---

Barbiturates in Anaesthesia

1. Historical Background

• Morgan and Mikhail's Clinical Anesthesiology, 7e
• Miller's Anesthesia, 10e

---

2. Chemistry & Formulation

• Thiobarbiturates — sulfur at position 2: thiopental, thiamylal
• Oxybarbiturates — oxygen at position 2: methohexital

• Longer alkyl chain at C5 → increased hypnotic potency but also toxicity
• Phenyl group at C5 → anticonvulsant activity (phenobarbital)
• Sulfur at C2 (thiobarbiturates) → increased lipid solubility and faster CNS onset
• N-methylation at position 1 → increased CNS excitability (methohexital's proconvulsant property)
• Branching at C5 → increased potency and shorter duration

• Supplied as sodium salts mixed with 6% anhydrous sodium carbonate, reconstituted in water, 5% dextrose, or normal saline
• Thiopental: 2.5% solution; thiamylal: 2% solution; methohexital: 1% solution
• Solution is highly alkaline (pH 10–11)
• Thiobarbiturates stable for 1 week refrigerated; methohexital stable for 6 weeks after reconstitution
• Cannot be mixed with acidic solutions (atracurium, vecuronium, rocuronium, suxamethonium, alfentanil, sufentanil, midazolam) — precipitation occurs as the free acid
• Miller's Anesthesia, 10e, p. 2492–2494
• Goodman & Gilman's, p. 1127

---

3. Mechanism of Action

1. Enhancement of inhibitory neurotransmission — potentiate GABA-A receptor–mediated Cl⁻ current (at a site distinct from benzodiazepines; at high concentrations they can directly activate the channel even without GABA)
2. Inhibition of excitatory neurotransmission — suppress glutamate and acetylcholine receptor activity

They do not possess analgesic properties; some evidence suggests they may even lower the pain threshold (hyperalgesia).

• Katzung's Basic & Clinical Pharmacology, 16e
• Miller's Anesthesia, 10e

---

4. Pharmacokinetics

• Katzung's Basic & Clinical Pharmacology, 16e, p. 710
• Miller's Anesthesia, 10e

---

5. Organ System Effects

5.1 Central Nervous System

• Dose-dependent CNS depression: sedation → hypnosis → general anaesthesia → burst suppression → isoelectric EEG
• No analgesia (may cause hyperalgesia)
• Potent cerebral vasoconstrictors → ↓ cerebral blood flow (CBF) → ↓ cerebral blood volume (CBV) → ↓ intracranial pressure (ICP)
• ↓ CMRO₂ (cerebral metabolic rate for O₂) in a dose-dependent manner up to EEG burst suppression
• Neuroprotection: effective against focal cerebral ischaemia (stroke, retraction, temporary clips during aneurysm surgery); not effective after global ischaemia (cardiac arrest)
• Anticonvulsant (except methohexital)
• Methohexital exception: activates epileptic foci → drug of choice for ECT anaesthesia and for identifying epileptic foci intraoperatively

5.2 Cardiovascular System

• ↓ Systemic blood pressure primarily from peripheral vasodilation (predominantly venodilation)
• Direct negative inotropic effect on the heart
• Reflex tachycardia (baroreceptor reflex is partially but less completely blunted compared to propofol)
• BP decrease is usually smaller than with propofol
• Thiopental maintains cardiac output better than equipotent doses of propofol
• Caution in hypovolaemia, haemorrhagic shock, cardiac disease

5.3 Respiratory System

• Dose-dependent respiratory depression — ↓ tidal volume, ↓ respiratory rate, ↓ hypercapnic and hypoxic ventilatory responses
• Apnoea is common after induction doses, especially with rapid injection
• Does NOT cause bronchodilation (unlike propofol and ketamine) — avoid in active bronchospasm

5.4 Hepatic/Renal

• Porphyria: Barbiturates stimulate aminolevulinic acid (ALA) synthetase → ↑ porphyrin production → can precipitate an acute porphyric crisis → absolute contraindication in acute intermittent porphyria (AIP)
• Katzung's, pp. 2403–2413
• Miller's Anesthesia, 10e

