Intracranial Pressure (ICP) Monitoring

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Normal ICP & Waveforms

What is normal intracranial pressure (ICP)?

  • 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)

Is ICP the same throughout the skull?

  • 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

What is a normal ICP waveform?

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

How does the normal ICP waveform vary with the cardiac and respiratory cycles?

Variation in ICP waveform with cardiac and respiratory cycles

Pathological ICP & Waveforms

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

  • 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

What happens to the ICP waveform in raised ICP?

  • 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

What are Lundberg Waves and what the different types?

  • 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

Description

Example

Implication

A-wave
  • Slow plateau waves for 5-15 minutes
  • Pressures of 50-100 mmHg
  • 1
    • 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
    • B-wave
      • Small oscillations over 30-120 seconds (freq. 0.5-2 / min)
      • Pressures up to 50 mmHg in amplitude
      • 2
        • Usually a sign of evolving cerebral injury
        • Thought to be due to vasomotor centre instability when CPP is unstable
        • C-wave
          • Low amplitude oscillations over 10-20 seconds (freq. 4-8 / min)
          • Pressures up to 20 mmHg in amplitude
          • 3
            • Present in healthy individuals and are of little clinical importance
            • Thought to occur because of interaction between cardiac and respiratory cycles
            • Indications & Techniques

              What are the indications for ICP Monitoring?

              • Clear evidence-based indications in head injury
              • Evidence lacking in other conditions and are often center or clinician specific
              • Indicated when the benefits of monitoring outweigh the risks of insertion or infection:
              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
                  • Which techniques can be used to measure and monitor ICP?

                    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
                        • 1
                          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
                          • External Ventricular Drain (EVD)

                            How does an EVD work to measure intracranial pressure?

                            • A small plastic catheter is inserted into the lateral ventricle:
                              • Passes through brain parenchyma
                              • Right frontal area usually used owing to its non-dominance in the majority of the population
                            • CSF in the catheter forms a continuous fluid column that is connected to a strain-gauge pressure transducer
                            • Zero-point for the transducer is the level of the external auditory meatus:
                              • Corresponds to the level of the foramen of Munro
                            • In addition to providing pressure measurements and EVD can be used to:
                              • Drain cerebrospinal fluid
                              • Administer intrathecal drugs such as antibiotics
                              • Drain intraventricular haemorrhage
                            • However, to measure ICP, the EVD must be ‘clamped’ – CSF cannot be simultaneously drained
                            Features of an external ventricular drain (EVD)

                            How can drainage of CSF be controlled?

                            • 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

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

                            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
                              • Which factors are associated with increased risk of infection with EVDs?

                                • Prolonged insertion time (>5 days)
                                • Frequent CSF sampling rate
                                • Intraventricular or subarachnoid haemorrhage
                                • Cranial fracture
                                • CSF leakage
                                • Nonsterile insertion

                                Microtransducer Devices

                                How are microtransducer devices used to monitoring ICP?

                                • Use a pressure transducer (strain gauge, pneumatic or fibreoptic device) at the tip of a wire
                                • Inserted via a small borehole and secured using a ‘bolt’
                                • Transducer can be sited in numerous anatomic locations:
                                  • Intraparenchymal region
                                  • Subdural, Subarachnoid or Epidural space
                                  • Intraventricular region
                                • Favoured location is commonly intraparenchymal:
                                  • Improved accuracy, easier insertion (not aiming for specific space) and reduced infection rates
                                  • Right frontal region often used owing to its non-dominance in the majority of the population
                                  • Other sites may be selected depending on regional pathology

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

                                Type
                                Strain Gauge
                                Fibreoptic
                                Pneumatic
                                Example
                                • Codman
                                • Raumedic
                                  • Camino
                                    • Spiegelberg
                                    • Description
                                      • 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
                                        • 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
                                          • 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
                                          • What are the advantages and disadvantages of using microtransducer devices as ICP monitors?

                                            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
                                              • Author

                                                The Guidewire
                                                Trainee in ICM & Anaesthesia

                                                Reviewer

                                                The Guidewire
                                                Trainee in ICM & Anaesthesia