2021A02 Describe the individual effects of propofol and sevoflurane on the cerebral metabolic rate of oxygen consumption (CMRO2) and cerebral blood flow (CBF).

 

Sevoflurane:

CMRO2 vs MAC

·  Dose-dependent reduction in electrophysiological function (60% of total)

·  No effect on basal function (40% of total) – only reduced by hypothermia

Diagram Description automatically generated

·  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 wins

Other

·  Luxury perfusion: due to ↓CMRO2 but ↑CBF

·  Hypoventilation: ↑PaCO2 may cause further vasodilatation (if spont vent)

 

 

Propofol:

CMRO2 vs Ce

·  Same as for sevoflurane

CMRO2 vs CBF

·  Coupling of CBF and CMRO2 unaffected

CBF vs Ce

·  Dose-dependent ↓CBF (ideal agent in intracranial hypertension)

·  Note exponential decay

Other

·  ↑↑Propofol Ce -> ↓SVR -> ↓mAP -> ↓perfusion

o   Autoregulation exhausted if mAP <50mmHg

 

 

Feedback welcome at ketaminenightmares@gmail.com

 

 

*Addendum

·       This graph is from Miller’s Anesthesia, Chapter 11

·       The data are from Anesthesiology (https://pubmed.ncbi.nlm.nih.gov/3740503/)

·       At 0.5 MAC, mean values for local blood flow were reduced in every grey matter tissue

·       However, none of the individual changes was statistically significant due to a large standard deviation

·       My personal opinion is:

o   It is unlikely that a universal decrement in mean blood flow between 0 MAC and 0.5 MAC was due to chance

o   Statistical significance would have been reached had sample sizes been larger

o   These findings are consistent with the curves for CBF vs CMRO2 and CMRO2 vs MAC

·       The alternative opinion is:

o   There is no evidence for significant change in CBF between 0 MAC and 1 MAC

o   In this range, the indirect vasoconstriction (via ↓CMRO2) and the direct vasodilation roughly cancel each other out

o   This is consistent with the leftward/upward shift in the CBF vs mAP curve that occurs with any concentration of volatile anaesthetic

o   See the Eger-Stoelting version below:

 

 

Special thanks to Dr. Stan Tay for his insights.