---

6. Clinical Uses

• Miller's Anesthesia, 10e, p. 2509–2511

---

7. Contraindications

• Miller's Anesthesia, 10e

---

8. Adverse Effects & Complications

---

9. Comparison: Thiopental vs. Methohexital vs. Propofol

• Miller's Anesthesia, 10e
• Katzung's, p. 2390

---

10. Current Status

• Rapid sequence induction where propofol is contraindicated
• Raised ICP management / barbiturate coma
• Neuroprotection during temporary vascular occlusion in neurosurgery
• Status epilepticus refractory to other agents
• Miller's Anesthesia, 10e; Katzung's Basic & Clinical Pharmacology, 16e; Morgan & Mikhail's Clinical Anesthesiology, 7e; Goodman & Gilman's Pharmacological Basis of Therapeutics

---

---

MD Anaesthesia Viva — Barbiturates

---

🔬 CHEMISTRY & PHARMACOLOGY (Basic Science Questions)

---

Barbituric acid — formed by condensation of malonic acid and urea (2,4,6-trioxohexahydropyrimidine). The nucleus itself is hypnotically inactive.

---

• Position 5 — aryl or alkyl substitution → hypnotic/sedative effect
• Position 2 — oxygen → oxybarbiturate; sulfur → thiobarbiturate (↑ lipid solubility, faster onset)
• Position 1 (N-methylation) — e.g. methohexital → ↑ CNS excitability, proconvulsant
• C5 phenyl group → anticonvulsant activity (e.g. phenobarbital)

---

---

Through keto-enol tautomerism, the C2 oxygen/sulfur becomes reactive in enol form, allowing formation of water-soluble sodium salts. Reconstituted in water with 6% anhydrous sodium carbonate → pH 10–11 (highly alkaline).

---

Rocuronium, suxamethonium, midazolam, alfentanil, and atracurium are all acidic solutions. Mixing with highly alkaline thiopental (pH 10–11) causes precipitation of the barbiturate as the free acid → can occlude the IV line, especially critical during RSI.

---

• Thiopental (thiobarbiturate): 1 week refrigerated
• Methohexital (oxybarbiturate): 6 weeks refrigerated

---

⚗️ MECHANISM OF ACTION

---

Two mechanisms:

1. Enhance inhibitory transmission — potentiate GABA-A receptor Cl⁻ channel (prolong channel opening duration); at high doses directly activate GABA-A even without GABA
2. Inhibit excitatory transmission — suppress AMPA glutamate receptors and nicotinic ACh receptors

Key point: Barbiturate binding site is distinct from the benzodiazepine site on the GABA-A receptor.

---

---

No. They are purely hypnotic. They may actually lower the pain threshold (hyperalgesia). Additional analgesia from opioids or volatile agents is always required during surgery.

---

🔄 PHARMACOKINETICS

---

After a single bolus, recovery is due to redistribution — not hepatic metabolism. Thiopental is highly lipid-soluble and rapidly redistributes from the highly perfused brain → muscle (vessel-rich group) → fat. The brain concentration falls below the threshold for anaesthesia within minutes.

Context: The elimination half-life is hours–days, but this is irrelevant after a single dose. This is why thiopental gives "rapid emergence" from a single induction dose despite slow metabolism.

---

After repeated boluses or infusion, the peripheral compartments (especially fat) become saturated. Recovery then depends on elimination (hepatic metabolism) rather than redistribution. The context-sensitive half-time increases markedly → prolonged sedation/hangover. Methohexital and propofol accumulate far less.

---

The time for plasma concentration to fall by 50% after terminating an infusion, as a function of infusion duration. Thiopental's context-sensitive half-time increases dramatically with infusion duration because its large fat reservoir slowly releases drug back into plasma. This makes it unsuitable for maintenance infusions.

---

Thiopental is ~85% protein-bound (primarily to albumin). In hypoalbuminaemia (liver disease, burns, malnutrition, nephrotic syndrome), free drug fraction increases → greater CNS effect from the same dose → reduce the induction dose.

---

Pentobarbital — a longer-acting barbiturate hypnotic. This accounts for a small fraction of thiopental's metabolism but contributes to prolonged sedation after large doses.

---

🫀 ORGAN SYSTEM EFFECTS

---

Sedation → Hypnosis → General Anaesthesia → Burst Suppression → Isoelectric EEG

At each level: ↓ CBF, ↓ CBV, ↓ ICP, ↓ CMRO₂ (all dose-dependent)

---

• Potent cerebral vasoconstrictors → ↓ cerebral blood volume (CBV) → ↓ ICP
• ↓ CMRO₂ → reduced metabolic demand → cerebral vasoconstriction (flow-metabolism coupling)
• Effect is dose-dependent and maximal at EEG burst suppression

---

• Focal cerebral ischaemia (stroke, surgical retraction, temporary clips during aneurysm surgery) — YES, barbiturates are neuroprotective
• Global cerebral ischaemia (cardiac arrest) — NO, barbiturates do not reduce injury

The distinction is because focal ischaemia has penumbral tissue where reducing CMRO₂ helps; global ischaemia has no zone of partial perfusion.

