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Neuro > Cerebral Blood Flow & ICP > Flashcards

Cerebral Blood Flow & ICP Flashcards

1
Q

This lecture deals with:

A
  • Cerebral Blood flow and its determinents
  • Cerebral Oedema
  • ICP & Compliance
  • ICP Monitoring
  • Cushing’s (Vasopressor) reflex
  • Cerebral Herniation
  • Management of Raised ICP
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2
Q

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

A
  • ~60ml/100gtissuemin

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

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3
Q

What factors affect Cerebral blood flow?

A
  • Cerebral Perfusion Pressure (MAP - ICP)

- PaCO2 & PaO2

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4
Q

How do arterial O2 & CO2 affect cerebral blood flow?

A

High PaCO2 causing cerebral arteries to dilate

High PaO2 causes cerebral arteries to constrict

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5
Q

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

A

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

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6
Q

In what cases does Cerebral Autoregulation fail?

A

When the capacity for autoregulation fails e.g. stroke

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

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7
Q

What is the monro-kelly doctrine

A

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.

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8
Q

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

A

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

This is called “Compliance”

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9
Q

What does a high vs low compliance mean?

A

High compliance= large change in volume per small change in pressure - up to ICP of 15mmHg

Low compliance= small change in volume per high change in pressure- up to ICP of 25mmHg

NO compliance= No change in volume per high change in ICP- above 25mmHg ICP

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

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10
Q

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

A

The Critical Volume

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11
Q

What are the types of cerebral oedema?

A
  • Vasogenic
  • Cytotoxic
  • Interstitial
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12
Q

Describe vasogenic cerebral oedema?

A

Local breakdown of the BBB, usualy traumatic.

Allows fluid to pass into the extracellular spaces of the brain- more plasma proteins in the fluid

Takes place in White matter
Due to metastasis, trauma, infarction
Steroids work on it
Mannitol works on it

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13
Q

Describe Cytotoxic Cerebral Oedema?

A

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

Occurs in grey and white matter
Mannitol effective
Increased amounts of water and sodium

E.g. during infarction

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14
Q

Describe Interstitial Oedema?

A

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

Related to communicating hydrocephalus

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15
Q

What would we see when monitoring ICP?

A

A three peaked Waveform

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16
Q

What are the 3 peaks in an ICP waveform?

A

P1 - Percussive Wave - Arterial pulsation

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

P3 - Dicrotic Wave - Venous Pulsation

17
Q

Describe a normal ICP waveform?

A

3 peaks of decreasing size with roughly equal distances between

18
Q

Describe an abnormal ICP waveform of increased arterial pulsation?

A

3 peaks of decreasing size

The 1st is much bigger than the other two

19
Q

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

A

3 Peaks
P2 is taller than P1

Indicates compliance has failed due to added mass or congestion

20
Q

What are A, B, C waves?

A

Types of abnormal ICP waveforms.

A waves:

  • abrupt elevation of the whole waveform lasting minutes to hours.
  • Amplitude between 50-100mmhg

B waves:

  • Lasting for a just minutes
  • Due to variations in breathing rythm
  • 0.5-2 amplitude per minute
  • not related to a decrease in CPP- necessarily

C waves:

  • Individual raised waves
  • Related to systolic blood waves
  • small amplitude
21
Q

What is Cushing’s Reflex?

A

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

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

22
Q

How does the Cushing’s Reflex occur?

A

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

23
Q

What are the main types of brain herniation?

A
  • 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)
24
Q

How doe we manage Intracranial Hypertension?

A
  • Elevate Head
  • Mannitol or Hypertonic Saline
  • Hyperventilation
  • Barbiturate Coma
  • Surgical Decompression
  • Brain tissue Oxygenation Monitoring
  • Micro-Dialysis
25
Q

Function of mannitol & Hypertonic saline?

A

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

26
Q

How does hyperventilation help ICP?

A

Prevents Hypercapnia, causing cerebral arteries to constrict

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

27
Q

How does a baribiturate coma help raised ICP?

A

Lowers brain metabolism thus decreasing blood flow

28
Q

How does brain tissue oxygen monitoring work?

A

A probes inserted to measure oxygenation of the tissue directly

29
Q

How does micro-dialysis work?

A

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

Neuro (20 decks)

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