Cerebral perfusion

Cerebral perfusion

CBF = CPP/CVR
* CBR = Cerebral blood flow
* CPP = Cerebral perfusion pressure
* CVR = Cerebral vascular resistance

 

Factors affecting CBF via CVR

CVR affected by:

• Metabolic autoregulation
• Pressure autoregulation (myogenic)
• Chemical (PCO2)
• Neural control
  * Note that neural control affects CPP, probably more significantly than its effect on CVR

1. CBF vs CMRO2

CMRO2 = cerebral metabolic rate of oxygen consumption

• Linear
• "flow-metabolism coupling"
• Ketamine disrupts this coupling
• Extremely important in minute to minute regional control of CBF
  * May affect global CBF in situations like generalised seizure
• Can affect global CBF in cases such as thiopentone induction or generalised convulsions
• Metabolites involved: pH, K+, adenosine
  [BL8:p250]

2. CBF vs MABP

Between 50-60mmHg to 130-150mmHg

--> CBF stays constant
* i.e. curve is flat

• Above this range, CBF increase with MABP
• Below this range, CBF decreases with MABP
• This pressure autoregulation is by myogenic mechanism
• Pressure autoregulation may take a few minutes to reach maximal effect
  * Carotid sinus responds more rapidly
  * [KB2:p66]

Curve shifts to right

Due to:

• Chronic hypertension
• Acute sympathetic stimulation

Plateau moves up

Due to:

• Anaemia
• Increase in PaCO2

Neonate

• CBF vs MABP curve is very LEFT displaced
• Plateau is narrower
  * From 30mmHg to 90mmHg
• MABP in neonate is ??50mmHg

3. CBF vs PaCO2

• 4% change in CBF per mmHg PaCO2 change
• Linear relationship between PaCO2
  --> 20mmHg to 80mmHg
• When PaCO2>80mmHg, no further increase in CBF due to maximal vasodilation
  * via nitric oxide
• When PaCO2<20mmHg, no further decrease in CBF due to hypoxic vasodilation
• Brain adapt to changes in PaCO2 and CBF returns back to normal in 4-6 hours

4. CBF vs PaO2

When PaO2>50-60 mmHg

--> No change in CBF with PaO2 changes
* because not much change in O2 content due to the shape of oxygen dissociation curve

When PaO2 <50-60 mmHg

--> CBF increase as PaO2 decreases
* because of the shape of ODC
* i.e.O2 content drops significantly and CBF increases to compensate

 

Factors affecting CBF via CPP

CPP = MABP - (ICP or JVP, whichever is higher)

However,

MABP is used as an index because

• Normally much larger than ICP or JVP
• Easy to measure

Thus,

Changes in MABP, ICP or JVP can affect CPP, and thus CBF

Role of sympathetic nervous system

• No important in controlling CVR
• But vitally important in controlling MABP, thus CPP, thus CBF

e.g. carotid sinus baroreceptor reflex
--> Faster than pressure autoregulation of CBF via CVR

 

Other notes

Reduction in CBF may be beneficial because

• ICP is reduced
• Blood loss during operation is minimised