Clinical Aspects of Cerebral Perfusion and ICP

Question 1

How much blood flow do white and grey matter receive per minute? What level of blood flow do ischaemia and permanent damage usually occur at?

Answer 1

• Grey matter: 75mL/100g/minute
• White matter: 45Ml/100g/minute
• Ischaemia: 20mL/100g/minute
• Permanent damage: 10mL/100g/minute
  (brain = 15% of CO)

Question 2

What are the most significant factors determining cerebral perfusion pressure? What is it determined by?

Answer 2

• CPP: cerebral perfusion pressure

- CPP = MAP - ICP

Question 3

What causes cerebral perfusion pressure to decrease?

Answer 3

Increased intracranial pressure (ICP)

Question 4

Factors that regulate cerebral blood flow under physiological conditions?

Answer 4

• CPP
• Concentration of arterial CO2
• Arterial PO2

Question 5

What is cerebral autoregulation?

Answer 5

• The ability to maintain adequate blood flow to the brain across a large range of CPP (50-150 mmHg). Done via constriction / dilation of arterioles
• CPP = MAP - ICP

Question 6

What is vasogenic oedema?

Answer 6

Oedema in the cerebrum due to disruption of the blood brain barrier (can be caused by too high of CPP)

Question 7

What is cerebral oedema often a cause of?

Answer 7

Intracranial hypertension

ICP is higher so MAP has to increase to maintain CPP?

Question 8

What forms the basis of the blood brain barrier? (BBB)

Answer 8

• Tight junctions between the endothelium of cerebral arterioles
• Astrocytic foot processes surround brain capillaries and induce the formation of these tight junctions during development

Question 9

What materials are usually naturally transported across the endothelial cells of the BBB?

Answer 9

• Lipid soluble substances (passive)

- Amino acids and sugars via carrier mediated transport

Question 10

What is the Monro-Kelly Doctrine? (cerebral volume)

Answer 10

when a new intracranial mass is introduced, there must be an accompanying decrease in CSF or venous blood volume to keep ICP constant

Question 11

What is compliance?

Answer 11

• Change in volume for a given change in pressure

- dV / dP

Question 12

What is elastance?

Answer 12

• Change in pressure observed for a given change in volume
• dP / dV

Represents the accommodation to outward expansion of an intracranial mass

Question 13

What are the mechanisms for maintaining homeostasis of ICP? Over what sort of pressure range are they effective?

Answer 13

• Venous collapse: squeezes blood out via jugular & emissary / scalp veins
• CSF displacement: out through foramina of Luschka and Magendie into spinal subarachnoid space
• Mechanisms work over pressure changes of about 8-15mmHg

Question 14

Once the innate homeostatic mechanisms regulating ICP are exhausted what happens?

Answer 14

• Once a “critical volume” is reached, additional small changes in volume produce significant changes in pressure
• ICP rises and intracranial hypertension ensues

Question 15

What is an extraventricular drain (EVD) used for?

Answer 15

• Treatment of hydrocephalus and elevated ICP

- Monitoring intracranial pressure

Question 16

What is Cushing’s Reflex?

Answer 16

• AKA Vasopressor response
• It is the nervous response to raised ICP, consisting of hypertension, irregular breathing and bradycardia
• Often seen at the terminal stages of brain trauma, may indicate brain herniation

Question 17

Explain the mechanism of the Cushing Reflex

Answer 17

• ICP > MAP, cerebral arteries compressed = ischaemia
• Sympathetic response: alpha-1 adrenergic compresses arterioles and induces tachycardia
• Baroreceptors detect increased MAP & induce bradycardia via vagus nerve
• Herniation / distortion of the medulla can result in irregular breathing

Question 18

Function of the Cushing Reflex?

Answer 18

It is a last ditch effort to maintain adequate CPP in the brain

Question 19

What are some different ways to manage increased ICP?

Answer 19

• Barbituate coma: decreases cerebral metabolism

- Surgical decompression (EVD)