoring

• High risk of respiratory failure post-extubation
• Extubate only when FVC > 20 mL/kg and bulbar function is adequate
• Low threshold for ICU admission
• May require prolonged mechanical ventilation

Intraoperative Management

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Postsurgical GBS

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🎯 Exam High-Yield Points for GBS

1. Succinylcholine is ABSOLUTELY CONTRAINDICATED — even in recovered GBS
2. Autonomic dysfunction → invasive arterial monitoring mandatory
3. 20-30-40 rule for mechanical ventilation
4. IVIG = Plasmapheresis in efficacy; steroids alone are NOT beneficial
5. Regional anaesthesia controversial — prefer GA
6. NDNMBs may cause prolonged block → quantitative TOF monitoring essential
7. GA is well tolerated; Regional is NOT
8. Miller Fisher Syndrome = Ophthalmoplegia + Ataxia + Areflexia (anti-GQ1b)
9. CSF: High protein + Normal cells = Albuminocytologic dissociation

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TOPIC C: ANAESTHETIC MANAGEMENT IN TRAUMATIC BRAIN INJURY (TBI)

Classification of TBI Severity — Glasgow Coma Scale (GCS)

The Glasgow Coma Scale (GCS) — Teasdale & Jennett 1974

Mnemonic for Motor: "Old Ladies Play Whist For Fun" (6→1)

TBI Severity Classification

⭐ GCS ≤ 7–8 = Intubation required (Miller's: "Patients with GCS scores of 7 to 8 or less require tracheal intubation and controlled ventilation")

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Primary vs. Secondary Brain Injury

Secondary Brain Injury Triggers (Must Prevent)

⭐ The most important principle: Prevent secondary brain injury by maintaining oxygen delivery and preventing hypotension

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ICP and CPP Management

Monroe-Kellie Doctrine

Normal Values

Brain Trauma Foundation (BTF) Guidelines — Key Targets

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Stepwise ICP Management (Lund-Rosner Concepts)

• Head of bed elevated 30°
• Head neutral (avoid jugular compression)
• Adequate sedation and analgesia
• Normocapnia, normoxia
• Avoid hyperthermia and seizures
• Osmotherapy: Mannitol 0.25–1 g/kg (20% solution) or Hypertonic Saline (3% or 23.4%)

• CSF drainage (if ICP monitor / EVD in place)
• Neuromuscular blockade
• Controlled hyperventilation (PaCO₂ 30–35 mmHg) — SHORT-TERM ONLY

• High-dose barbiturate coma (thiopental/pentobarbital) — requires EEG monitoring
• Decompressive craniectomy
• Induced hypothermia (limited evidence)

EUROTHERM Trial: Therapeutic hypothermia after TBI showed NO outcome benefit — not routinely recommended

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Hyperventilation in TBI — Miller's

⭐ Critical exam topic: Hyperventilation is a double-edged sword

• Mechanism: Hypocapnia → cerebral vasoconstriction → ↓ CBV → ↓ ICP
• Benefit: Rapid ICP reduction (effective within minutes)
• Harm: The same vasoconstriction causes cerebral ischaemia, especially when baseline CBF is already low (as it typically is in the first 24–48 hours after TBI)

Brain Trauma Foundation Recommendations on Hyperventilation (Miller's, p. 8220–8221):

1. "Prolonged prophylactic hyperventilation with PaCO₂ of 25 mmHg or less is NOT recommended"
2. "Hyperventilation is recommended as a temporising measure only for elevated ICP"
3. "Hyperventilation should be avoided in the first 24 hours after injury when CBF is often critically reduced"
4. "If hyperventilation is used, SjvO₂ or brain tissue PO₂ measurements are recommended to monitor oxygen delivery"

• Use hyperventilation only for acute herniation or ICP crisis not responding to other measures
• Target PaCO₂ 30–35 mmHg (not < 25 mmHg)
• It is a bridge to definitive treatment, not a therapeutic strategy

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Airway Management in TBI

Challenges (Miller's Box — Intubation of the Head-Injured Patient)

