CSF and ICP Flashcards
What is the Munro Kellie Doctrine
- It is determined by the three components of the Monro-Kellie relationship, which states that an increase in the volume of one intracranial compartment will lead to a rise in ICP unless it is matched by an equal reduction in the volume of another compartment
Draw an intracranial pressure volume vurve
What is the 3 volume s of the compartments making up the Munro Kellie doctrine
◦ Brain tissue: 1400ml on average
◦ Cerebral blood volume: 150ml
◦ Cerebrospinal fluid: 150ml
What is a normal ICP? How does it vary with position?
- Normal ICP 7-15mmHg or <20mmHg
◦ On average 9mmHg across 24 hours
◦ Head down 14-19mmHg
◦ With heads lifted 20 degrees the ICP was 3-4mmHg
◦ Interestingly if someone is totally upright variation can be -2 to +2
CSF pressure = (equation)
CSF pressure = resisatnce to CSF outflow x CSF formation + pressure in sagittal sinus
What is CSF production dependent on
CPP (not ICP
How is CSF production related to ICP
It is related to CPP
What is the normal variation in the the ICP trace due to?
- percussion
- Tidal
- Dicrotic
- Respiratory
What are the names for P 1-3 on the ICP trace
- percussion
- Tidal
- Dicrotic
What does increased amplitude of all waves indicate
Increased CSF volume
Missing bone flap
What does diminished P1 indicate
Reduced CPP
Prominent P1 menas
Systolic BP too high
Prominent P2
Oedema - reduction in cerebral compliance
How does ICP change with posture
- Intracranial pressure is normally ~ 10 mmHg in the supine person, and probably 0-2 mmHg in the upright person
How is ICP regulated 3
- Displacement of venous blood out of the CNS
- Displacement of CSF out of the CNS - to the psinal cord, venting to veinous circulations
- Meningeal distension
What is the most important minute to minute ICP regulation mechanism
Displacement of veinous blood out fo CNS
◦ Displacement of venous blood out of the CNS - large and mobile component, and can shift at short notice ‣ Arterial volume is smaller and higher pressure so shfits in it are of less significance ‣ Venous displacement likely accounts for the majority of second to second changes
What are the 2 ways CSF can be displaced from the cranium due to raised ICP?
- Into spinal cord due to better compliance
- Venting to veinous circulation - low velocity
What are factors affecting ICP
- Munro Kellie doctrine assumptions - that you have an intact cranial vault - sometimes not true
- CSF volume
- CSF production
- VSF drainage - Arterial and veinous volume
- Arterial - cerebral metabolic rate, vasoactive agents, systemic
- Veinous - outflow obstruction either pressure or physical obstriction - Brain - blood, tumour
How can CSF efflux be affeted
eg. hydrocephalus, EVD, VP shunt)
‣ ability to drain the CSF –> into the spinal cord inthe case of hydrocephalus, out of the CNS in EVD, VP shunt or inadvertant dural tear or into the venous ciruclation when arachnoid granulations are occluded by meningitis or SAH
Why might CSF production be reduced
(acetazolamide, diuretics, dehydration)
‣ CSF production directly
‣ Reduced volume status
What factors affect brain volume
◦ Age (decreased mass)
◦ Space occupying lesions (eg. tumour, abscess)
◦ Cerebral oedema
What 3 factors affect arterial brain volume
◦ Arterial blood volume
‣ Cerebral metabolic rate (eg. hyper or hypthermia, seizures, sedation)
‣ Cerebral arterial vasoactive agents (CO2, hypoxia)
‣ Systemic increases in blood flow or blood pressure (eg. pain, anxiety)
What 2 factors affect veinous volume
◦ Venous volume
‣ Venous outflow obstruction (eg. raised intra-thoracic pressure (valsalva, cough, childbirth), shivering, coughing, C-spine collar, jugular or sinus venous thrombosis, poor RV compliance
‣ Venous reflux - head down
‣ Venous efflux - upright
Mechanisms of agents used to reduce ICP are 34
- Cerebral metabolic demand
- Sedation
- Seizure - Improve veinous flow - reduced cough, muscle realxant, diuretic
- Reduce brain tissue volume - osmotherpay, dexamethasone
- CSF production - Acetazolamide inhibits carbonic anydrase
What are the 2 drugs that can reduce brain volume
Dexamethasone
Osmotic therapy
5 methods of measurnig ICP
CLinical
EVD
Codman
Epidrual catheter
Lumbar puncture
What is CSF
ultrafiltrate of plasma contained within the ventricles of the brain and the subarachnoid spaces of the cranium and spine.
How much CSF is produced per day
25ml/hr
400-600ml per day
How much CSF do you have at any one time
150ml
How is CSF distributed
25ml ventricles
30ml in spinal cord
remaining in subarachnodi space
What are the 3 sources of CSF production
CHoroid plexus in lateral ventricles 75%
Brain intersitial fluid - brain cpaillaruy endothelial cells
Circumventricular organ
How is CSF produced
Highly regulated ultrafiltration and secretion
How is there an ultrafiltrate when there is a BBB
- Ultrafiltrate of plasma is formed by the fenestrated choroidal capillaries
◦ The choroid plexus is the highest surface area of BBB fenestration and the largest fluid output area for CSF
◦ lack tight junctions
What determines the fluid in the intersitial space within the choroid
Starlings forces
How is the intersitial space of the choroid translated to CSF?
CHoroidal epithelium
Choroidal epithelium histological feature
Microvilli in the ventricle
How is the ionic content of the CSF regualted
- Na - actively secreted from apical membrane + Sodium co transport with Cl
- Carbonic anhydrase provides H+ to power Na/H+ exchange at basal membrane and makes HCO3 for transport into the CSF
- Cl/HCO3 exchanger
- Na/K/2Cl cotransporter
- Aquaporins then allow water to flow transcelluarly in basal and apical membrane
How is sodium transported in the CSF?
- Na - actively secreted from apical membrane + Sodium co transport with Cl
How is carbonic anhydrase involved in CSF production
- Carbonic anhydrase provides H+ to power Na/H+ exchange at basal membrane and makes HCO3 for transport into the CSF
The HCO3 is then exchanged for Cl
How does water cross into the CSF
Aquaporins then allow water to flow transcelluarly in basal and apical membrane
Flow down the gradient created by ionic transport
CSF production vs pressure
Pressure independnet unless CPP <55mmHg and then it decreases
Describe the cation content of CSF with reference to plasma?
- CSF sodium potassium and calcium are slightly lower than plasma
◦ Calcium lower due to less albumin, ionised calcium similar, tihgtly regulated
◦ Tightly regulated CSF K to be slightly lower to not interfere with transmembrane gradient, independent of blood K - 2.9 mmol/L
◦ Sodium varies with plasma levels, activity is essentially the same 135-140
Ca 1.12mmol/L
Mg 1.1mmolL
When descibring CSF content what are the 6 domains
- Cations
- Anions
- Acid base
4, Cells - Protein - total protein and specific beta 2 transferrin
- Glucose
How does sodium activity in the CNS compare with plasma for osmolality?
Similar
Level is very slightyl lower 135-140mmol/L