Intracranial pressure measurement

Describe the measurement of intracranial pressure

Accurate measurement of intracranial pressure (ICP) is fundamental in neurocritical care, allowing early detection of intracranial hypertension and guiding therapy.

Several monitoring modalities are available, broadly divided into:

ventricular systems (external ventricular drains)
intraparenchymal devices (fibreoptic and strain-gauge monitors)

Each technique differs in:

accuracy and calibration
ability to provide therapeutic intervention (CSF drainage)
susceptibility to error and complications

Understanding these differences is essential to selecting the appropriate monitor and interpreting ICP values correctly in clinical practice.

ICP measurement devices

External ventricular drain (EVD)

Feature	External ventricular drain (EVD)
Components	Ventricular catheter Fluid-filled non-compressible tubing External transducer (Wheatstone bridge) Processor / monitor Drainage chamber
Mechanism	Ventricular pressure is transmitted via a fluid column to an external transducer, where it is converted into an electrical signal and displayed as ICP waveform and value.
Benefits	Gold standard Measures ventricular (global) ICP Can be re-zeroed Diagnostic (CSF sampling) Therapeutic (CSF drainage) Relatively inexpensive
Limitations	Requires expertise Invasive Risk of infection and haemorrhage Difficult with small ventricles Can block Risk of over-drainage
Sources of error	Patient: Movement Coughing Shivering Seizures Catheter blockage (clot/debris) Operator: Incorrect levelling Incorrect zeroing Incorrect placement Incorrect drain height Environment: Electrical interference Tubing movement Air bubbles in system Machine: Transducer calibration error Damping / resonance Monitor malfunction

Intraparenchymal ICP monitors

Feature	Fibreoptic ICP monitor (e.g. Camino®)	Strain-gauge ICP monitor (e.g. Codman®)
Components	Pressure-sensitive diaphragm Fibreoptic cable Processor / monitor	Tip-mounted strain gauge sensor Electrical cable Processor / monitor
Mechanism	Pressure deforms diaphragm Alters reflected light signal Converted to ICP waveform	Pressure deforms strain gauge Alters electrical resistance Converted to ICP waveform
Benefits	Easier and faster to insert than EVD Less invasive (no ventricular cannulation) Can be placed intraparenchymal, subdural or epidural Continuous ICP waveform Not prone to blockage Useful when ventricles are small or inaccessible Lower infection risk than EVD
Limitations	Expensive Measures local parenchymal pressure Cannot drain CSF Cannot sample CSF Cannot be re-zeroed after insertion Calibration drift (notably after ~72 h) Small risk of haemorrhage and infection
Sources of error	Patient: Movement Coughing Shivering Seizures Local pressure gradients around sensor tip Operator: Incorrect placement Poor fixation Sensor or cable damage during insertion Environment: Electrical interference Cable kinking or tension Temperature-related signal variation Machine: Calibration drift Baseline drift Signal processing error Monitor malfunction

Interpreting the ICP waveform

Each cardiac cycle produces three peaks:

P1 (percussion wave)
- Represents arterial pulsation transmitted from the choroid plexus
- Normally the highest peak
- Reflects arterial pressure
P2 (tidal wave)
- Reflects intracranial compliance
- Sensitive to changes in intracranial volume buffering
- Normally lower than P1
P3 (dicrotic wave)
- Corresponds to aortic valve closure
- Smallest peak

Normal waveform

P1 > P2 > P3
Indicates normal intracranial compliance
Arterial pulsations are effectively buffered by:
- CSF displacement
- Venous outflow

Raised ICP / reduced compliance

P2 increases and may exceed P1
Indicates reduced intracranial compliance and hence progression toward the decompensated (steep) part of the pressure–volume curve

Other waveform changes:

Increased overall amplitude of the entire waveform
Rounded / broadened peaks
Reduced distinction between P1, P2 and P3

ICP waveform

Variation in the ICP waveform with normal or raised pressures

By Harary et al. Intracranial Pressure Monitoring—Review and Avenues for Development.

ON THIS PAGE

#Intracranial pressure measurement

#Describe the measurement of intracranial pressure

#External ventricular drain (EVD)

#Intraparenchymal ICP monitors

#Interpreting the ICP waveform

#Normal waveform