Intracranial Pressure and cerebral blood flow

Question 1

perfusion pressure

Answer 1

MAP-CVP

Question 2

cerebral perfusion pressure

Answer 2

=MAP-ICP

raised ICP will reduce cerebral perfusion pressure

Question 3

the monro-kellie hypothesis

Answer 3

volume of the cranium is fixed
the contents of the cranium=brain,csf and blood and they are incompressible
any increase in volume of one of these must be compensated by a decrease in volume of another eg cerebral atrophy

Question 4

why do you have a headache after a lumbar puncture

Answer 4

if CSF escapes from an LP site, a low pressure headache can result as the brain sags, pulling on its meningeal attachments

Question 5

why contraindicated in raised ICP?

Answer 5

when the iCP is markedly raised, if CSF is allowed to escape via an LP this can result in the brain suddenly being pulled down into the foramen magnum=dangerous as causes pressure on the reticula activation system

Question 6

ICP regulation -short term

Answer 6

blood-veins readily change their calibre
css-easily moved between cranial and spinal spaces
if ICP is raised ,pressure on ventricles moves CSF to the spinal space

Question 7

ICP regulation-long term

Answer 7

volume of CSF balanced between:
production:
-constant rate
-energy dependent
reabsorption:
-variable:rate increases as ICP rises

Question 8

CSF formation

Answer 8

produced mainly by the choroid plus in lateral and 3rd ventricles

Question 9

CSF circulation

Answer 9

• circulates into 3rd ventricle via foramina of monro
• flows into 4th ventricle via aqueduct
• CSF circulates over surface of brain and spinal cord
• reabsorbed on brain surface via arachnoid granulations

Question 10

What are the two types of hydrocephalus?

Answer 10

communicating 
-blocked CSF reabsorption at arachnoid granulations
-CSF circulation is normal 
non-communicating
-tumour,haemorrhage
-commonly at Aqueduct

Question 11

How is hydrocephalus relieved?

Answer 11

pressure is relieved by a V-P shunt

Question 12

local-autoregulation

Answer 12

• local vascular control mechanism that maintains constant blood flow despite varying blood pressure
• achieved by changing vasomotor tone

Question 13

equation for working out blood pressure

Answer 13

Q=P/R

blood flow=perfusion pressure/vascualr resistance

Question 14

Cerebral metabolism and CBF

Answer 14

• as brain activity increases, greater production of metabolites(especially CO2) leads to vasodilation
• reducing cerebral metabolism helps to keep ICP down
• worst situation-fitting brain

Question 15

Cushing’s response

Answer 15

-rise in ICP will result in a fall in CPP
CPP=MAP-ICP
-higher ICP sensed in 4th ventricle ,generating an increase in MAP to restore CPP
-commonly associated with a reflex bradycardia

Question 16

Extradural haematoma

Answer 16

• looks like a lemon
• midline shifts
• ventricle compressed

Question 17

chronic subdural haematoma

Answer 17

minor midline shift and is liquefied and so a light shadow appears -looks like a river

Question 18

Subarachnoid haemorrhage

Answer 18

• looks like a banana

- blood spreads widely through CSF

Question 19

measures shown to improve outcome in head injury

Answer 19

• prevent hypotension

- avoid hypoxia

Question 20

Glasgow Coma Scale

Answer 20

ranges from 3-15
GCS of 8–> time for action
lower the score the more in danger they are

Question 21

what happens if the brain continues to swell

Answer 21

• no more CSF transfer can occur

- icp goes up markedly ,leading to vascular compression and impaired perfusion

Question 22

what is coning?

Answer 22

• a continuing rise in ICP forces cerebellar tonsils down into foramen magnum:
• brainstem and cranial nerves compressed
• when ICP>systolic BP,blood flow ceases