Cerebral Blood Flow & ICP

Question 1

This lecture deals with:

Answer 1

• Cerebral Blood flow and its determinents
• Cerebral Oedema
• ICP & Compliance
• ICP Monitoring
• Cushing’s (Vasopressor) reflex
• Cerebral Herniation
• Management of Raised ICP

Question 2

What is the normal rate of cerebral blood flow and what constitutes ischaemia?

Answer 2

• ~60ml/100gtissuemin

Ischaemia is considered to be when it hits 20ml/100g/min

Question 3

What factors affect Cerebral blood flow?

Answer 3

• Cerebral Perfusion Pressure (MAP - ICP)

- PaCO2 & PaO2

Question 4

How do arterial O2 & CO2 affect cerebral blood flow?

Answer 4

High PaCO2 causing cerebral arteries to dilate

High PaO2 causes cerebral arteries to constrict

Question 5

Since cerebral perfusion pressure can vary how does the body ensure the brains blood supply remains fairly constant?

Answer 5

Cerebral Autoregulation

High CPP -> Arteriolar Constriction
Low CPP -> Arteriolar Dilation

This enables the body to maintan a constant Cerebral blood flow over CPPs of 50-150mmHg

Question 6

In what cases does Cerebral Autoregulation fail?

Answer 6

When the capacity for autoregulation fails e.g. stroke

When the CPP because too high or low e.g. hypertensive crisis

Question 7

What is the monro-kelly doctrine

Answer 7

It states that since the cranium is a rigid cage, any increase in intracranial volume (e.g. bleed) will will cause an increase in pressure.

Question 8

How does the brain compensate for increases in intracranial volume so the pressure doesnt increase?

Answer 8

By Forcing out venous blood and CSF into the Jugulars and spinal subarachnoid place.

This is called “Compliance”

Question 9

What does a high vs low compliance mean?

Answer 9

High compliance means an increase in intracranial volume wont increase ICP much.

As intracranial volume increases compensatory mechanisms are used up and Compliance falls until it fails

Question 10

What do we call the point at which cerebral compliance fails?

Answer 10

The Critical Volume

Question 11

What are the types of cerebral oedema?

Answer 11

• Vasogenic
• Cytotoxic
• Interstitial

Question 12

Describe vasogenic cerebral oedema?

Answer 12

Local breakdown of the BBB, usualy traumatic.

Allows fluid to pass into hte extracellular spaces of the brain

Question 13

Describe Cytotoxic Cerebral Oedema?

Answer 13

Damage to cells in the brain or altered metabolisms causing them to retain water
–> Intracellular Oedema

E.g. during infarction

Question 14

Describe Interstitial Oedema?

Answer 14

Disruption of the CSF-brain Barrier allowing CSF to flow into the interstitial spaces of the brain

Question 15

What would we see when monitoring ICP?

Answer 15

A three peaked Waveform

Question 16

What are the 3 peaks in an ICP waveform?

Answer 16

P1 - Percussive Wave - Arterial pulsation

P2 - Tidal Wave - Intracranial Compliance (Lower is more compliant)

P3 - Dicrotic Wave - Venous Pulsation

Question 17

Describe a normal ICP waveform?

Answer 17

3 peaks of decreasing size with roughly equal distances between

Question 18

Describe an abnormal ICP waveform of increased arterial pulsation?

Answer 18

3 peaks of decreasing size

The 1st is much bigger than the other two

Question 19

Describe an abnormal ICP waveform of a non-compliant system?

Answer 19

3 Peaks
P2 is taller than P1

Indicates compliance has failed due to added mass or congestion

Question 20

What are A, B & C waves?

Answer 20

Types of abnormal ICP waveforms.

A waves are abrupt elevation of the whole waveform lasting minutes to hours

B waves are the same lasting for a just minutes

C waves are individual raised waves

Question 21

What is Cushing’s Reflex?

Answer 21

An end stage response to raised ICP, when ICP exceeds MAP

It results in a triad of Hypertension, Bradycardia & Irregular Breathing

Question 22

How does the Cushing’s Reflex occur?

Answer 22

ICP exceeds MAP

• > Compresses the cerebral arteries
• > Cerebral blood flow drops
• > Sympathetic system activated
• > Alpha-1 receptors trigger increased contraction strength and HR
• > Aortic baroreceptors detect rising BP and stimulate parasympathetic Vagus fibres
• > Bradycardia

Hence the Hypertension, bradycardia and irregular breathing

Question 23

What are the main types of brain herniation?

Answer 23

• Sub-falcine (Cingulate gyrus herniates under falx)
• Uncal (Temporal herniates over tentorium pressing down on it)
• Tonsilar herniation through foramen magnum
• Central or Transtentorial (Herniates through tentorium)

Question 24

How doe we manage Intracranial Hypertension?

Answer 24

• Elevate Head
• Mannitol or Hypertonic Saline
• Hyperventilation
• Barbiturate Coma
• Surgical Decompression
• Brain tissue Oxygenation Monitoring
• Micro-Dialysis

Question 25

Function of mannitol & Hypertonic saline?

Answer 25

Both increase blood volume (decrease viscosity) to increase cerebral blood flow without decreasing blood tonicity and so exacerbating cerebral oedema as pure fluids would

Question 26

How does hyperventilation help ICP?

Answer 26

Prevents Hypercapnia, causing cerebral arteries to constrict

This decreases CBF, lowering the blood in the cranial vault and so the ICP

Question 27

How does a baribiturate coma help raised ICP?

Answer 27

Lowers brain metabolism thus decreasing blood flow

Question 28

How does brain tissue oxygen monitoring work?

Answer 28

A probes inserted to measure oxygenation of the tissue directly

Question 29

How does micro-dialysis work?

Answer 29

Collecting molecular size peptides etc from the brain through special catheter
Allows you to monitor brain metabolism