---

1. ↓ BP — primarily from peripheral vasodilation (venodilation)
2. Direct negative inotropy (↓ cardiac contractility)
3. Reflex tachycardia (partial baroreceptor blunting)
4. BP decrease is less than propofol; cardiac output is better maintained than propofol
5. Caution in hypovolaemia, haemorrhagic shock, cardiac disease

---

Two reasons:

1. Reflex sympathetic activation in response to hypotension (baroreflex — though partially blunted)
2. Direct vagolytic effect of thiopental (mild)

---

• Dose-dependent ↓ tidal volume and respiratory rate
• Apnoea common after induction dose (rate- and dose-dependent)
• ↓ hypercapnic and hypoxic ventilatory drives
• NO bronchodilation — unlike propofol and ketamine
• Risk of laryngospasm under light depth with airway stimulation

---

💊 CLINICAL USE

---

Standard: 3–4 mg/kg IV (ED50 ~2.2–2.7 mg/kg)

Dose is reduced in:

• Elderly patients
• Haemorrhagic shock / ↓ cardiac output
• Hypoalbuminaemia (burns, liver disease, malnutrition, uraemia)
• Opioid or benzodiazepine premedication
• Severe anaemia, malignancy, obesity, extremes of lean body mass

---

• It is a proconvulsant — activates epileptic foci (N-methylation at position 1)
• Produces longer, better-quality seizures during ECT compared to thiopental or propofol
• Propofol shortens seizure duration; thiopental is anticonvulsant and reduces seizure quality
• Faster recovery due to higher plasma clearance
• Note: propofol can still be used if methohexital is unavailable, but seizure monitoring is important

---

25 mg/kg rectally as a 10% solution through a 14F catheter inserted 7 cm into the rectum. Sleep onset is rapid; mean peak plasma levels occur within 14 minutes.

---

High-dose barbiturate (thiopental or pentobarbital) infusion titrated to EEG burst suppression to maximally reduce CMRO₂ and ICP.

Indications: Refractory raised ICP (severe TBI, subarachnoid haemorrhage) not responding to other measures.

Monitoring: Continuous EEG (aim for burst suppression pattern), haemodynamic monitoring (vasopressors often required).

---

⚠️ CONTRAINDICATIONS & COMPLICATIONS

---

Acute Intermittent Porphyria (AIP)

Barbiturates stimulate aminolevulinic acid (ALA) synthetase (the rate-limiting enzyme in haem synthesis) → ↑ porphyrin production → acute porphyric crisis → abdominal pain, neurological manifestations, cardiovascular instability, potentially fatal.

Safe alternatives for induction in porphyria: Propofol or ketamine

---

Mechanism: Crystallisation of thiopental as free acid in the acidic arterial blood → microcrystal embolism → intense vasospasm → endarteritis obliterans → thrombosis → distal gangrene

Management:

1. Do NOT remove the needle/cannula (use it for treatment)
2. Dilute with normal saline
3. Inject papaverine (vasodilator) through the same cannula
4. Sympathetic block (stellate ganglion block or brachial plexus block) to relieve vasospasm
5. Systemic anticoagulation (heparin)
6. Warm soaks, analgesia

---

Highly alkaline solution (pH 10–11) → chemical cellulitis and tissue necrosis

Management: Hyaluronidase infiltration (to disperse the drug), warm soaks, elevation, analgesia

---

• Hiccup, tremor, myoclonic movements
• Due to N-methylation at position 1 → subcortical excitation (disinhibition of inhibitory circuits)
• More common with methohexital than thiopental
• Can be reduced by opioid premedication

---

🔍 HIGHER-ORDER / TRICKY VIVA QUESTIONS

---

Because initial recovery is determined by redistribution, not by elimination. High lipid solubility means thiopental rapidly distributes from brain (highly perfused, rapid equilibration) to muscle, then slowly to fat. The brain concentration falls below the anaesthetic threshold within minutes, even though the drug is still present in the body and the elimination half-life is long. This is the "redistribution principle."

