Neurophysiology - Cerebral Blood Flow

Question 1

Name the main arteries that supply the brain and state the approximate proportion of the brain supplied by these vessels

Answer 1

Internal carotid arteries x 2 –> supply 80% (40% x 2)
Vertebral arteries x 2 –> supply 20% (10% x 2)
(the two vertebral arteries fuse to form the basilar artery)

Question 2

What are ‘watershed areas’

Answer 2

Watershed locations are those border-zone regions in the brain supplied by the major cerebral arteries where blood supply is decreased –> these overlapping areas determine the severity of focal ischaemia following systemic hypotension, focal vasospasm and major vessel occlusion.

Question 3

What is mean cerebral blood flow and state the CBF at which neurons can survive

Answer 3

Cerebral blood flow ± 50 ml/100g/minute

CBF 18 -25 ml/100g/minute –> nerves survive but don’t function

CBF < 18 ml/100g/minute –> nerves die

Question 4

In which settings is venous drainage to the brain relevant to the anaesthetist

Answer 4

1. Jugular venous bulb SaO2 monitoring

2. Ensuring venous drainage in RICP to optimise CPP

Question 5

Which sinuses form the internal jugular

Answer 5

Transverse Sinuses

Inferior Petrosal Sinuses

Question 6

What proportion of blood in the internal jugular comes from the ipsilateral and contralateral sides of the brain

Answer 6

Right internal jugular: 70% ipsilateral and 30% contralateral

Question 7

What percentage of the blood in the jugular veins is from extracranial sources?

Answer 7

3%

Question 8

What percentage of adults have unilateral transverse sinus drainage?

Answer 8

5 - 12%

Question 9

What are normal jugular venous SaO2 and what is implied if this is lower or higher than this?

Answer 9

65%
Higher –> Hyperemia or failure of utilization
Lower –> indicative of ischaemia and are an indication for increasing CPP and assessing the effect on further jugular venous saturations.

Question 10

Draw the graph representing the effect of PaCO2 and PaO2 on Cerebral blood flow

Answer 10

See Page 1 of FCA workbook notes

Question 11

What effect do systemic vasodilators have on cerebral blood flow

Answer 11

Unchanged or increased CBF due to cerebral vasodilation

Question 12

What effects to alpha 1 blockers have on CBF

Answer 12

Minimal

Question 13

What effects do beta agonists have on CBF

Answer 13

Low dose - no effect

High dose - Increase CBF and CMR

Question 14

What effects do beta blockers have on CBF

Answer 14

Minimal

Question 15

What effects does dopamine have on CBF

Answer 15

Probable vasodilation. high dose –> vasoconstriction

Question 16

Which anaesthetic agents decrease CMR/CBF

Answer 16

1. Barbiturates
2. Propofol
3. Etomidate
4. Benzodiazepines (decrease CMR and increase CBF)

Question 17

Which anaesthetic agents increase CMR/CBF

Answer 17

Ketamine

Question 18

Which anaesthetic agents uncouple CMR

Answer 18

Inhaled agents –> luxury perfusion with POORLY MAINTAINED Autoregulation

Question 19

Which anaesthetic agents have minimal effect on CMR/CBF/Autoregulation

Answer 19

1. Morphine
2. Fentanil
3. Alfentanil
4. Sufentanil
5. Remifentanil

Question 20

Which anaesthetic agents change the effect of CO2 on CBF

Answer 20

None

Question 21

What specific effect do alfentanil, sufentanil and remifentanil have on the CNS and why

Answer 21

Increase ICP briefly if given as a stat dose. IVI infusion does not cause increase ICP. Sudden MAP change –> Cerebral VC as part of autoreg response. If given slowly no change to MAP.

Question 22

Compare the brain’s relative weight to its proportion of cardiac output and O2 consumption

Answer 22

2% of the body weight
15% of Cardiac output
- CBF = 50 ml/100g/min - 40ml to grey and 10ml to white matter
20% of total O2 consumption

Question 23

What is the O2 consumption of the brain

Answer 23

3 - 3.8 ml/100g/minute = 35 - 70 ml/minute

• 75% by neurons
• 10% by glial cells

Wide range is due to different activation states of the CNS

Question 24

List the normal values for the following:

1. CBF
2. CMRO2
3. CO to brain
4. Brain glucose utilization
5. ICP
6. CPP

Answer 24

1. CBF –> 50 ml/100g/minute
2. CMRO2 –> 50ml/100g/minute
3. CO to brain –> 20% (2% TBW)
4. Brain glucose utilization –> 5g/100g/minute
5. ICP –> 8 - 12 mmHg
6. CPP –> 80 - 100mmHg

Question 25

Describe the levels of CBF and CPP at which reduced and isoelectric activity occur. Include also the CBF and CPP at which irreversible brain death occurs

Answer 25

CBF
Normal: 50 ml/100g/minute
Slow EEG: 20 - 25 ml/100g/minute
Isolelectric EEG: 15 - 20 ml/100g/minute
Irreversible brain death: < 15 ml/100g/minute

CPP
Normal: 80 - 100 mmHg
Slow EEG: 40 - 50 mmHg
Isoelectric EEG: 25 - 40 mmHg
Irreversible brain death: < 25 mmHg

Question 26

What is the optimal viscosity for O2 blood delivery to the brain and how does viscosity affect CBF

Answer 26

Increased viscosity reduces cerebral blood flow. But O2 delivery depends on Hct.

Hct of 30 - 34 % is optimal viscosity for O2 delivery to the brain.

Question 27

How does noradrenalin affect cerebral autoregulation - draw the graph

Answer 27

It extends the plateau phase leading to a prolonged interval of stable CBF over a greater range of MAP. See page 4 FCA workbook.

Question 28

How does the administration of vasodilators such as hydralazine/captopril affect cerebral autoregulation

Answer 28

These agents shorten the plateau phase leading to a diminished range of MAP for which CBF remains stable. Page 4 FCA workbook

Question 29

What factors impair cerebral autoregulation

Answer 29

1. Sepsis
2. Acidosis
3. Hypoxia
4. Intracranial disease (TBI/SAH)
5. Epilepsy
6. Anaesthesia (But autoreg maintained with MAC < 1 for isoflurane and less than 1.5 for sevoflurane)

Question 30

What is the “paradoxical CBF effect’ or ‘steal effect’ related to changes in CBF with PaCO2

Answer 30

High PaCO2 –> No increase CBF to diseased vessel. Increased CBF to non-diseased vessels. Therefore stealing blood from already relatively ischaemic areas.

Question 31

How is CBF influenced by hypoxia?

Answer 31

Hypoxia does not impact CBF significantly until the PaO2 drops below 8 kPa at which point CBF rises exponentially. FCA work book page 3.

Question 32

How does temperature affect CMRO2

Answer 32

For each drop of 1 degree C –> 5 - 7% decrease CMRO2

At 20 deg C –> Isoelectric EEG

Tb 37 - 42 deg C –> increased CBF

Tb > 42 deg C –> dramatic decrease CBF (protein degradation)

Question 33

What role does the autonomic nervous system play in cerebral autoregulation

Answer 33

About 10% of the VD/VC

Apparently protects BBB under conditions of very high MAP.