10.2.1 Raised ICP Flashcards
What contributes to intracanial pressure?
Brain 80%
Blood 10%
CSF 10%
What is the normal range of ICP?
5-15 mmHg
Varies between different ages
What is the Monro-Kellie Doctrine?
Sum of volumes (brain,blood,CSF) must remain constant to avoid a raised ICP
Increased volume of one, or volume addition, must be offset by equal decrease in volume of the others
What is first to reduce in an attempt to prevent raised ICP?
CSF then venous blood
Describe the relationship between intracranial pressure and volume
Non-linear
May have large volume change before ICP increases
Compensatory mechanisms of CSF and venous blood prevent pressure rising too much
After compensatory mechanisms deplete what happens to ICP?
Rapid rises in ICP for a given increase in volume
What can caused raised ICP?
Too much CSF (hydrocephalus)
- Congenital
- Acquired
Too much blood
Outside Cerebral Vessels
- Intracranial haemorrhage
- Haemorrhagic stroke
Inside Cerebral Vessels (rare)
- Increased arterial pressure (malignant hypertension)
- Increased venous pressure (SVC obstruction)
Cerebral oedema
- Secondary to:
- Trauma
- Infection
- Ischaemia and infarct
Mass lesion
- Space occupying lesion e.g. tumour, cerebral abcess
Other
- Idiopathic intracranial hypertension, affects overweight women of childbearing age (can cause headaches, blurred vision, they are alert and conscious)
What happens in hydrocephalus in infants?
Increased head circumfrence due to unfused bones
Depending on age fontanelles will bulge
Fontanelles fuse at :
Posterior 1-3 months
Anterior 18-24 months
What sign occurs in hydrocephalus when eyes are displaced downwards in infants?
Sunsetting sign
How can hydrocephalus be treated?
Acutely- removal of CSF or external ventricular drain
Long term- shunts form ventricular system to peritoneum or right atrium
How can hydocephalus appear on a CT scan?
Dilation of the lateral ventricles
What are the two major conssequences of raised ICP?
Brain ischaemia- due to imparied cerebral perfusion
Compression and herniation of the brain, leading to death
What is cerebral blood flow dependent on?
Cerebral perfusion pressure
What is the equation for cerebral perfusion pressure?
CPP= Mean arterial pressure - ICP
Normal values
MAP~90mmHg
ICP~10mmHg
CPP >70mmHg
How is cerebral perfusion pressure maintained?
Cerebral autoregulation
Ensures CPP and cerebral blood flow can be steadily maintained despite MAP variations
What changes occur when MAP is decreased and increased?
Reduced MAP
Maximal vasodilation of cerebral arterioles
Increased MAP
Maximal vasoconstriction of cerebral arterioles
Outline the limitations of autoregulation
MAP can be too low or high and the CPP cannot be maintained
Leads to reduced blood flow
What can happen to cerebral autoregulation in damaged brain tissue?
Impaired or absent
What would happen without autoregulation?
CPP directly dependent and responsive to changes in MAP
Therefore changes in MAP would have direct effects on the brain
What happens to cerebral prefusion pressure in increased ICP?
CPP will decrease as
CPP= MAP-ICP
What mechanisms help to mitigate CPP reduction in increased ICP?
Cerebral arterioles vasodilate to increase cerebral blood flow to maintain CPP
Elevation of MAP by increasing systemic BP
Why is increased cerebral blood flow to maintain CPP problematic?
Increasing cerebral blood volume does not help rising ICP
What happens when an expanding mass causes rising ICP?
- Compensation by extrusion of CSF and venous blood to decrease volume
- Rising ICP starts to reduce CPP, which reduces cerebral blood flow, this causes cerebral vasodilation and increase increased blood pressure to oppose decreased CPP
- ICP continues to rise, CPP unable to compete, hypoxic brain
- Cerebral hypoxia causes cerebral oedema, further rise in intracranial volume and pressure
- Brain and brainstem compression