---

Propofol has a very high plasma clearance (20–30 mL/kg/min, exceeds hepatic blood flow — extrahepatic metabolism). Its context-sensitive half-time remains short even after prolonged infusions. Thiopental, despite hepatic metabolism, has a large volume of distribution and low clearance → prolonged context-sensitive half-time → hangover effect.

---

• Induction: Propofol (safe in porphyria) or ketamine
• Maintenance: Propofol TIVA or volatile agents (isoflurane, sevoflurane — generally considered safe)
• Avoid: All barbiturates, etomidate (questionable), some opioids
• Regional anaesthesia preferred if feasible

---

It has no antiemetic properties (unlike propofol) — but also no direct emetogenic effect. PONV rates are intermediate between propofol (antiemetic) and volatile agents (emetogenic). Thiopental's PONV profile is neutral.

---

---

A review of anaesthetic agents and awareness risk found:

• Benzodiazepines reduce awareness most effectively
• Ketamine and etomidate reduce wakefulness compared to thiopental
• Thiopental has an intermediate awareness profile — not the best for preventing awareness when used as a maintenance agent
• Propofol TIVA has a lower awareness risk than volatile agent-based techniques

---

pKa = 7.6 — very close to physiological pH (7.4). This means at body pH, a significant fraction exists in the unionised (lipid-soluble) form → rapid CNS penetration → fast onset of action.

In acidosis (↓ pH), more drug is in unionised form → enhanced CNS effect → dose should be reduced in acidotic patients.

---

📝 RAPID-FIRE VIVA ROUND

---

Examiner tip: The questions most commonly asked in MD viva on barbiturates are: (1) Why does thiopental have short action despite long half-life? (2) Why is it contraindicated in porphyria? (3) Intra-arterial injection management. (4) Why methohexital for ECT? (5) Mechanism of action vs benzodiazepines.

---

---

CSF Formation, Circulation & Anaesthetic Importance

---

1. Introduction & Overview

• It determines intracranial pressure (ICP)
• It is the medium through which intrathecal drugs are administered and distributed
• Its dynamics are directly altered by anaesthetic drugs, ventilation strategy, and patient positioning
• Its volume, along with blood and brain parenchyma, follows the Monro-Kellie doctrine

---

2. Anatomy of the CSF Compartments

Ventricles

---

3. Formation of CSF

Site of Production

• Cerebral endothelial cells
• Fluid produced as a by-product of neuronal metabolic activity

Rate of Production

• ~450–500 mL/day (some sources: 500 mL/day; Miller's: 450 mL/day)
• Rate of formation: ~0.35 mL/min (≈ 20 mL/hr)
• Total CSF volume: ~150 mL (intracranial ~100 mL + spinal ~50 mL)
• Therefore, CSF turns over approximately 3–4 times per day

Mechanism of Formation

1. Hydrostatic efflux — fluid moves from choroidal capillaries into the perivascular space by hydrostatic pressure gradient
2. Active transport — choroid plexus epithelial cells (resembling renal distal tubular cells) actively transport ions and water into the ventricles:
   • Secreted into CSF: Na⁺, Cl⁻, HCO₃⁻, water
   • Reabsorbed from CSF into blood: K⁺
   • Proteins, cholesterol, and large molecules are excluded (large molecular size)

The Blood-CSF Barrier (Choroid Plexus Barrier)

1. Capillary endothelial cells + basement membrane
2. Neuroglial membrane
3. Epithelial cells of the choroid plexus (joined by tight junctions)

Circadian Rhythm

---

4. CSF Composition

---

5. Circulation of CSF

Choroid plexus (lateral ventricles)
        ↓ [Foramina of Monro]
    Third ventricle
        ↓ [Cerebral aqueduct of Sylvius]
    Fourth ventricle
        ↓ [Foramina of Luschka (×2) + Foramen of Magendie (×1)]
    Subarachnoid space (around brain and spinal cord)
        ↓ [Convection/bulk flow upward over cerebral convexities]
    Arachnoid granulations (in dural venous sinuses)
        ↓ [One-way bulk flow]
    Superior sagittal sinus → Venous blood

Additional Drainage Routes (Minor)

• Along cranial and peripheral nerve sheaths
• Perivascular (perivenous) routes
• Along white matter tracts (transependymal flow)
• Meningeal and cervical lymphatic vessels

Driving Force for Circulation

• Choroidal arterial pulsations
• Respiratory pressure variations
• Cilia on ependymal cells lining the ventricles

