Neurophysiology - Cerebral Blood flow

Question 1

What is normal Cerebral Blood flow

Answer 1

50 ml / 100g brain tissue / minute

Question 2

What is the cerebral perfusion pressure and what are the equations

Answer 2

CPP = MAP - ICP

CPP = CBF x CVR

CVR - Cerebral Vascular Resistance

Question 3

Describe using numbers what happens to the brain as cerebral perfusion drops

Answer 3

CBF (ml / 100 g / min)

< 50 - Acidosis
< 40 - impaired protein synthesis
< 30 - Oedema
< 20 - Membrane pumps fail --> loss of ion gradients
< 10 - Cell death

Question 4

What is Flow metabolism Coupling

Answer 4

CBF is diverted to most metabolically active regions

E.g.
Grey matter - 70 ml / 100g / min
White matter - 20 ml / 100 g / min

CO2 / K+ / H+ / Adenosine –> Cerebral VD in metabolically active tissue

Question 5

Which factors affect global cerebral blood flow (CBF)

Answer 5

1. Status epilepticus / Sepsis –> Increased CMRO2 and CBF
2. Increased PaCO2 (max 8 kPa) –> increase CBF
3. Decreased PaCO2 (min 3.5 kPa) –> decrease CBF
4. Decrease PaO2 < 8 –> increase CBF
5. Hct increase –> decrease CBF (ideal Hct to maintain DO2 is 0.3)

Question 6

Describe the effects of intravenous general anaesthetic agents on cerebral blood flow

Answer 6

Propofol / thiopental / Etomidate decrease CMRO2 –> decrease CBF

Ketamine increase CMRO2 –> increase CBF

Question 7

Describe the effects of volatile general anaesthetic agents on cerebral blood flow

Answer 7

Uncouple CMRO2 and CBF

All decrease CMRO2 (expected to decrease CBF) but also case cerebral arteriolar vasodilatation which increases CBF:

0. 5 MAC –> effect on CMR exceeds Cerebral VD –> decrease CBF
1. 0 MAC –> both effects are equal CBF is unchanged
2. 5 MAC –> Vasodilation > CMRO2 reduction which leads to increased CBF

Question 8

Which Volatile anaesthetic has the greatest and which has the lowest propensity to induce Cerebral arteriolar vasodilation

Answer 8

Halothane - greatest

Sevoflurane lowest

Question 9

How does N2O affect CBF

Answer 9

Potent cerebral vasodilator AND also increases CMR

Significant increase in CBF

Question 10

What is the effect of opioids on CBF

Answer 10

None direct but will suppress resp center Ve in spontaneously breathing patients –> hypercapnoea and increased CBF

Question 11

What are the two most common ways of measuring cerebral blood flow. Describe these techniques

Answer 11

1. Transcranial doppler ultrasonography
   - By far the most common method
   - Doppler effect used to determine the velocity of blood in the middle cerebral artery
   - CBF in one half of the brain can be estimated.
   - Can also be used to detect emboli during carotid endarterectomy and to diagnose vasospasm following subarachnoid haemorrhage
2. Jugular bulb cathetrization
   - Dilatation of jugular just below base of skull
   - Seldinger catheterization cranially
   - Sample blood for: O2 tension / HbO2 sats ? lactate.
   - Measure overall adequacy of CBF to ipsilateral side of the brain
   - No information about regional blood flow