Effects of Anesthetic Agents on CBF, CMRO₂, and ICP
🔹 Introduction
Anesthetic agents significantly influence cerebral physiology by altering:
- Cerebral Blood Flow (CBF)
- Cerebral Metabolic Rate for Oxygen (CMRO₂)
- Intracranial Pressure (ICP)
These effects can protect or exacerbate brain injury depending on the clinical scenario, making their understanding crucial in neuroanesthesia and critical care.
🔹 Basic Principles
- CBF and CMRO₂ are coupled under normal physiology.
- Anesthetics can uncouple this relationship.
- Changes in CBF directly impact ICP, especially when compliance is low (e.g., in tumors, TBI).
- Autoregulation and CO₂ reactivity may be preserved or blunted depending on the agent.
🔹 Summary Table: Effects on CBF, CMRO₂, ICP
|
Agent/Class |
CBF |
CMRO₂ |
ICP |
Autoregulation |
|
Propofol |
↓ |
↓↓↓ |
↓ |
Preserved |
|
Etomidate |
↓ |
↓↓ |
↓ |
Preserved |
|
Thiopentone |
↓ |
↓↓↓ |
↓ |
Preserved |
|
Ketamine |
↑ or ↔ |
↑ or ↔ |
↑ |
Variable |
|
Benzodiazepines |
↓ |
↓ |
↔ or ↓ |
Preserved |
|
Dexmedetomidine |
↓ |
↓ |
↔ or ↓ |
Preserved |
|
Volatile agents |
|
|
|
|
|
(Sevo/Iso/Desflurane) |
↑ (dose-dependent) |
↓ (mild) |
↑ |
Impaired >1 MAC |
|
Nitrous Oxide (N₂O) |
↑ |
↑ |
↑ |
Impaired |
|
Opioids |
↔ or ↓ (high dose) |
↓ |
↔ or ↓ |
Preserved |
🔹 Intravenous Agents
🟢 Propofol-
Excellent for neuroprotection and TBI patients
🟢 Etomidate
- Adrenocortical suppression limits long-term use
- Useful in hemodynamically unstable patients
🟢 Thiopentone
- Uses: Neuroprotection, refractory intracranial hypertension, burst suppression for seizure control
🟠 Ketamine
- Controversial: Recent data suggest safe use in controlled ventilation
- May benefit hypotensive or asthmatic patients
🔹 Volatile Anesthetics
⚠️ General Trends (dose-dependent)
|
Parameter |
Low Dose (<1 MAC) |
High Dose (>1 MAC) |
|
CBF |
Mild ↑ or unchanged |
↑↑ (vasodilation) |
|
CMRO₂ |
↓ |
↓↓ |
|
ICP |
↔ or ↑ |
↑↑ |
🔹 CO₂ Reactivity and Autoregulation
- Most agents preserve CO₂ reactivity → Hyperventilation remains effective to reduce ICP
- Volatile agents >1 MAC may impair autoregulation
- IV agents like propofol and thiopentone preserve autoregulation and CO₂ responsiveness
🔹 Clinical Implications
|
Scenario |
Preferred Agent |
|
TBI with high ICP |
Propofol, thiopentone |
|
Neuroprotection |
Barbiturates, etomidate |
|
Awake craniotomy |
Dexmedetomidine |
|
Hypotensive patient |
Etomidate, ketamine (with caution) |
|
Neurovascular surgery (AVMs, aneurysms) |
Avoid N₂O, limit volatile agents |
|
Pediatrics (sevo favored) |
Sevoflurane with IV adjuncts |
🧠 Viva Flashcards
Q: Which anesthetic reduces CMRO₂ the most?
A: Thiopentone
Q: Which agent increases both CMRO₂ and CBF?
A: Ketamine, Nitrous Oxide
Q: Why is propofol ideal in raised ICP?
A: ↓ CBF, ↓ CMRO₂, ↓ ICP, preserves autoregulation
Q: Which volatile is safest in raised ICP?
A: Sevoflurane (at <1 MAC with controlled ventilation)
🔍 References
- Miller’s Anesthesia, 9th ed., Chapters on Cerebral Physiology & Neuroanesthesia
- Cottrell & Young’s Neuroanesthesia
- BJA Education: Effects of Anesthetics on Brain
- StatPearls: Anesthetics and Cerebral Hemodynamics
- Brash Neuroscience Review