---

6. The Glymphatic System — Anaesthetic Relevance

1. CSF enters periarterial spaces (bounded by vessel walls and astrocyte end-feet)
2. Aquaporin-4 channels on astrocyte end-feet facilitate water exchange
3. CSF is transported by convection to brain parenchyma
4. Accumulates in perivenous space → drains into cervical lymphatics and along cranial nerves

• The periarterial space increases significantly during sleep AND during general anaesthesia → waste clearance enhanced
• Among anaesthetic agents:
  • Volatile agents → reduce lymphatic/glymphatic transport
  • Dexmedetomidine → less reduction; better preserves glymphatic transport
  • This may be relevant to the pathogenesis of postoperative cognitive dysfunction (POCD)
• Miller's Anesthesia, 10e, p. 930

---

7. Absorption of CSF

Primary Route: Arachnoid Granulations

• One-way valve-like projections into the dural venous sinuses (mainly superior sagittal sinus)
• Bulk flow from CSF to venous blood when CSF pressure > venous pressure
• Open at a threshold pressure of ~68 mmH₂O

Secondary Routes

• Perivenous and perineural pathways
• Spinal arachnoid granulations
• Nasal mucosal lymphatics (via cribriform plate)

---

8. Monro-Kellie Doctrine & ICP

• Brain parenchyma: ~80%
• Blood (cerebrovascular): ~12%
• CSF: ~8% (~100 mL intracranial)

1. Displacement of CSF into spinal subarachnoid space (most readily displaced)
2. Reduction in cerebral venous blood volume
3. ↓ CSF production
4. Once compensation is exhausted → exponential rise in ICP (decompensation)

CPP = MAP − ICP (or MAP − CVP, whichever is higher)

Target: CPP ≥ 60 mmHg (brain-injured patients: ≥ 70 mmHg)

---

9. Anaesthetic Importance of CSF — Comprehensive

---

9.1 Spinal (Subarachnoid) Anaesthesia

CSF specific gravity: 1.003–1.008 at 37°C

Hyperbaric bupivacaine (0.5% + 8% glucose) = heavy bupivacaine, most commonly used

• Baricity — most important factor
• Dose (volume × concentration)
• Patient position at and immediately after injection
• Level of injection (L3-4 vs L4-5)
• Speed of injection
• CSF volume — reduced in pregnancy (epidural venous engorgement compresses subarachnoid space), obesity, elderly → unpredictable high block risk

• CSF leaks through dural puncture site → ↓ CSF volume and pressure → traction on pain-sensitive intracranial structures
• Positional: worse sitting/standing, relieved lying flat
• Prevented by: small-gauge pencil-point needles (Whitacre, Sprotte)
• Treated by: hydration, caffeine, epidural blood patch (gold standard)

---

9.2 Epidural Anaesthesia and CSF

• Epidural space is external to the dura — no direct CSF contact
• Accidental dural puncture (wet tap) → PDPH (especially with large Tuohy needle, 16–18G)
• Epidural blood patch: 20 mL autologous blood injected epidurally → seals CSF leak → clot increases epidural pressure temporarily and seals dural puncture → 85–90% success rate
• Total spinal: accidental intrathecal injection of epidural volume → high/total spinal block

---

9.3 Anaesthesia in Raised ICP

1. Maintain CPP ≥ 60 mmHg (MAP 80–100 + ICP control)
2. Reduce ICP by:
   • Reducing cerebral blood volume (CBV)
   • Reducing CSF volume
   • Reducing brain water/oedema

• Volatile anaesthetics have modest, variable effects on CSF production and resorption — "clinically far less important than their effects on CBF" (Barash, 9e)
• The dominant mechanism of ICP change with volatiles is through changes in CBV (cerebral blood flow), not through direct effects on CSF

---

9.4 Hyperventilation and ICP

• ↓ PaCO₂ → cerebral vasoconstriction → ↓ CBV → ↓ ICP
• Each 1 mmHg ↓ PaCO₂ → ↓ CBF by ~3%
• Target PaCO₂: 35 mmHg (routine) or 30–35 mmHg (acute ICP crisis)
• Prophylactic hyperventilation to PaCO₂ <30 mmHg is not recommended (cerebral ischaemia risk)
• Effect is temporary — CSF pH normalizes within 6–12 hours (bicarbonate compensates)

