clinical aspects of cerebral perfusion and ICP

Question 1

what % of cardiac output is cerebral outflow

Answer 1

15%

Question 2

what is the normal cerebral blood flow average

Answer 2

55-60mL/100brain tissue per minute

Question 3

when does ischaemia occur

Answer 3

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

Question 4

when does permanent damage occur

Answer 4

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

Question 5

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

Answer 5

cerebral perfusion pressure (CPP)

= the effective blood pressure gradient across the brain

Question 6

what does an increased ICP do to the CPP

Answer 6

causes it to decrease

Question 7

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

Answer 7

CPP
concentration of arterial CO2
arterial PO2

Question 8

what is cerebral auto regulation

Answer 8

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

Question 9

how does cerebral auto regulation occur

Answer 9

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

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

Question 10

when can the autoregulatory system fail

Answer 10

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

Question 11

what is cerebral oedema

Answer 11

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

Question 12

what can cerebral oedema cause

Answer 12

subacute to chronic intracranial hypertension

Question 13

what is the blood brain barrier

Answer 13

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

Question 14

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

Answer 14

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

Question 15

what materials can be naturally transported across the BBB and how

Answer 15

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

Question 16

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

Answer 16

total = 1700mL

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

Question 17

what is the moro-kelly doctrine

Answer 17

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

Question 18

what is compliance

Answer 18

change in volume observed for a given change in pressure

dV/dP = change in volume/change in pressure

Question 19

what is elastance

Answer 19

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

Question 20

how can blood be displaced from the brain

Answer 20

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

Question 21

how can CSF be displaced from the brain

Answer 21

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

Question 22

when can small changes in volume produce significant changes in pressure

Answer 22

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

Question 23

how can ICP be measured

Answer 23

manometry

connected to electric monitor to see ICP waveforms

Question 24

what are the waves seen ICP manometry

Answer 24

Lundberg waves:
A waves
B waves
C waves

Question 25

when are A waves seen

Answer 25

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

Question 26

when are B waves seen

Answer 26

related to rhythmic variations in breathing

Frequency of 0.5 to 2 waves per minute

Question 27

what are C waves

Answer 27

Rhythmic variations related to waves of systemic blood pressure and have smaller amplitude

Question 28

what is Cushing’s reflex and what does it involve

Answer 28

a vasopressor response

triad:
hypertension
irregular breathing
bradycardia

Question 29

what is the physiology that causes the triad in Cushing’s response

Answer 29

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

Question 30

how is raised ICP managed

Answer 30

Head end elevation: facilitate venous return

Mannitol/ Hypertonic saline

Hyperventilation: decrease CBF (temporary measure)

Barbiturate coma: decrease cerebral metabolism, CBF

Surgical decompression