Cerebral Blood Flow & ICP

Question 1

What is the normal rate of cerebral blood flow and what constitutes ischaemia?
Units: ml/g of tissue/min

Answer 1

• ~60ml/100g tissue/min

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

Question 2

What are 4 ways that the brain regulates cerebral blood flow?

What is the equation/formulae that shows the relationship between MAP, ICP and Cerebral Perfusion Pressure (CPP)?

Answer 2

• Autoregulation via myogenic mechanism
• PaCO2 & PaO2
• Cerebral metabolism
• Neurohumeral factors

Cerebral Perfusion Pressure (MAP - ICP)

Question 3

How do arterial O2 & CO2 affect cerebral blood flow? [2]

What is the relationship between vessel radius in the brain and cerebral blood flow?

Answer 3

High PaCO2 causing cerebral arteries to dilate

High PaO2 causes cerebral arteries to constrict

Larger vessel diameter, increased cerebral blood flow

Question 4

Describe the physiology of cerebral auto regulation or myogenic mechanism.
What happens when high CPP and what happens when low CPP
2 functions of the myogenic mechanism

Answer 4

Cerebral Autoregulation

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

This enables the body to maintain constant CBF over CPPs of 50-150mmHg [1] and prevents vascular hemorrhage [1] as it prevents high MAP from reaching smaller blood brain vessels

Question 5

In what cases does Cerebral Autoregulation fail? [3]

What happens when this mechanism is impaired? [2]

Answer 5

Stroke, SAH, hypertensive crises

When impaired the cerebrovascular system becomes pressure dependent [1] so increase in MAP leads to increase in CBF [1]

Question 6

What is the Monro-kelly doctrine [2]

Answer 6

It states that since the cranium is a rigid cage, any increase in intracranial volume [1] (e.g. bleed) will cause an increase in pressure [1]

Question 7

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

Answer 7

By forcing out venous blood and IJV and CSF into thecal sac.

This is called “Compliance”

Question 8

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

Answer 8

The Critical Volume

Question 9

What are the 3 types of cerebral oedema?

Answer 9

• Vasogenic
• Cytotoxic
• Interstitial

Question 10

Describe vasogenic cerebral oedema [2]

Answer 10

Local breakdown of the BBB, usually traumatic.

Allows fluid to pass into extracellular spaces of the brain

Question 11

Describe Cytotoxic Cerebral Oedema [2]

Answer 11

Damage to cells in the brain or altered metabolisms causing them to retain water
–> Intracellular Oedema
E.g. during infarction

Question 12

Describe Interstitial Oedema? [2]

Answer 12

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

Question 13

What would we see when monitoring ICP? [1]

Answer 13

A three peaked Waveform

Question 14

What are the 3 peaks in an ICP waveform?

Answer 14

P1 - Percussive Wave - Arterial pulsation

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

P3 - Dicrotic Wave - Venous Pulsation

Question 15

Describe a normal ICP waveform? [2]

Answer 15

3 peaks of decreasing size [1] with roughly equal distances between [1]

Question 16

Describe an abnormal ICP waveform of increased arterial pulsation? [2]

Answer 16

3 peaks of decreasing size [1]

The 1st is much bigger than the other two [1]

Question 17

Describe an abnormal ICP waveform of a non-compliant system? [2] What would this clinically suggest? [1]

Answer 17

3 Peaks [1]
P2 is taller than P1 [1]

Indicates compliance has failed due to added mass or congestion [1]

Question 18

What are A, B & C waves? [3]

Answer 18

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 19

What is Cushing’s Reflex? [2]
What is the triad that it presents with? [3]
A MEDICAL EMERGENCY

Answer 19

An end stage response to raised ICP [1] when ICP exceeds MAP [1]
It results in a triad of Hypertension, Bradycardia & Irregular Breathing

Question 20

How does Cushing’s Reflex occur? [5]

Explain the vicious cycle that this engenders [3]

Answer 20

When ICP exceeds MAP

• > Compresses the cerebral arteries, CBF drops
• > Sympathetic system activated
• > Aortic baroreceptors detect rising BP, MAP
• -> This stimulates parasympathetic Vagus fibres
• > Reflex Bradycardia occurs
• -> Compression of brainstem (respiratory centers in medulla) causes irregular breathing

The MAP rise > to ICP further increasing > further drop in CPP

Question 21

What are the main types of brain herniation? [4]

Answer 21

• 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 22

How doe we manage Intracranial Hypertension? [6]

Answer 22

• Elevate Head 30 degrees
• Mannitol or Hypertonic Saline
• Hyperventilation
• Barbiturate Coma
• Surgical interventions
• Brain tissue Oxygenation Monitoring
• Micro-Dialysis

Question 23

Function of mannitol & Hypertonic saline? [4]

Answer 23

Both increase blood volume (decrease viscosity) [1] to increase cerebral blood flow [1] without decreasing blood tonicity [1] and so exacerbating cerebral oedema as pure fluids would [1]

Question 24

How does hyperventilation help ICP? [4]

Answer 24

Prevents Hypercapnia [1] which causes cerebral arteries to constrict [1]
This decreases CBF [1] lowering the blood in the cranial vault and so the ICP [1]

Question 25

How does a baribiturate coma help raised ICP? [2]

Answer 25

Lowers brain metabolism [1] thus decreasing blood flow [1]

Question 26

How does brain tissue oxygen monitoring work?

Answer 26

Probes are inserted to measure oxygenation of the tissue directly

Question 27

How does micro-dialysis work? [2]

What does it monitor? [1]

Answer 27

Collecting molecular size peptides [1] etc from the brain through special catheter [1]
Allows you to monitor brain metabolism [1]

Question 28

If decreased MAP and increased ICP co-exist, what are the possible sequelae on CPP and the brain [2]

Answer 28

Significantly reduced CPP

Risk of brain ischemia

Question 29

Describe cerebral metabolism and neurohumeral factors as a mechanism that regulates CBF [5]

Answer 29

Cerebral metabolism: detection of substances in blood [1] passing through the BBB [1]
Neurohumeral factors: sympathetic tone on cerebral arteries [1] lead to mild tonic vasoconstriction [1] allowing for higher limits on auto regulation curve [1]

Question 30

Surgical interventions for raised ICP [3]

Answer 30

Craniotomy, evacuation of clot
Eternal ventricular drainage
Decompressive craniotomy

Question 31

Intracranial hypertension: approach to management - avoid pyrexia - reasons [2]

Answer 31

Avoid pyrexia - increases ICP and is an independent predictor of poor outcome after severe head injury

Question 32

Intracranial hypertension: approach to management - manage seizures - why and how, who

Answer 32

Why? Contribute to raised ICP
How? manage aggressively using standard anticonvulsant, consider prophylactic mx
Who? Children with significant head injury and neuroinfections are at increased risk