2021A Question 2
Describe the individual effects of propofol and sevoflurane on the cerebral metabolic rate of oxygen consumption (CMRO2) and cerebral blood flow (CBF).
35.9% of candidates achieved a pass in this question.
The major domains assessed in this question were:
- The effect of propofol on CMRO2
- The effect of propofol on CBF
- The effect of sevoflurane on CMRO2
- The effect of sevoflurane on CBF
Significant weight was given to a clear demonstration of:
- The triphasic nature of the MAC/CBF relationship for sevoflurane
- A clear description of the direct association of decreased CMRO2 caused by propofol with a proportional decrease in CBF due to flow metabolism coupling.
The commonest misconception was that a decrease in CBF associated with propofol was due to its systemic circulatory effects causing hypotension. This is only relevant at the lower limit of the auto regulatory curve. A number of candidates suggested that reduction in CBF caused by propofol was “prevented” because auto regulation “was preserved”. Some candidates suggested that the CMRO2 is determined by CBF, reflecting a major misunderstanding of cerebral physiology.
|CMRO2 vs MAC||
- Dose-dependent reduction in electrophysiological function (60% of total)
- No effect on basal function (40% of total) – only reduced by hypothermia
- Exponential decay → Greater effect at low dose
|CBF vs CMRO2||
- Dose-dependent vasodilatation (↓ L-Ca2+ activity, ↑ NO activity)
- Coupling of CBF and CMRO2 impaired (not ablated)
- Slope ∝ dose → Greater effect at high dose
|CBF vs MAC||
- At low concentration: Indirect vasoconstriction (via ↓ CMRO2) wins
- At high concentration: Direct vasodilatation more significant
- Luxury perfusion: Due to ↓ CMRO2 but ↑ CBF
- Hypoventilation: ↑ PaCO2 may cause further vasodilatation (if spont vent)
|CMRO2 vs Ce||
- As sevoflurane
|CMRO2 vs CBF||
- Coupling of CBF and CMRO2 unaffected
|CBF vs Ce||
- Dose-dependent ↓ CBF (ideal agent in intracranial hypertension)
- Exponential decay
- Autoregulation exhausted if MAP ≤50mmHg