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SOE 442: Intracranial Pressure (ICP) & Monitoring

Regarding intracranial pressure (ICP) and monitoring of ICP…

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Question No. 2

Q: What is the Monro-Kellie Doctrine?

Answer No. 2

The skull of an adult is in effect a rigid box which contains brain tissue, blood and CSF. An increase in the volume of one component invariably results in an increase in ICP unless the volume of another component decreases

Question No. 3

Q: What are the volumes of the different ‘compartments’ contained within the skull (i.e. brain, CSF & blood)?

Answer No. 3

Monro Kellie Doctrine and the intracranial contents

Question No. 4

Q: Draw a graph to show the relationship between intracranial pressure (ICP) and intracranial volume?

Answer No. 4

  • The relationship between ICP and intracranial volume can be demonstrated using the intracranial compliance curve
  • Compliance reflects the ability to compensate for changes in volume without subsequent rises in ICP - with reducing compliance comes large rises in ICP
  • The curve can be described in three stages:
Intracranial compliance curve demonstrating the relationship between intracranial volume and intracranial pressure (ICP)
Stage 1
(Initial stage)
  • Characterised by high compliance and low ICP
  • Compensatory mechanisms act to prevent rises in ICP associated with increased intracranial volume:
    • CSF moves into the spinal subarachnoid space (primary mechanism)
    • Blood displaces into extracranial vessels
    • Brain itself has minimal compressibility therefore is very limited in scope for compensation
  • Reserve volume varies with age due to ongoing cerebral atrophy:
    • 60–80 mL in young persons and approximately 100–140 mL in geriatric population
Stage 2
(Transition stage)
  • Characterised by low compliance and low ICP
  • Gradual depletion of compensatory reserve as intracranial volume rises with slow increase in ICP
Stage 3
(Ascending stage)
  • Characterised by low or null compliance and high ICP
  • Compensatory mechanisms stop working and small changes in volume elicit high increases in pressure
  • Results in a high risk of cerebral ischaemia and herniation

Question No. 5

Q: What is normal intracranial pressure (ICP)?

Answer No. 5

  • Normal ICP in adults is 5-15mmHg
  • It is constantly varying:
    • Throughout the cardiac cycle
    • Throughout the respiratory cycl
    • With exercise, coughing and straining (can reach 50mmHg in the normal brain)

Question No. 6

Q: Is ICP the same throughout the skull?

Answer No. 6

  • Cannot be assumed to be evenly distributed particularly in pathological states with intracranial hypertension
  • Pressure gradients may exist between different tentorial compartments
  • CSF pressure usually represents global ICP but relies on unobstructed flow

Question No. 7

Q: What is a normal ICP waveform?

Answer No. 7

  • The normal ICP trace looks similar to an arterial trace but has 3 distinct peaks:
Normal intracranial pressure (ICP) waveform
P1
Percussion Wave
The arterial systolic pressure transmitted from the choroid plexus
P2
Tidal wave
Thought to represent true ICP proper as a correlate of the arterial pulses reflected off the brain parenchyma
P3
Dicrotic notch
The arterial dicrotic notch transmitted from the choroid plexus

Question No. 8

Q: What is raised ICP and when should active treatment be undertaken?

Answer No. 8

  • ICP >15 mm Hg is elevated
  • Management is mostly extrapolated from TBI where treatment is recommended for ICP >22 mm Hg (Brain Trauma Foundation Guidelines):
    • Values above this level are associated with increased mortality
  • However, this single threshold is probably an oversimplification:
    • Time spent over the value and its intensity is more important (the 'ICP dose')
    • Prolonged exposure to values below the threshold may still be harmful
    • Also influenced by cerebral perfusion pressure (CPP):
      • If CPP low (<50mmHg) ICP no longer a predictor
      • Moderate elevations in ICP may be better tolerated at higher CPP

Question No. 9

Q: What happens to the ICP waveform in raised ICP?

Answer No. 9

  • With increasing ICP there is decreased compliance of the brain resulting in a change in the morphology of the ICP waveform:
    • Increases in amplitude
    • P2 exceeds P1
    • Becomes broader and 'rounded off'
Changes in the morphologu of the ICP waveform with rising ICP

Question No. 10

Q: What are Lundberg Waves and what the different types?

