Clinical aspects of cerebral perfusion & ICP Flashcards by Heo Mun Ho

Cerebral blood flow is what percentage of cardiac output

15%

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Brain ischaemia occurs at what perfusion rate

20ml/100g tissue/min

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What pressure determines cerebral blood flow at any given time

Cerebral perfusion pressure

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Cerebral perfusion pressure (CPP) =

MAP - ICP (intracranial pressure)

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MAP =

Diastolic pressure + 1/3 pulse pressure

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Increased ICP causes cerebral perfusion pressure to increase or decrease

Decrease

Remember CPP = MAP - ICP

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Factors that regulate cerebral blood flow (3)

CPP
Conc. of arterial PCO2
Conc. of arterial PO2

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Weight of brain

1300-1400g

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Name 3 types of oedema

Vasogenic (extracellular)
Cytotoxic (intracellular)
Interstitial

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Vasogenic oedema

what is it due to
location of oedema
composition of oedema
extracellular fluid levels increased or decreased
effect of steroids
effect of mannitol

Increased capillary permeability
Mainly white matter
Plasma filtrate
Increased
Effective
Effective

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Cytotoxic oedema

what is it due to
location of oedema
composition of oedema
extracellular fluid levels increased or decreased
effect of steroids
effect of mannitol

Cellular swelling (of neurons and glial cells) due to failure of Na/K pump

Grey and white matter

Increased intracellular water and sodium

Decreased

No effect of steroids

Mannitol effective

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Interstitial oedema

what is it due to
location of oedema
composition of oedema
extracellular levels increased or decreased
effect of steroids
effect of mannitol

increased brain water due to impairment of absorption of CSF

periventricular white matter

CSF based

Extracellular fluid increased

No effect of steroids

Mannitol sometimes has an effect, sometimes doesn’t

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Causes of vasogenic oedema (4)

Tumour
Abscess
Late stage infarction
Trauma

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Causes of cytotoxic oedema (2)

Early stages of infarction

Water intoxication

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Cause of interstitial oedema

Hydrocephalus (communicating or non-communicating)

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What condition do you get interstitial oedema

Hydrocephalus

Percentage of brain substance, blood and CSF within the cranium

80%
10%
10%

What is the munro-kelly doctrine

Pressure-volume relationship that aims to keep a dynamic equilibrium among the essential non-compressible components inside the rigid compartment of the skull

Essentially meaning that the sum of the volumes of brain substance , CSF, and intracranial blood is constant (i.e. total intracranial volume is always constant)

When a new intracranial mass is introduced, what happens in respect to the munro-kelly doctrine

a compensatory change in volume must occur through a reciprocal decrease in venous blood or CSF to keep the total intracranial volume constant

Define compliance

change in volume observed for a given change in pressure

Define elastance + what does the term mean in association with intracranial masses

change in pressure observed for a given change in volume (INVERSE OF COMPLIANCE)

Refers to the accommodation to outward expansion of an intracranial mass

Homeostatic compensatory mechanisms in response to intracranial masses (2)

1) Venous system can collapse to squeeze blood out jugular veins or emissary and scalp veins
2) CSF can be displaced into subarachnoid space

What happens when homeostatic compensatory mechanisms in response to intracranial masses have been exhausted

The innate homeostatic pressure-buffering mechanisms keep compliance flat until a “critical volume” is reached

After this critical volume, small changes in volume produce significant increase in pressures, and intracranial hypertension naturally follows

As compliance decreases, does ICP increase or decrease

Increase

What is the cushing's reflex

AKA vasopressor (anti-hypotensive) response physiological response to raised ICP (causing decreased CPP), resulting in CUSHING’S TRIAD

What does the Cushing's triad consist of

Hypertension Irregular breathing Bradycardia

Cushing's reflex comes about due to raised ICP (causing decreased CPP), the decreased CPP activates what system (1) which brings about what response (2) to cause the cushing's triad

AUTONOMIC NERVOUS SYSTEM Sympathetic response – a1 adrenergic receptors stimulated to cause HYPERTENSION & TACHYCARDIA Aortic baroreceptors stimulate vagus nerve --> BRADYCARDIA

Management of raised ICP (6)

Head elevation - to improve venous return to heart Mannitol/hypertonic saline - short acting Hyperventilation - to decrease CPP, short acting Barbiturate coma - to decrease cerebral metabolism & CPP Surgical decompression Brain tissue oxygenation monitoring with probe

Normal ICP range

7-15 mmHg

3 types of ICP pulse waveforms (Correlate to arterial pressure)

P1 wave = percussion wave P2 wave = tidal wave P3 wave = dicrotic wave

What are Lundberg A waves

Steep increases in ICP lasting for 5 to 10 minutes. They are always pathological and represent reduced intracranial hypertension indicative of early brain herniation

What are Lundberg B waves

oscillations of ICP at a frequency of 0.5 to 2 waves/min and are associated with an unstable ICP

What are Lundberg C waves

oscillations with a frequency of 4-8 waves/min. They have been documented in healthy subjects and are probably caused by interaction between the cardiac and respiratory cycles. Related to waves of systemic BP

What is cerebral autoregulation

ability to maintain constant blood flow to the brain over a WIDE RANGE OF CPP (50-150mmHg)

If CPP low, what do cerebral arterioles do

dilate to compensate for the decreased pressure and vice versa if CPP is high

In some pathologies, cerebral blood flow can't be autoregulated (ability to maintain constant blood flow to the brain over CPP range of 50-150mmHg) - describe some of these pathologies (3)

if CPP exceeds 150mmHg, such has in HYPERTENSIVE CRISIS, the autoregulatory system fails as it’s not within the range Toxins like CO2 can cause diffuse cerebrovascular dilation and inhibit proper autoregulation During first 4-5 days of head trauma, many patients can experience disruption in cerebral autoregulation