Sequence

1. Pre-oxygenate
2. Cricoid pressure + manual in-line axial stabilisation (MIAS) — not traction
3. Modified RSI: propofol (small dose) + lidocaine 1.5 mg/kg IV + fentanyl + rocuronium (or succinylcholine if urgent)
4. Video laryngoscopy preferred
5. Confirm placement; begin controlled ventilation

Succinylcholine in TBI

• Any ICP increase from succinylcholine is small and probably does not occur with serious cerebral injuries
• If urgency demands it (full stomach, rapid sequence), succinylcholine is appropriate
• Rocuronium + sugammadex is the modern alternative

Cervical Spine Clearance

• Modern multislice helical CT alone is sufficient to rule out unstable cervical spine injuries
• MRI required if ligamentous injury is suspected
• In-line axial stabilisation must be maintained until cleared

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Fluids in TBI

⭐ Albumin 4% is CONTRAINDICATED in TBI — SAFE trial showed increased mortality (hyponatraemia, cerebral oedema)

Haemoglobin Targets in TBI

• Hb < 7 g/dL → impaired brain function in animal models
• Hb < 10 g/dL → may be detrimental to TBI recovery
• Transfuse to maintain Hb 7–10 g/dL in severe TBI

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Decompressive Craniectomy

• CPP cannot be maintained despite all therapies including barbiturate coma
• Specific anatomic TBI patterns
• Uncontrolled refractory ICP

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ARDS in TBI — Double-Hit Model (Miller's)

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Monitoring in Severe TBI

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CPP Management Approaches (Historical Controversy — Exam Favourite)

Currently accepted: CPP 60–70 mmHg; normovolaemia; phenylephrine or norepinephrine for MAP support. Rosner approach has few current advocates.

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🎯 Exam High-Yield Points for TBI

1. Single episode of SBP < 90 mmHg doubles mortality in severe TBI
2. GCS ≤ 7–8 = intubate
3. ICP target < 22 mmHg; CPP target 60–70 mmHg
4. Hyperventilation is a temporising measure only — avoid in first 24 hours
5. PaCO₂ 30–35 mmHg acceptable; < 25 mmHg is harmful and prohibited
6. Succinylcholine is NOT contraindicated in TBI (Miller's)
7. Albumin 4% is CONTRAINDICATED (SAFE trial)
8. Glucose solutions are CONTRAINDICATED
9. PEEP is safe in TBI with adequate volume
10. Hypothermia showed no benefit (EUROTHERM trial)
11. Monroe-Kellie Doctrine underpins all ICP physiology

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TOPIC D: ANAESTHETIC MANAGEMENT FOR FRONTAL LOBE TUMOUR (SUPRATENTORIAL TUMOUR)

Classification of Brain Tumours (WHO 2021)

WHO CNS Tumour Grade

Common Supratentorial / Frontal Lobe Tumours

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Preoperative Assessment

ICP Status — The Critical Question

Neurological Deficits from Frontal Lobe Involvement

Karnofsky Performance Scale (KPS)

KPS ≥ 60 associated with better surgical outcome

ECOG Performance Status (Eastern Cooperative Oncology Group)

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Preoperative Optimisation

Steroids (Miller's, p. 8402)

⭐ Key principle: Patients with significant tumour-related mass effect and oedema must receive preoperative dexamethasone

• Dexamethasone is the agent of choice — minimal mineralocorticoid activity
• Ideal course: 48 hours preoperatively
• Minimum: 24 hours (sufficient for clinical effect)
• Typical regimen: Dexamethasone 10 mg IV/orally → then 4–10 mg every 6 hours
• Mechanism: Reduces peritumoral vasogenic oedema (BBB stabilisation)
• Does NOT reduce cytotoxic oedema (no role post-ischaemia/infarction)

Anti-epileptics

• Frontal lobe tumours → high seizure risk (motor cortex proximity)
• Prophylactic anticonvulsants: Levetiracetam preferred (fewer drug interactions, no enzyme induction)
• Phenytoin: enzyme inducer — affects other drugs; still used but less preferred
• Check drug levels; continue perioperatively

