Cerebral O2 Supply

Question 1

3 components that make us cerebral o2 supply

Answer 1

CVR - cerebral vascular resistance. Influenced by vessel diameter and ICP. Vessel diameter in term is influenced by autoregulation and cerebral metabolic demand

CPP - cerebral perfusion pressure, BP of the brain. CPP=MAP-ICP. Normal CPP 60-100mmHg. MAP in the brain moderated by the ICP. Normal ICP 0-15mmHg

CBF - cerebral blood flow 750cc/min ~15% CO. CBF=CPP/CVR - Cerebral blow flow rates correspond directly to the metabolism of the cerebral tissue via changes in CVR and CPP

Question 2

Autoregulation

Answer 2

In the brain, when MAP >80, blood vessels constrict, inc CVR, thus decreasing CBF. When MAP <60, blood vessels dilate increasing CBF.

Thus CBF is maintained despite changes in MAP

Only works when:
CPP>60
ICP<30
MAP 60-100

Ie in brain injuries when these parameters are out of whack, the autoregulation breaks down and causes more problems than it fixes. Think shock for the brain, but different pathway. Blood flow no longer moderated based on demand, so need to modulate CPP with MAP goals to maintain adequate CBF and thus perfusion

Question 3

What significant about ICP waveform

Answer 3

Has 3 little bumps, the second bump P2 should be lower than the first bump P1 as it indicated intracranial compliance. So if second bump is higher, we know that intracranial compliance is lower, therefore even minor changes in MAP and other pressures will have an exaggerated effect on ICP. Can see ICP rising directly with MAP. That’s bad.

Question 4

How to manage ICP increase without decreasing o2 supply, essentially when all else fails

Answer 4

• Manage CO2 levels - can hyperventilate but only in short term and only keep CO2 within normal range
• decrease demand with sedation
• keep normothermic
• reduce environmental stimuli

Question 5

Normal range for SjO2

CERO2

Answer 5

55-75%
SaO2-SjO2 / SaO2 X100
Normal 25-35%