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).
Examiner Report
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
Both agents decrease CMRO2 by inhibition neuronal activity through enhanced GABA mechanisms. The maximum reduction is 50-60% because a baseline of 40% CMRO2 is required for neuronal maintenance.
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.
Discussions of other physiological or pathological factors affecting CBF and CMRO2 not relating to the effect of propofol or sevoflurane did not attract marks.
Model Answer
Sevoflurane
| Relationship | Detail |
|---|---|
| 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 |
| Other | - Luxury perfusion: Due to ↓ CMRO2 but ↑ CBF - Hypoventilation: ↑ PaCO2 may cause further vasodilatation (if spont vent) |
Propofol
| Relationship | Detail |
|---|---|
| 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 |
| Other | - ↑↑ Propofol Ce → ↓ SVR → ↓ MAP → ↓ Perfusion - Autoregulation exhausted if MAP ≤50mmHg |