Avoid sedative premedication in patients with raised ICP

• Sedatives → respiratory depression → hypercapnia → ↑ CBF → ↑ ICP
• Miller's: "Sedative premedication outside of the operating room is usually avoided" in patients with impaired intracranial compliance

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Monitoring

Routine

• 5-lead ECG
• SpO₂
• EtCO₂
• Temperature
• Urinary catheter

Invasive (Miller's, p. 8406)

⭐ "We almost invariably place arterial catheters for craniotomies under general anaesthesia"

• Arterial line:
  • Preinduction placement if severe mass effect and reduced compensatory reserve
  • At minimum, before pin fixation (highest-risk period for hypertension)
  • Continuous beat-to-beat monitoring during induction + emergence (most critical phases)
• Central venous line:
  • If peripheral access limited and large blood loss expected
  • Tumours encroaching on sagittal sinus → VAE risk → right atrial catheter if near posterior half of sagittal sinus

ICP Monitoring

• Rarely warranted just for induction
• Once cranium is opened: surgical field observation provides equivalent information

Neurophysiological Monitoring

• Awake craniotomy + cortical mapping: if tumour is near eloquent cortex (motor strip, Broca's area)
• MEPs, SSEPs for motor cortex monitoring if under GA
• EEG for seizure monitoring

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Anaesthetic Management

Goals of Anaesthesia for Supratentorial Tumours

The "Four Ls" of Neuroanesthesia:

1. Loose brain (good brain relaxation) — avoid ↑ ICP, ↑ CBV
2. Low ICP — maintain CPP, use ICP-reducing strategies
3. Labile haemodynamics prevented — avoid hyper/hypotension
4. Lucid emergence — rapid awakening for neurological assessment

Induction

• Ketamine (↑ CBF, ↑ ICP, ↑ CMRO₂ — traditionally avoided, though newer evidence is less clear)
• Etomidate (myoclonus; adrenal suppression)
• Large doses of benzodiazepines (interfere with neurological monitoring; prolonged sedation)

Maintenance

⭐ TIVA (Propofol + Remifentanil) is the gold standard for supratentorial tumour surgery with MEP/SSEP monitoring

Pin Fixation (Mayfield Head Holder)

• Ensure deep anaesthesia before pins are inserted
• Additional propofol bolus + remifentanil bolus just before pinning
• Infiltration of pin sites with local anaesthetic (bupivacaine/ropivacaine)
• Arterial line must be in place before pins

Intraoperative Brain Relaxation Techniques

1. Moderate hypocapnia — PaCO₂ 35 mmHg; mild vasoconstriction reduces CBV
2. Head-up 15–30° — improves venous drainage; reduces cerebral venous congestion
3. Mannitol 0.5–1 g/kg IV — given after induction, before dura opening (osmotic decompression)
4. Hypertonic saline — alternative/adjunct to mannitol for brain relaxation
5. Furosemide 0.5 mg/kg — may be added to mannitol for additive effect
6. Avoid hypotonic fluids — maintain osmolality
7. CSF drainage (EVD or lumbar drain) — deflates brain dramatically
8. Avoid excessive volatile agents (> 1 MAC) — vasodilate cerebral vasculature
9. Dexamethasone — to reduce peritumoral oedema (preoperatively and intraoperatively)

Ventilation

• Controlled ventilation throughout
• Target PaCO₂ 35–40 mmHg (mild hypocapnia acceptable)
• Avoid PaCO₂ < 30 mmHg — excessive vasoconstriction causes ischaemia
• PEEP usually kept low (but see TBI section — adequate volume makes PEEP safe)

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Frontal Lobe Specific Considerations (Miller's, p. 8368)

Subfrontal Approach — Specific Issues

Miller's: "Patients who undergo a craniotomy involving a subfrontal approach sometimes manifest a disturbance of consciousness in the immediate postoperative period."

