Clinical aspects of cerebral perfusion and ICP Flashcards
What is the normal average blood flow though the overall brain tissue
55 to 60ml/100g brain tissue per minute
What is the normal average blood flow though the grey matter
75ml/100g/minute
What is the normal average blood flow though the white matter
45ml/100g/minute
What is the blood flow in ischaemia through the brain
20ml/100g/minute
What is the blood flow rate where permanent damage can occur
drops below 10ml/100g/minute
What is the most significant factor in determining cerebral blood flow
Cerebral perfusion pressure - the effective blood pressure gradient across the brain
What is the equation for cerebral perfusion pressure
CPP = MAP - ICP
What affect does increasing the ICP have on the CPP
Increased ICP causes the CPP to decrease
What is the equation for MAP: Mean arterial pressure
MAP = DP +1/3PP
or
MAP = 2/3DP + 1/3SP
(PP = SP-DP)
What is three factors that regulate cerebral blood flow under physiological conditions
Cerebral prefusiion pressure
Concentration of arterial CO2
Arterial PO2
Define cerebral auto-regulation
The ability to maintain constant blood flow to the brain over a wide range of CPP (50-150 mm Hg)
What is the cerebral auto-regulation when the CPP is low
cerebral arterioles dilate to allow adequate flow at the decreased pressure
What is the cerebral auto-regulation when the CPP is high
cerebral arterioles constrict
How does the concentration of arterial CO2 and PaO2 regulate cerebral blood flow
Decreased levels causes arteriolar constrictions therefore lowering cerebral blood flow
eg hyperventilation
What are the Pathological conditions where cerebral blood flow can not be regulated
CPP exceeds 150mmHg - hypertensive crisis
Exudate of fluid from vascular system with resultant vasogenic edema
Toxins: CO2 - diffuses cerebrovascular dilation and inhibit proper auto-regulation
During the first 4 to 5 days of head trauma, many patients can experience a disruption in cerebral autoregulation
What is cerebral edema
state of increased brain volume as a result of increase on water content
What is the three types of edema in the brain
Vasogenic
Cytotoxic
Interstitial
What is the pathogenesis and location of vasogenic edema (extracellular)
Increased capillary permeability located mainly in white matter
Composed of plasma filtrate (plasma proteins)
What is the pathogenesis and location of cytotoxic edema (intracellular`)
Cellular swelling in the grey and white matter
Composed of increased intracellular water and sodium (due to failure of membrane transport)
What is the pathogenesis interstitial deem
Increased brain water due to impairment in absorption of CSF, composed of the CSF
and Located in the periventricular white matter in hydrocephalus
What types of edema have an increase in extracellular fluid
Vasogenic
Interstitial
What type of edema is effective to steroids
Vasogenic
What type of deem is effective to mannitol
Vasogenic
Cytotoxic
(Interstitial ??)
What is the potential pathological lesion resulting in vasogenic edema
Primary or metastatic tumour
Abscess
Late stages of infarction
Trauma
What is the potential pathological lesion resulting in cytotoxic edema
Early stages of infarction
water intoxication
What is the potential pathological lesion resulting in interstitial edema
Obstructive or communicating hydrocephalus
What is the composition and purpose of the blood brain barrier
Composed of astrocytic foot process wrapping around a capillary endothelium composed of tight junctions
Endothelial tight junctions are the barrier to the passive movement of many substances in order to protect the sensitive neural tissue from toxic materials
How are lipid soluble substance transported across the blood brain barrier
usually penetrate all capillary endothelial cell membranes in a passive manner
How are amino acids and sugars transported across the blood brain barrier
transported across the capillary endothelium by specific carrier-mediated mechanisms
What is the three components of the cranium
Brain
Blood
CSF
How does the intra- cranium respond to changes in pressure and volume
Compliance - change in volume observed for a given change in pressure
Elastance - change in pressure observed for a given change in volume
How is compliance shown when a new intracrhail mass is introduced increasing the pressure in the brain
a compensatory change in volume must occur through a reciprocal decrease in venous blood or CSF to keep the total intracranial volume constant
How is elastane of the intra-cranium represented
accommodation to outward expansion of an intracranial mass
What is the homeostatic mechanism of the venous system in compliance to decrease volume in the cranium
The venous system collapses easily and squeezes venous blood out through the jugular veins or through the emissary and scalp veins
What is the homeostatic mechanism of CSF in compliance to decrease volume in the cranium
CSF can be displaced from the ventricular system through the foramina of Luschka and Magendie into the spinal subarachnoid space
How long does the “pressure buffering” mechanism of venous blood and CSF compliance last
Until critical volume is reached
What natural happens at when homeostatic mechanisms causes “critical volume”
Intracranial hypertension as small volumetric changes result in significant increase in pressure
What are the three waves, on an ICP waveform
P1= Percussion wave
P2 = Tidal wave
P3 = Dicrotic wave
What does P1: Percussion wave represent
Arterial pulsation (systolic pressure, large intracranial arteries, and choroid plexus)
What does P2: tidal wave represent
Intra cranial compliance
What does P3: dicrotic wave represent
Venous pulsation
What affect does an increased/decreased systolic BP have on P1
Increased: Big P1
Decreased: P1 decreases and eventually disappears
What does a prominent P2 wave indicate
Mass lesion is increasing volume
Intracranial compliance is decreased
Inspiratory breath is being held
If P2 and P3 waves is not present what does this indicate
Hyperventilation
How is intracranial hypertension indicated in in ICP waveforms
When P2 is higher than P1
How do you define Lindbergh A waves
Abrupt elevation in ICP for 5 to 20 minutes followed by a rapid fall in the pressure to resting levels
The amplitude may reach as high as 50 to 100 mm Hg
How do you define Lindbergh B waves
Frequency of 0.5 to 2 waves per minute, are related to rhythmic variations in breathing
How do you define Lindbergh C waves
Rhythmic variations related to waves of systemic blood pressure and have smaller amplitude
What is the cause of A waves
ICP Exceeds the limits of cerebral compliance
What is the cause of B waves
Respiratory changes
Variation in cerebral blood flow
How do you mange increased ICP
Head end elevation: facilitate venous return
Mannitol/ Hypertonic saline
Hyperventilation: decrease CBF (temporary measure)
Barbiturate coma: decrease cerebral metabolism, CBF
Surgical decompression
What occurs in stage 1 cushings relfex
ICP increases more than MAP
This compresses the cerebral arterioles and decreases cerebral blood flow, and the ischaemic conditions activates the autonomic nervous system
How does a sympathetic response occur in cushings reflex
Sympathetic response via alpha 1-adrenergic receptors,
causing constriction of body’s arteries and increases the rate of heart contractions and cardiac output causing tachycardia
What is the affect of cushing contraction of bodies arteries
raises the total resistance of blood flow, elevating blood pressure to high levels, which is known as hypertension.
How is a parasympathetic response triggered in cushions reflex
baroreceptors in the aortic arch detect the increase in blood pressure and trigger a parasympathetic response via the Vagus nerve.
What occurs due to parasympathetic stimulation in cushions reflex
This induces bradycardia signifying the second stage of the reflex
How does bradycardia also occur in cushing reflex
Due to mechanical distortion of medulla
What is the cushions triad of symptoms
Increased blood pressure
irregular breathing
Bradycardia
What occurs in brain tissue oxygenation monitoring
Probe to monitor oxygenation of tissue
Detect and treat low oxygenation, increasing Cerebral perfusion pressure
What occurs in micro dialysis
Investigate brain metabolism
By implantation of specially designed catheters collecting small-molecular-weight substances to help measure and identify neurotransmitters, peptides, and other substances
