Clinical Aspects of Cerebral Perfusion and ICP Flashcards
What does the intracranial cavity contain
Nervous tissue
Blood vessels
Venous channels
CSF
What is the normal intracranial pressure
5-15 mmHg
What raise in icp will be pathological
Sustained increase above 20mmHg
can increase and decrease during transient activities (sneezing, straining, getting up from lying down)
What is the Monro-Kellie hypothesis
Intracranial volume is fixed due to non-compliant skull
Contents non-compressible
Change in volume of contents or addition of new space occupying lesions = increased pressure
unless reduction of other content
What are the percentages of the contents in the intracranial cavity
CSF volume: 150ml, 10%
Blood volume: 150ml, 10%
Brain parenchyma volume: 1400ml, 80%
What is the trend of increasing ICP
Brain has minimal compliance
ICP starts increasing very minimally
Once increasing volume cannot be compensated further
ICP increases exponentially
What can the Intracranial cavity do to try compensate raising ICP
Reduction of venous blood
Displacement of CSF towards spine
Reduction CSF volume
Once these cannot compensate further for the raising ICP it will increase exponentially
What are types of pathologies that can cause a raised ICP
localized shifts of brain across various compartments
focal or global reduction of blood flow
Focal or global ischemia
Reduced energy production
Pump failure on cell membrane
Cellular dysfunction
Interstitial microenvironment changes (toxic metabolites)
membrane damage and cell death
What are the 3 peaks in ICP waveform
P1: percussion wave, arterail pulsation
P2: Tidal wave, intracranial compliance
P3: Dicrotic wave, venous pulsation
What does a normal ICP waveform look like
P1 highest upstroke, then P2, then P3
What would suggest intracranial hypertension suggest on ICP waveforms
P2 is higher than P1
What does P2>P1 indicate
Intracranial hypertension
What are the two different types of waveform pulses
Vascular pulse: one waveform, top to bottom of wave
Respiratory pulse: many waverforms, top valve to bottom value
What can measure intracranial pressure
Subarachnoid (richmond) bolt, prefered method
EVD, tunneled EVD (external ventricular drainage)
epidural bolt
what is normal cerebral blood flow
700ml/min, 50ml/100g/min cerebral blood flow
roughly 14% of cardiac outflow
what is the normal cerebral blood flow to white and grey matter
Grey matter: 70ml/100g/min
White matter: 20ml/100g/min
What does CBF depend on
MAP
Intracranial pressure
Vascular resistance
Autoregulatory mechanisms of brain
What is the cerebral perfusion pressure
MAP - ICP
what is the cerebro-vascular resistance (CVR)
resistance from cerebral vascular to flowing blood
What do the autoregulation mechanisms do
maintain constant blood flow over wide range of pressures
Brain can also increase blood flow to specific regions when they are active
What is mean arterial pressure (MAP)
Systolic - diastolic pressure
What are the four autoregulation mechanisms
Autonomic neurogenic theory
endothelial mechanims
myogenic autoregulation
Metabolic autoregulation (PaCO2 significant influencer on CBF, PaO2 influences if it drops significantly)
Why is CBF, ICP and autoregulation so important
Brain uses glucose and ketones for metabolism
Has no stored energy source
Impaired blood flow: mitochondrial energy production failure –> cell death within minutes
Injured brain loses ability to autoregulate: hyper/hypotension, hypoxia and raised ICP lead to secondary insult.
What does neuromonitoring involve
ICP measurements and maintenance of CPP
Measures CPP by meausuring MAP and ICP
preventing insult: reduce ICP and prevent high or low CPP
