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Flashcards in Cerebral Blood Flow & ICP Deck (32)
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1
Q

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

A
  • ~60ml/100g tissue/min

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

2
Q

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)?

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

Cerebral Perfusion Pressure (MAP - ICP)

3
Q

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

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

A

High PaCO2 causing cerebral arteries to dilate

High PaO2 causes cerebral arteries to constrict

Larger vessel diameter, increased cerebral blood flow

4
Q

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

A

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

5
Q

In what cases does Cerebral Autoregulation fail? [3]

What happens when this mechanism is impaired? [2]

A

Stroke, SAH, hypertensive crises

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

6
Q

What is the Monro-kelly doctrine [2]

A

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]

7
Q

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

A

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

This is called “Compliance”

8
Q

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

A

The Critical Volume

9
Q

What are the 3 types of cerebral oedema?

A
  • Vasogenic
  • Cytotoxic
  • Interstitial
10
Q

Describe vasogenic cerebral oedema [2]

A

Local breakdown of the BBB, usually traumatic.

Allows fluid to pass into extracellular spaces of the brain

11
Q

Describe Cytotoxic Cerebral Oedema [2]

A

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

12
Q

Describe Interstitial Oedema? [2]

A

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

13
Q

What would we see when monitoring ICP? [1]

A

A three peaked Waveform

14
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

15
Q

Describe a normal ICP waveform? [2]

A

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

16
Q

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

A

3 peaks of decreasing size [1]

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

17
Q

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

A

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

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

18
Q

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

A

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

19
Q

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

A

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

20
Q

How does Cushing’s Reflex occur? [5]

Explain the vicious cycle that this engenders [3]

A

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

21
Q

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

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)
22
Q

How doe we manage Intracranial Hypertension? [6]

A
  • Elevate Head 30 degrees
  • Mannitol or Hypertonic Saline
  • Hyperventilation
  • Barbiturate Coma
  • Surgical interventions
  • Brain tissue Oxygenation Monitoring
  • Micro-Dialysis
23
Q

Function of mannitol & Hypertonic saline? [4]

A

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]

24
Q

How does hyperventilation help ICP? [4]

A

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]

25
Q

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

A

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

26
Q

How does brain tissue oxygen monitoring work?

A

Probes are inserted to measure oxygenation of the tissue directly

27
Q

How does micro-dialysis work? [2]

What does it monitor? [1]

A

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

28
Q

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

A

Significantly reduced CPP

Risk of brain ischemia

29
Q

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

A

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]

30
Q

Surgical interventions for raised ICP [3]

A

Craniotomy, evacuation of clot
Eternal ventricular drainage
Decompressive craniotomy

31
Q

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

A

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

32
Q

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

A

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