Why is 0.5 Minimum Alveolar Concentration (MAC) of sevoflurane used for craniotomies in adult or elderly patients with potential comorbidities like hypertension, diabetes, or cardiovascular disease?

Why 0.5 MAC Sevoflurane is Used for Craniotomies

The use of 0.5 MAC sevoflurane during craniotomies represents a deliberate strategy to minimize cerebral vasodilation and intracranial pressure elevation while maintaining adequate anesthesia depth, particularly when combined with other agents in a balanced technique.

Cerebrovascular Physiology Rationale

The primary reason for limiting sevoflurane to 0.5 MAC during craniotomies relates to its dose-dependent effects on cerebral blood flow and intracranial pressure:

• Sevoflurane causes cerebral vasodilation in a dose-dependent manner, which can increase cerebral blood volume and intracranial pressure 1
• At lower concentrations (0.5 MAC or less), sevoflurane maintains cerebrovascular autoregulation and CO2 responsiveness intact in patients with cerebrovascular disease 2
• Sevoflurane is less vasoactive than older volatile agents (halothane, enflurane, isoflurane, desflurane), making it more suitable for neurosurgical cases when used at appropriate concentrations 1

Balanced Anesthesia Approach

The 0.5 MAC concentration allows sevoflurane to be part of a balanced anesthetic technique:

• Low-dose volatile anesthetics (0.25-0.5 MAC) combined with opioids provide cardioprotection while minimizing cerebrovascular effects 3, 4
• This concentration provides adequate anesthesia when combined with intravenous agents (opioids, propofol) without excessive cerebral vasodilation 1
• The combination allows for rapid emergence at the end of surgery, which is critical for immediate neurological assessment 1

Intracranial Pressure Considerations

The dose limitation is particularly important in patients with reduced intracranial compliance:

• In patients with space-occupying lesions or elevated intracranial pressure, even modest increases in cerebral blood volume can be problematic 1
• Sevoflurane at 0.88 MAC (approximately 1.5% end-tidal) maintains cerebral blood flow at 28 ml/100g/min in patients with ischemic cerebrovascular disease, demonstrating preserved autoregulation 2
• Higher concentrations risk increasing intracranial pressure, while propofol decreases it—making the choice concentration-dependent 1

Monitoring and Depth of Anesthesia

The use of 0.5 MAC aligns with modern anesthesia monitoring standards:

• Age-adjusted MAC should be monitored during use of inhaled anesthetic drugs throughout craniotomy procedures 3
• Processed EEG monitoring should be considered to ensure adequate anesthesia depth while avoiding excessive volatile agent administration 3
• Avoiding volatile anesthetic overdose by close MAC monitoring is critical to prevent hypotension and other adverse effects 3

Clinical Decision Algorithm

For patients with normal intracranial pressure and no mass effect:

• Sevoflurane 0.5 MAC combined with opioids and/or propofol is appropriate 1
• Maintain cerebral perfusion pressure and avoid hypotension 2

For patients with elevated intracranial pressure or reduced intracranial compliance:

• Consider total intravenous anesthesia (propofol-based) as first choice 1
• If using sevoflurane, strictly limit to ≤0.5 MAC and ensure adequate hyperventilation 2, 1

For awake craniotomies:

• Propofol-based asleep-awake-asleep technique is preferred over volatile agents 5

Common Pitfalls to Avoid

• Do not exceed 0.5 MAC in patients with known intracranial hypertension or mass lesions, as this risks further ICP elevation 1
• Do not rely solely on volatile agents for anesthesia maintenance—use a balanced technique with opioids to minimize required volatile concentration 4
• Do not assume all volatile agents are equivalent—sevoflurane has less cerebrovascular reactivity than isoflurane or desflurane at equivalent MAC values 1
• Ensure adequate depth monitoring to avoid both awareness (with low concentrations) and excessive depth (with higher concentrations) 3