Cerebral Physiology and Pathophysiology

Cerebral Blood Flow & Metabolism - Brain's Balancing Act

• Cerebral Blood Flow (CBF):
  • Normal: 50 mL/100g/min (≈15% Cardiac Output).
  • Thresholds: EEG changes <20-25; Irreversible damage <10 mL/100g/min.
  • Formula: $CBF = CPP / CVR$. (CPP = MAP - ICP or CVP).
• Cerebral Metabolic Rate (CMRO2):
  • Normal: 3.0-3.8 mL O2/100g/min.
  • Coupled with CBF (flow-metabolism coupling).
  • Temperature: CMRO2 ↓ by 7% per 1°C ↓ in body temperature.
  • Anesthetics: Most ↓ CMRO2 (e.g., propofol, barbiturates); Ketamine ↑ CMRO2.
• Autoregulation:
  • Maintains CBF constant over MAP range 50-150 mmHg.
  • Curve shifts right in chronic hypertension.
• Key CBF Modulators:
  • $PaCO_2$: Most potent. CBF changes 1-2 mL/100g/min per 1 mmHg $PaCO_2$ change (effective range 20-80 mmHg).
  • $PaO_2$: Significant CBF↑ only if $PaO_2$ < 50 mmHg.

⭐> Hyperventilation (↓$PaCO_2$) rapidly ↓CBF and can be used to acutely ↓Intracranial Pressure (ICP), but prolonged use risks cerebral ischemia.

Intracranial Pressure Dynamics - Skull's Squeeze

• Intracranial Pressure (ICP): Pressure within the cranium. Normal: 5-15 mmHg.
• Monro-Kellie Doctrine: Skull is a rigid box. $V_{Total} = V_{Brain} + V_{Blood} + V_{CSF} = Constant$.
  • Increase in one component requires decrease in another(s) to maintain normal ICP.
  • Compensation: CSF displacement, ↓ venous blood.
  • Decompensation: Small volume ↑ → large ICP ↑.
• Intracranial Hypertension (ICH): Sustained ICP > 20-25 mmHg.
  • Leads to ↓ Cerebral Perfusion Pressure (CPP), ischemia, herniation.
• ICP Waveform: P1 (percussion), P2 (tidal), P3 (dicrotic). P2 > P1 indicates ↓ compliance.
• Cushing's Triad: Hypertension, Bradycardia, Irregular Respirations. 📌 Mnemonic: HBI.

  ⭐ Cushing's triad is a LATE sign of severely increased ICP, indicating brainstem compression.

Cerebral Protection - Brain Shield Tactics

• Core Goal: Preserve neuronal integrity, prevent secondary brain injury.
• Key Strategies (Brain SHIELD 📌):
  • Supply Oxygen & Optimize Perfusion:
    • CPP: Target $CPP = MAP - ICP > \textbf{60-70 mmHg}$.
    • Oxygenation: PaO₂ $>\textbf{100 mmHg}$; Normocapnia (PaCO₂ 35-40 mmHg).
  • Hypothermia (Therapeutic):
    • Mild (32-34°C) for specific conditions (e.g., post-cardiac arrest).
  • ICP Control:
    • Osmotic agents: Mannitol (0.25-1 g/kg), hypertonic saline.
    • CSF drainage (EVD); surgical decompression.
  • Electrical Stability & Glucose:
    • Seizure prophylaxis (TBI).
    • Glycemic control.
  • Lower Metabolic Demand (↓CMRO₂):
    • Sedatives: Barbiturates, propofol (burst suppression).
  • Drugs (Specific):
    • Lidocaine: blunts ICP rise (airway).

⭐ Barbiturates (e.g., thiopental) induce "pharmacological coma," maximally reducing CMRO₂ by ~50%, decreasing excitotoxicity, offering potent neuroprotection.

Neuro-Pathophysiology Snippets - Anesthetic Alerts

• Raised ICP (>20 mmHg)
  • Normal: 5-15 mmHg.
  • Signs: Headache, vomiting, papilledema, ↓GCS.
  • Cushing's Triad (late): Hypertension, bradycardia, irregular respirations.
  • Anesthetic: Maintain CPP ($CPP = MAP - ICP$), avoid ICP spikes (smooth induction/extubation, no coughing).
• Cerebral Edema
  • Types: Vasogenic, Cytotoxic.
  • Rx: Head up, mannitol, hypertonic saline, hyperventilation (PaCO2 30-35 mmHg, transient).
• Cerebral Vasospasm
  • Post-SAH (days 4-14).
  • Rx: Maintain normovolemia, induced hypertension.
• Seizures
  • Intraop Rx: Propofol, thiopental, benzodiazepines.
  • EEG if high risk.

⭐ Cushing's triad (hypertension, bradycardia, irregular respirations) is a late sign of severely raised ICP, indicating brainstem compression.

High‑Yield Points - ⚡ Biggest Takeaways

Error: Failed to generate content for this concept group.