Answer No. 10

  • Rhythmic variations in ICP described first described by Lundberg in the 1960's
  • Clinically less useful in modern practice, with an emphasis on earlier recognition and treatment of raised ICP meaning A waves rarely seen:

Type

A-wave
B-wave
C-wave

Description

  • Slow plateau waves for 5-15 minutes
  • Pressures of 50-100 mmHg
  • Small oscillations over 30-120 seconds (freq. 0.5-2 / min)
  • Pressures up to 50 mmHg in amplitude
  • Low amplitude oscillations over 10-20 seconds (freq. 4-8 / min)
  • Pressures up to 20 mmHg in amplitude

Example

Implication

  • Always associated with intracranial pathology - represent critical hypoperfusion and early brain herniation
  • Thought to be due to reflex vasodilatation in response to decreased cerebral perfusion leading to a vicious circle of increased intracranial volume and pressure
  • Usually a sign of evolving cerebral injury
  • Thought to be due to vasomotor centre instability when CPP is unstable
  • Present in healthy individuals and are of little clinical importance
  • Thought to occur because of interaction between cardiac and respiratory cycles

Question No. 11

Q: What are the indications for monitoring ICP?

Answer No. 11

Trauma (according to the Brain Trauma Foundation)
  • All 'salvageable' patients with severe TBI (GCS 3-8) with abnormal CT findings
  • Patients with severe TBI (GCS 3-8) with normal CT findings and 2 of:
    • Age >40
    • Motor posturing
    • Systolic BP <90
Non-Trauma
Any non-traumatic insult complicated by raise ICP:
  • Intracerebral haemorrhage
  • Subarachnoid haemorrhage
  • Hydrocephalus
  • Malignant MCA syndrome
  • Hepatic encephalopathy
  • Cerebral oedem
  • Intracranial infection

Question No. 12

Q: Which techniques can be used to measure and monitor ICP?

Answer No. 12

Invasive
'One-Time'
  • Lumbar puncture and manometry
Continuous
  • External ventricular drain (Gold standard - BTF Guidelines)
  • Microtransducer ICP monitoring devices:
    • Fibreoptic, strain gauge or pneumatic design
    • Most commonly intraparenchymal placement
    • May be intraventricular, epidural, subdural, or subarachnoidal placement

Possible sites for continuous ICP measurement
Non-Invasive (Surrogate markers)
  • Transcranial doppler ultrasound
  • Tympanic membrane displacement
  • Optic nerve sheath diameter
  • Imaging (CT & MRI)
  • Fundoscopy & papilloedema
  • Anterior fontanelle pressure monitoring

Question No. 13

Q: What type of microtransducer devices are available and how do they work?

Answer No. 13

Type
Example
Description
Strain Gauge
  • Codman
  • Raumedic
  • Uses a piezoelectric strain gauge
  • Changes in ICP distort the device causing changes in the resistance across it
  • Degree of resistance translated into an ICP value
Fibreoptic
  • Camino
  • Uses light transmitted via fibreoptics towards a displaceable mirror
  • Changes in ICP move the mirror causing differences in intensity of reflected light
  • Intensity of reflection can be translated into an ICP value
Pneumatic
  • Spiegelberg
  • Uses a small balloon in the distal catheter tip
  • Changes in ICP result in pressure changes within the balloon
  • Allows quantitative measurement of intracranial compliance

Question No. 14

Q: What are the advantages and disadvantages of using microtransducer devices as ICP monitors?

Answer No. 14

Advantages
  • Relatively easy to insert and use
  • Allows continuous monitoring
  • Lower risk of infection than EVD (absence of fluid coupled system)
  • Lower risk of haemorrhagic complications than EVD
  • Accuracy generally considered comparable with that of EVD
Disadvantages
  • Prone to drift (though don’t require routine replacement)
  • Cannot be recalibrated once sited
  • May only represent local rather than global pressures
  • Does not allow for CSF drainage

Question No. 15

Q: What are the advantages and disadvantages of using external ventricular drains (EVDs) as ICP monitors?

Answer No. 15

Advantages
  • Gold standard accuracy
  • Can be recalibrated in situ and at the bedside
  • Allows removal of CSF as part of treatment for raised ICP
  • Allows administration of intrathecal drugs
Disadvantages
  • Conduit for intraventricular infection (~5%)
  • Associated with risk of haemorrhage during insertion (0.9 – 1.2%)
  • Can easily become blocked due to their small calibre
  • Insertion may be difficult in younger patients with small ventricles or in cerebral oedema
  • Involve traversing brain tissue which has rarely led to inadvertent injury to cerebral structures

Question No. 16

Q: How can drainage of CSF be controlled using an EVD?

Answer No. 16

  • CSF drainage can be controlled by changing the height of the EVD measuring cylinder above the zero set point:
    • Set a prescribed distance in cm above the zero-point
    • Requires the pressure of CSF in cmH2O to equal the set height before it will drain (a drain set at a height of 15cm will require an intracranial pressure (ICP) >15cm H2O) before CSF will drain
  • The level at which the EVD flow chamber should be set is usually determined by the neurosurgical team

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