Intracranial Pressure Monitoring: Invasive intracranial pressure monitoring is mainly limited to neurological and trauma intensive care units. Insertion of these devices is performed in specialist units. The presentation here is to aid understanding of the mechanisms and indications for use of these techniques.
Indications of Intracranial Pressure Monitoring
• Cerebral trauma:
• GCS ≤8.
• Compression of basal cistern on brain CT scan.
• Midline shift >0.5 mm on brain CT scan.
• Raised ICP not requiring surgery.
• After intracranial haemorrhage (SAH or intracerebral).
• Acute liver failure (grade 4 coma with signs of raised ICP).
• Metabolic diseases with raised ICP (e.g. Reye’s syndrome).
• Post-operative oedema (after neurosurgery).
Intracranial pressure monitoring in patients who are at risk of unexpected rises in ICP should ideally be started before secondary brain injury has occurred and where it can influence the management of the patient. As facilities in neurosurgical centres may be limited, it has been suggested that these patients may be effectively managed in general intensive care units.
Contraindications of Intracranial Pressure Monitoring
• Uncorrectable coagulopathy.
• Local infection near placement site or meningitis.
Method of Intracranial Pressure Monitoring
There are many intracranial pressure devices available, including subdural, extradural, parenchymal, and intraventricular monitors. Parenchymal and intraventricular monitors are more accurate but associated with greater risk than extradural monitors. They should be implanted by experienced persons only.
Pre-packaged kits are available (e.g. Codman subdural bolt). This monitor is inserted in the prefrontal region, and the kit contains the necessary screws for creating a burrhole and spinal needles to perforate the dura.
The ICP waveform obtained is a dynamic real-time pressure recording that looks superficially similar to the pulse waveform. It is created by the pulsations of the cerebral blood vessels within the confined space of the cranium, with the effects of respiration superimposed.
Cerebral perfusion pressure = mean arterial pressure – ICP.
The normal resting mean ICP measured in a supine patient is less than 10 mmHg (<1.3 kPa). The level which requires treatment depends, to some extent, on the disease or trauma.
• In benign intracranial hypertension, values of ~40 mmHg may not be associated with neurological symptoms.
• In cerebral trauma, treatment should be initiated with a mean ICP >25 mmHg, though this value is debated.
Several types of pressure wave are described, of which the most significant are ‘A waves’. These are sustained increases of the ICP of up to 50–100 mmHg (6–13 kPa), lasting 10–20 minutes. They are associated with a poor prognosis.
The readings of the ICP monitors should always be accompanied by careful neurological examination. Treatment of raised ICP is discussed in Chapters 8 and 18.
Complications of intracranial pressure monitoring
• Infection (up to 5%).
• Bleeding (e.g. local, subdural, extradural, intracerebral).
• CSF leakage.
• Misreading of ICP pressures.