• Retraction and irritation of the inferior surfaces of the frontal lobes → delayed emergence or disinhibition
• More likely with bilateral subfrontal retraction
• Procedures requiring subfrontal approach: Olfactory groove meningiomas, craniopharyngiomas, pituitary tumours with suprasellar extension

⭐ Critical anaesthetic implication:

• "The clinician should be more inclined to confirm return of consciousness before extubating the patient rather than to extubate expectantly"
• A less liberal use of long-acting IV anaesthetic drugs (fentanyl, propofol infusion) is appropriate when bilateral subfrontal retraction is planned — low residual drug concentrations that are tolerated by normal brains may cause prolonged unresponsiveness in this population

Hypothalamic Involvement (Craniopharyngioma / Pituitary Tumours)

⭐ DI typically has a delayed onset (12–48 hours) — not usually seen in the OR but in the ICU/ward

Awake Craniotomy for Frontal Lobe Tumours Near Eloquent Cortex

• Primary motor cortex / motor strip
• Broca's area (dominant frontal)
• Frontal eye fields

1. Asleep-Awake-Asleep (AAA) — LMA or ETT → wake up for mapping → re-anaesthetise for closure
2. Monitored Anaesthesia Care (MAC) — dexmedetomidine + propofol + local infiltration throughout
3. Awake-Awake-Awake — sedation throughout without GA phase

• Dexmedetomidine (excellent sedation, minimal respiratory depression, no effect on cortical mapping)
• Remifentanil (titratable analgesia)
• Propofol (titrated for sedation)
• Avoid benzodiazepines — raise seizure threshold, may interfere with speech mapping

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Emergence from Neurosurgery — Critical Phase

To Minimise Coughing/Straining at Emergence:

1. Include generous narcotics (antitussive) at closure stage — as much as consistent with spontaneous ventilation at end
2. IV Lidocaine 1.5 mg/kg IV just before head movement for dressing application
3. Withdraw volatile agents — last agent to be withdrawn: N₂O (patients wake more rapidly and smoothly)
4. Propofol bolus or low-dose infusion as supplement during dressing
5. Delay neuromuscular reversal until late — premature reversal → patient moves and coughs

VAE Risk in Supratentorial Tumours

• VAE is infrequent for most supratentorial tumours
• Exception: Convexity meningiomas encroaching on the posterior half of the superior sagittal sinus
• Full VAE precautions (right atrial catheter) reserved for these cases only

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Postoperative Management

Immediate Post-op

• Neurological assessment as soon as awake (GCS, focal deficits, speech)
• Blood pressure control — prevent hypertension (Bayliss response → ↑ ICP, haemorrhage)
• Continue dexamethasone
• Maintain normoglycaemia
• Continue anticonvulsants

ICU Indications

• Prolonged/difficult surgery
• Significant mass effect
• Bilateral frontal lobe involvement
• Posterior fossa proximity
• Haemodynamic instability
• Subfrontal retraction → possible delayed awakening

Postoperative Complications of Frontal Lobe Surgery

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🎯 Exam High-Yield Points for Frontal Lobe Tumour

1. Dexamethasone 10 mg → 4–10 mg q6h for 24–48 hrs preop — reduces peritumoral oedema
2. Avoid sedative premedication in raised ICP (CO₂ retention → ↑ ICP)
3. Arterial line BEFORE pin insertion — this is the most stimulating phase
4. Lidocaine 1.5 mg/kg IV before laryngoscopy AND before head dressing
5. Subfrontal retraction → confirm consciousness before extubation; minimise long-acting drugs
6. DI onset is DELAYED (12–48 hrs postop) — not seen intraoperatively
7. VAE risk is ONLY with tumours near posterior half of superior sagittal sinus
8. TIVA (propofol + remifentanil) = gold standard for neuromonitoring cases
9. Desflurane > 1.5 MAC is CONTRAINDICATED in raised ICP
10. Awake craniotomy for eloquent cortex — dexmedetomidine is the drug of choice
11. KPS ≥ 60 = better surgical prognosis; Hoehn-Yahr scale is for Parkinson's

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Cross-Topic Summary Table (Quick Revision)

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