---

9.5 Drugs That Reduce CSF Production

---

9.6 CSF Drainage as a Neurosurgical Adjunct

• Lumbar CSF drain or ventricular drain (EVD): deliberate removal of CSF during surgery to reduce ICP and improve surgical access to deep structures
• External Ventricular Drain (EVD): catheter in lateral ventricle → continuous ICP monitoring + CSF drainage
• Used in: subarachnoid haemorrhage (SAH), TBI, post-craniotomy ICP management, hydrocephalus
• Anaesthetic relevance: sudden rapid CSF drainage → risk of brain herniation (especially with posterior fossa tumours and aqueductal obstruction)

---

9.7 Lumbar Puncture — Anaesthetic Considerations

• Performed at L3-4 or L4-5 (below conus medullaris, which ends at L1-2 in adults)
• Patient position: lateral decubitus (knees to chest) or sitting flexed
• Normal opening pressure: 70–180 mmH₂O
• Contraindications to LP:
  • Raised ICP with papilloedema or mass lesion (risk of transtentorial herniation)
  • Local infection at puncture site
  • Coagulopathy (INR >1.5, platelets <50,000, therapeutic anticoagulation)
  • Anticoagulant therapy — timing per regional anaesthesia guidelines (ASRA/ESRA)

• Blood-tinged CSF: traumatic tap (clears with successive samples) vs SAH (xanthochromia)
• CSF glucose: always compare with blood glucose — ratio <0.5 = bacterial meningitis

---

9.8 CSF and Neuraxial Drug Spread

• Lipophilic (fentanyl, sufentanil): rapid uptake into cord → segmental analgesia, minimal rostral spread, shorter duration
• Hydrophilic (morphine, diamorphine): slow uptake → rostral spread in CSF → prolonged analgesia + risk of delayed respiratory depression (up to 18–24 hours with morphine)
• Requires 24-hour respiratory monitoring after intrathecal morphine

---

9.9 Position and CSF Pressure

---

9.10 The Glymphatic System and POCD

• General anaesthesia increases glymphatic transport (periarterial space enlarges)
• Volatile agents reduce glymphatic function relative to sleep
• Dexmedetomidine (which produces sleep-like state) better preserves glymphatic flow
• Accumulation of amyloid-β and tau (AD pathology markers) may relate to impaired glymphatic clearance
• This underlies ongoing research linking choice of anaesthetic agent to long-term cognitive outcomes

---

10. Summary Table — Anaesthetic Effects on CSF/ICP

---

11. Key Viva Points

---

---

---

Autoregulation in the CNS — Types, Tests & Anaesthetic Importance

---

1. What is Cerebral Autoregulation?

However, the current view in Miller's Anesthesia (10e) emphasizes that the "static, flat plateau" of Lassen is outdated. Autoregulation is now understood as a dynamic, integrative process influenced by multiple interdependent variables — not a simple pressure-passive or pressure-independent switch.

---

2. Regulation of Cerebral Blood Flow — Overview

1. Myogenic (pressure autoregulation)
2. Metabolic / Chemical (CO₂, O₂, pH)
3. Neurogenic
4. Neurovascular coupling (flow-metabolism coupling)
5. Endothelial (NO, prostaglandins, endothelin)

---

3. Types of Autoregulation

---

3.1 Pressure Autoregulation (Myogenic Autoregulation)

• The Bayliss myogenic response — smooth muscle in cerebrovascular walls responds to transmural pressure changes
• ↑ perfusion pressure → active vasoconstriction (prevents overperfusion)
• ↓ perfusion pressure → active vasodilation (prevents underperfusion)
• Operates predominantly in pial arteries and arterioles

• The plateau is not flat — has a gentle upslope
• The range of pressure-passivity is wider than classically believed
• Lower and upper limits vary considerably between individuals
• Measured autoregulatory curves show variation in lower limit, plateau, and upper limit

---

3.2 Metabolic / Chemical Autoregulation

3.2.1 CO₂ Reactivity (PaCO₂)

• CO₂ freely diffuses across the blood-brain barrier
• Alters extracellular pH in the perivascular space
• H⁺-mediated NO release, adenosine, arachidonic acid metabolites, reactive oxygen species → vasodilation
• Acidosis (↑CO₂) → vasodilation → ↑CBF
• Alkalosis (↓CO₂) → vasoconstriction → ↓CBF

• Each 1 mmHg ↑ PaCO₂ → ↑ CBF ~3% (linear response ~20–80 mmHg)
• Below PaCO₂ ~25 mmHg: further CBF reduction is limited (vasospasm limit)
• Above PaCO₂ ~75–80 mmHg: response is attenuated

