Cerebral blood flow

Question 1

What factors affect cerebral blood flow?

Answer 1

CPP = MAP - ICP
Cerebral blood flow is impaired by (see figure 33-8 in Ganong 24th ed):
- raised ICP
- lowered MAP
- local cerebral arteriole constriction / thrombus
- viscosity (think about how fetal MCA Doppler reflects fetal anaemia when the flow is high, meaning low viscosity)

Question 2

What is the mechanism of the Cushing response?

Answer 2

Increase in ICP results in:
Reduced cerebral blood flow –> Ischaemia to Vasomotor Area –> Increased sympathetic output –> Raised BP –> stimulation of baroreceptors –> Lowered HR.
RR is also lowered.
Cushing triad: High BP, Low HR, Low RR.

Question 3

What is the Monroe-Kellie doctrine?

Answer 3

The internal volume of the skull is a fixed volume, containing blood (100mL), brain (1400g) and CSF (100mL). If there is any addition to the contents, there must be a compensatory decrease in the volume of one or more blood, brain and CSF.