Neuro Physiology - Cerebral Blood Flow and ICP Flashcards
What is the Monroe-Kellie doctrine?
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The skull is a fixed container with three types of content:
Brain 85%, Blood 5%, CSF 10%.
If one of these increases in volume, another must reduce otherwise ICP will rise.
Initially venous blood from the sinuses is pushed out of the skull.
Secondly, CSF via the foramen magnum
Thirdly, *arterial blood volume reduces *(but after a point this will not make much difference)
At this point, small increases in volume dramatically increase ICP, compressing brain parenchyma and causing ischaemia. Above 20mmHg there is focal ischaemia, and above 45-50mmHg there is global ischaemia.
Normal ICP is 8-12mmHg supine, directly related to ITP, with a respiratory swing. PEEP, coughing and straining all increase ICP.
What are the causes of increased ICP?
Increased CSF (Hydrocephalus)
Blockage, overproduction or
under-absorption
Mass effect of brain
Oedema, contusion, abscess, tumour
Increased intracranial blood volume
Venous obstruction (Venous sinus thrombosis)
Haemorrhage (IC/SA/SD/ED)
Benign intracranial HTN - unclear cause, associated with obesity
Draw and explain the intracranial pressure waveform
**Time (s) along X axis, mmHg along Y axis
Curve broadly similar to that of iABP, and is measured through an intracranial pressure transducer (Eg. Ventricular drain/SA bolt)
Three waves of gradually reducing amplitude
In normal physiology, the pulse wave has 3 components:
P1 - Percussive transmitted arterial pressure
P2 - Tidal Arterial wave reflecting off brain tissue, varying with compliance. Approx 80% amplitude of P1, and ends at point of dichrotic notch
P3 - Dicrotic wave Venous pressure wave, increases as CVP increases.
Respiration causes a variation in baseline pressure.
There are other slow (Lundberg) waves, usually pathological:
Type A - Plateau waves Rapid increase in baseline pressure to more than 50mmHg for 2-20 minutes, severely reduced compliance - always pathological
Type B 0.5-2 waves/minute, increases in pressure of around 20-30mmHg before returning to baseline, variable ICP usually present too. Can be due to vasospasm
Type C 4-8 waves/minute, up to 20mmHg, may be normal
