Intracranial Pressure and cerebral blood flow Flashcards
what is intracranial pressure?
Dynamic balance between these contents
Normal ICP 5 to 15 cm of water
what does the intracranial cavity contain?
contains nervous tissue, blood vessels venous channels and CSF
when is transient increase or decrease in ICP common?
Transient increase or decrease is common during activities like straining, sneezing or getting up from recumbent position
when is increase in ICP considered pathological?
Sustained increase beyond 20 cm of water is pathological
what is the monro-kellie hypothesis?
Intracranial volume is fixed due to non-compliant skull
Contents are non-compressible
Change in volume of contents or addition of new space occupying lesion increases pressure unless compensated by reduction of other content
how would a large extradural haematoma affect ICP?
Compensation
Reduction of venous blood
Displacement of CSF towards spine
Reduction CSF volume
Brain has minimal compliance
If these cannot compensate for the increase in volume then ICP rises exponentially
what are pathologies of 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 would happen if we put a pressure transducer inside the brain?
we will find that the ICP can be traced as waves.
Under normal circumstances the normal ICP wave consists of three components.
The wave consists of P1, P2 and P3.
These are the three notches that are present in a normal ICP wave and
these respond to the arterial wave of the pulse.
even with respiration, there is a gradual increase and decrease of the ICP.
So essentially, ICP varies based on two components.
One is the arterial pulse, and second is the respiration.
what is normal cerebral blood flow?
50ml/100g/min or 700 ml/min
what percentage of cardiac output is cerebral blood flow?
14 % of the cardiac output.
how does cerebral blood flow range between white and grey matter?
Range from 20ml/100g/min in white matter to 70ml/100g/min in grey matter
Regional variation in flow depending on demand
what is mean arterial pressure (MAP)?
diastolic pressure plus one third of the pulse pressure (difference between the systolic and diastolic)
what is the cerebral perfusion pressure (CPP)?
Difference between mean arterial pressure (MAP) and intracranial pressure (ICP)
what is the cerebro-vascular resistance (CVR)?
Resistance offered by cerebral vasculature to flowing blood
what is autoregulation?
ability of brain to maintain constant blood flow over wide range of pressures
what four factors does the physiology of cerebral blood flow depend on?
Mean arterial pressure (driving blood into intra-cranial cavity)
Intracranial pressure (creates a gradient against incoming blood)
Vascular resistance (low resistance higher flow or vise-versa)
Autoregulatory mechanisms of the brain
what does the brain have the capacity to do in terms of autoregulation?
ability to increase blood flow to specific regions in brain when these are active e.g. temporal lobe during speech
what are the different mechanisms of autoregulation?
The autonomic neurogenic theory
Endothelial mechanisms
Myogenic autoregulation
Metabolic autoregulation
PaCO2 has significant influence on CBF
PaO2 influences CBF only if it drops significantly
what does brain tissue use exclusively for metabolism?
glucose and ketones
what do brain tissues not have?
a stored energy source
what effect will impaired blood flow have on energy production?
Impaired blood flow will lead to mitochondrial energy production failure within seconds and cell death within minutes
what does loss of the brains ability to autoregulate result in?
Injured brain loses its ability to autoregulate hence hypotension, hypertension, hypoxia or raised ICP can lead to significant secondary insult
what effect does head injury have on autoregulation?
Severe traumatic brain injury causes raised ICP
Hypotension and hypoxia is common in polytrauma
Autoregulation is lost
Neuromonitoring involves ICP measurements and maintenance of CPP as therapy
what is neuromonitoring?
Continuous monitoring of ICP with surgically implanted pressure transducer
Continuous measurement CPP by measuring MAP and ICP
Optimizing CPP to prevent brain insult by reducing high ICP and preventing high or low CPP
