clinical aspects of cerebral perfusion and ICP Flashcards

1
Q

what % of cardiac output is cerebral outflow

A

15%

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2
Q

what is the normal cerebral blood flow average

A

55-60mL/100brain tissue per minute

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3
Q

when does ischaemia occur

A

when cerebral blood flow at 20ml/100g/minute

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4
Q

when does permanent damage occur

A

when cerebral blood flow drops below 10ml/100g/minute

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5
Q

what is the most important factor that determines cerebral blood flow at any given time

A

cerebral perfusion pressure (CPP)

= the effective blood pressure gradient across the brain

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6
Q

what does an increased ICP do to the CPP

A

causes it to decrease

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7
Q

what are the three factors that regulate cerebral blood flow under physiological conditions

A

CPP
concentration of arterial CO2
arterial PO2

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8
Q

what is cerebral auto regulation

A

the ability to maintain constant blood flow to the brain over a wide range of CPP

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9
Q

how does cerebral auto regulation occur

A

CPP low - cerebral arterioles dilate to allow flow at decreased pressure

CPP high - cerebral arterioles constrict to allow flow at increased pressure

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10
Q

when can the autoregulatory system fail

A

if CPP exceeds 150mm/Hg
- eg hypertensive crisis

vasogenic oedema from exudation fluid from the vascular system

toxins (eg CO2) diffuse cerebrovascular dilation and inhibits autoregualtion

during first 4-5 days of head trauma

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11
Q

what is cerebral oedema

A

state of increased brain volume as a result of an increase in water content

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12
Q

what can cerebral oedema cause

A

subacute to chronic intracranial hypertension

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13
Q

what is the blood brain barrier

A

barrier composed of astrocytic foot processes wrapping around a capillary endothelium composed of tight junctions

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14
Q

what is the specific role of the endothelial tight junctions in the brain

A

stop passive movement of substances to the neural tissues to protect i from toxic materials

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15
Q

what materials can be naturally transported across the BBB and how

A

lipid-soluble substances
- penetrate all capillary endothelial cell membranes in a passive manner

amino acids and sugars
- transported across the capillary endothelium by specific carrier-mediated mechanisms

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16
Q

what is the total volume of the brain/blood/CSF and what re their respective volumes

A

total = 1700mL

brain - 1400mL
blood - 150mL
CSF - 150mL

17
Q

what is the moro-kelly doctrine

A

When a new intracranial mass is introduced, a compensatory change in volume must occur through a reciprocal decrease in venous blood or CSF to keep the total intracranial volume constant

ie 100ml mass - 100ml decrease in blood/CSF in skull

18
Q

what is compliance

A

change in volume observed for a given change in pressure

dV/dP = change in volume/change in pressure

19
Q

what is elastance

A

inverse of compliance

change in pressure observed fro a given change in volume

dP/dV
= change in pressure/change in volume

**represents the accommodation to outward expansion of an intracranial mass

20
Q

how can blood be displaced from the brain

A

venous system collapses easily and squeezes venous blood out through the jugular veins or through the emissary and scalp veins

21
Q

how can CSF be displaced from the brain

A

displaced from the ventricular system through the foramina of Luschka and Magendie into the spinal subarachnoid space

22
Q

when can small changes in volume produce significant changes in pressure

A

when a critical volume is reached - any small increase in volume over this level will cause drastic increases in pressure

23
Q

how can ICP be measured

A

manometry

connected to electric monitor to see ICP waveforms

24
Q

what are the waves seen ICP manometry

A

Lundberg waves:
A waves
B waves
C waves

25
when are A waves seen
Abrupt elevation in ICP for 5 to 20 minutes followed by a rapid fall in the pressure to resting levels The amplitude may reach as high as 50 to 100 mm Hg
26
when are B waves seen
related to rhythmic variations in breathing Frequency of 0.5 to 2 waves per minute
27
what are C waves
Rhythmic variations related to waves of systemic blood pressure and have smaller amplitude
28
what is Cushing's reflex and what does it involve
a vasopressor response triad: hypertension irregular breathing bradycardia
29
what is the physiology that causes the triad in Cushing's response
Increased ICP more than MAP causes compression of cerebral arterioles Decreased CBF activates autonomic nervous system Sympathetic response: alpha-1 adrenergic receptors -> Hypertension and tachycardia Aortic baro-receptors stimulate vagus nerve -> Bradycardia Bradycardia also due to mechanical distortion of medulla
30
how is raised ICP managed
Head end elevation: facilitate venous return Mannitol/ Hypertonic saline Hyperventilation: decrease CBF (temporary measure) Barbiturate coma: decrease cerebral metabolism, CBF Surgical decompression