• Hypercarbia → cerebral vasodilation → attenuates autoregulatory response to hypertension → autoregulation becomes pressure-passive at lower MAP
• Hypocapnia → vasoconstriction → autoregulation maintained over wider MAP range

• Effect is not sustained — despite maintained arterial pH change, CBF returns toward normal in 6–8 hours as bicarbonate is extruded from CSF (pH normalizes)

• After prolonged hyperventilation: acute return to normal PaCO₂ → CSF acidosis → ↑CBF → ↑ICP
• After prolonged hypoventilation: acute normalization → CSF alkalosis → risk of ischaemia

• Moderate hypotension (MAP ↓ <33%): CO₂ responsiveness is significantly attenuated
• Severe hypotension (MAP ↓ ~66%): CO₂ responsiveness is abolished

---

3.2.2 Oxygen (PaO₂)

• PaO₂ 60–300 mmHg: little influence on CBF
• PaO₂ < 60 mmHg → rapid ↑ CBF (critical threshold corresponding to fall in SpO₂)
• Relationship between CBF and haemoglobin saturation is inversely linear
• Mechanism: hypoxia → local adenosine release, ↓pH, NO release → vasodilation
• Haematic hypoxia (e.g., anaemia): reduction in oxygen content with normal PaO₂ also triggers compensatory ↑CBF

• Haematocrit ↓ → ↓ viscosity + ↓ O₂ content → ↑CBF
• At low haematocrit, the benefit of ↓ viscosity is outweighed by ↓O₂ carrying capacity
• Optimal haematocrit for cerebral O₂ delivery: ~30–35% (balance of viscosity and O₂ content)

---

3.2.3 Metabolic/Flow-Metabolism Coupling (Neurovascular Coupling)

• Neuronal activity → release of K⁺, H⁺, adenosine, CO₂, arachidonic acid metabolites → perivascular vasodilation
• Astrocytes (glial cells) act as intermediaries → release vasoactive substances (NO, arachidonic acid derivatives)
• Potassium siphoning by astrocytes also contributes

• Volatile agents cause uncoupling — they increase CBF despite decreasing CMRO₂ (direct vasodilation exceeds metabolic suppression)
• IV agents (propofol, thiopental) maintain or improve coupling — they reduce CMRO₂ and CBF in parallel

• Progressive reduction in CMRO₂ and CBF with increasing barbiturate dose
• Maximum effect at isoelectric EEG (electrophysiologic silence)
• Beyond this point, no further reduction — only basal metabolic activity (cellular homeostasis) persists
• Additional barbiturate causes no further CMRO₂ or CBF reduction

---

3.3 Neurogenic Autoregulation

• Cerebral vessels receive sympathetic, parasympathetic, and sensory innervation
• Sympathetic: from superior cervical ganglia → travels with cerebral arteries → supplies large + penetrating arteries
• Parasympathetic: from pterygopalatine ganglion → cholinergic vasodilator input
• Sensory: trigeminal nerve → peptide-mediated (CGRP, substance P)

• Mild-to-moderate sympathetic stimulation causes little change in CBF — myogenic and metabolic mechanisms override
• Sympathetic influence becomes important in protecting against acute severe hypertension — prevents high pressure from reaching small vessels and causing haemorrhage (stroke prevention)

• Medications affecting sympathetic tone (β-agonists, α₂-agonists, ACE inhibitors, ARBs, calcium channel blockers) all modulate autoregulation
• Dexmedetomidine (α₂-agonist): reduces sympathetic outflow → affects cerebrovascular tone

---

3.4 Endothelial Autoregulation

---

4. Static vs Dynamic Autoregulation — Detailed Comparison

---

5. Tests of Cerebral Autoregulation

5.1 Transcranial Doppler (TCD) — Most Used Clinically

• Scale of 0–9 (Tiecks et al.)
• 0 = complete pressure-passivity (no autoregulation)
• 9 = perfect autoregulation
• ARI ≥ 4 considered normal; ARI < 4 = impaired

---

5.2 Pressure Reactivity Index (PRx) — ICP-Based

• Continuous simultaneous recording of ABP and ICP
• Calculate rolling correlation coefficient between MAP and ICP over 5-minute windows
• PRx = Pearson correlation coefficient (MAP vs ICP)

• U-shaped relationship between PRx and CPP
• The MAP at the nadir (lowest PRx) = optimal CPP where autoregulation is most effective
• Managing TBI patients to CPPopt is associated with better outcomes

