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