Intracranial Pressure (ICP) Monitoring
- Elwishi, BJA Ed (2018); Monitoring the brain
- Pattinson, BJA Ed (2005): Monitoring intracranial pressure, perfusion and metabolism
- Raboel, Crit Care Prac & Res (2012); Intracranial Pressure Monitoring: Invasive versus Non-Invasive Methods—A Review
- Wijdicks, Neurocrit Care (2019); Lundberg and his Waves
- Steiner, BJA, (2006); Monitoring the injured brain: ICP and CBF
- Nag, World J Clin Cases
(2019); Intracranial pressure monitoring: Gold standard and recent innovations
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:
How does the normal ICP waveform vary with the 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’
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:
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:
Which techniques can be used to measure and monitor ICP?
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
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?
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
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?
What are the advantages and disadvantages of using microtransducer devices as ICP monitors?
Trainee in ICM & Anaesthesia
Trainee in ICM & Anaesthesia