---

5.3 Cerebral Oximetry Index (COx / TOx)

• Correlation between MAP and regional cerebral oxygen saturation (rSO₂) measured by Near-Infrared Spectroscopy (NIRS)
• COx = correlation coefficient between MAP and rSO₂

• COx near 0 or negative = intact autoregulation (rSO₂ buffered from MAP changes)
• COx > 0.3–0.4 = impaired autoregulation
• Used in cardiac surgery, TBI — can derive optimal MAP from U-shaped COx-MAP curve

---

5.4 Laser Doppler Flowmetry

• Continuous cortical CBF measurement
• Invasive (intraoperative) or perioperative
• Provides real-time CBF data for autoregulation assessment
• Used in experimental and intraoperative neurosurgical settings

---

5.5 Xe-133 Clearance / PET / MRI

• Gold standard measurement of absolute CBF
• Used in research and select clinical settings
• Static autoregulation curves derived from multiple BP levels

---

5.6 Near-Infrared Spectroscopy (NIRS) — Non-Invasive Bedside

• Non-invasive; measures oxygenated vs deoxygenated Hb changes with BP fluctuations
• Used in neonates, cardiac surgery, TBI
• Optimal MAP for cardiac surgery derived from NIRS-based autoregulation monitoring

---

6. Conditions Affecting Autoregulation

6.1 Physiological Shifts

6.2 Disease States

---

7. Anaesthetic Agents and Autoregulation

7.1 Intravenous Agents

7.2 Volatile Agents

• Mechanism: Direct cerebral vasodilation → pressure-passive CBF at high doses
• Net effect = sum of indirect vasoconstriction (via ↓CMRO₂) and direct vasodilation

At high anaesthetic doses (>1.5–2 MAC), CBF becomes essentially pressure-passive with all volatile agents.

---

8. The Flow-Metabolism Uncoupling with Volatiles

• Volatile agents decrease CMRO₂ but simultaneously cause direct vasodilation
• This is called "functional uncoupling" — CBF ↑ or → unchanged despite CMRO₂ ↓
• From a mechanistic standpoint, true uncoupling may not occur: there is still a coupled CMRO₂-driven vasoconstriction, opposed by a direct vasodilatory effect
• Net result depends on which effect dominates at a given MAC

---

9. Factors Modulating Autoregulation — Summary

---

10. Clinical Applications of Autoregulation in Anaesthesia

10.1 Blood Pressure Management Intraoperatively

• Within autoregulatory range: BP changes have minimal CBF effect
• Outside autoregulatory range (pressure-passive): every MAP change directly changes CBF and ICP
• Vasopressor choice matters:
  • Phenylephrine (α₁-agonist): ↑ SVR → may ↓ CO → compromised CBF (especially with bolus)
  • Ephedrine (α+β): maintains or ↑ CO + MAP → better CBF maintenance
  • After volume optimization, prefer agents that maintain/increase both CO and MAP

10.2 Anaesthetic Agent Selection for Neurosurgery

• Prefer TIVA (propofol + opioid) for raised ICP → preserved autoregulation, ↓ CBV
• If volatile required: sevoflurane ≤ 1 MAC + normocapnia → autoregulation maintained
• Avoid desflurane in patients with raised ICP
• Ketamine: avoid in raised ICP (uncoupling, ↑ CBF, ↑ CMRO₂)

10.3 Targeted MAP Management — CPPopt

• In TBI patients: use PRx monitoring to identify CPPopt
• Manage MAP to keep CPP within optimal range where PRx is most negative
• Prevents both: ischaemia (CPP too low) and hyperaemia/oedema (CPP too high)

10.4 CO₂ Management

• Normocapnia (PaCO₂ 35–40 mmHg): standard
• Hyperventilation (PaCO₂ 30–35 mmHg): for acute ICP crisis only — bridge therapy
• PaCO₂ < 30 mmHg: risk of ischaemia (vasoconstriction beyond safety threshold)
• Chronic hyperventilation not sustained — pH normalizes in 6–8 hours

10.5 Cardiac Surgery

• Prolonged CPB → potential impairment of autoregulation
• NIRS-based autoregulation monitoring (COx/HVRx) allows determination of individual optimal MAP
• Above the ULA: risk of cerebral hyperaemia, oedema, intracerebral haemorrhage
• Below the LLA: risk of ischaemia, cognitive dysfunction, stroke

---

11. Key Viva Questions

---

---