44. ICP Flashcards
What is normal intracranial pressure?
Around 10 mmHg or less
Sustained pressure of >15mmHg is termed ‘intracranial hypertension’
Areas of focal ischaemia if ICP > 20
Global ischaemia if ICP > 50
Treatment usually considered if ICP > 20
Cerebral perfusion pressure:
CPP = MAP − ICP(or CVP if greater)
Normal CPP = 80–100mmHg.
Remember that adequate tissue perfusion is not just a function of CPP
but depends on blood flow and oxygen content.
What are the causes of raised intracranial pressure?
According to the Munro–Kelly hypothesis (1852) the contents of the cranium
are not compressible. Any significant increase in the volume of brain tissue,
blood or CSF within the cranium will lead to a rapid rise in intracranial
pressure (ICP).
The causes of raised ICP may therefore be divided into three broad
categories:
- Brain
Tumours
Infection/abscess
Oedema – three types: vasogenic, cytotoxic, interstitial - Blood
Extradural
Subdural
Intracerebral
Subarachnoid
Venous congestion (e.g. cavernous sinus thrombosis) - CSF
Outflow obstruction, e.g. SAH, meningitis, tumours
Increased production – choroid plexus papilloma (extremely rare !)
Benign intracranial hypertension (usually young, obese women).
What are the symptoms and signs of raised intracranial pressure?
Symptoms
Headache
Vomiting
Drowsiness
Confusion
Neck stiffness
Seizures.
Signs
Depressed GCS
Papilloedema (late sign)
Cushing reflex (hypertension and bradycardia)
Irregular respirations
Seizures
Hemiparesis
IV cranial nerve palsy (false localising sign due to long intracranial course)
III cranial nerve palsy due to tentorial coning
Coning Two major types:
- Tentorial – part of the temporal lobe compresses the
brainstem on the contralateral side.
This results in an ipsilateral IIIn palsy, decreased GCS, Cushing reflex and
decerebrate rigidity. - Cerebellar- the medulla is compressed by the cerebellar
tonsils passing through the foramen magnum resulting in
Cheyne–Stokes breathing, sudden apnoea and neck stiffness.
What are the indications for ICP monitoring following head injury?
GCS<8 with an abnormal CT scan
Normal CT scan but two or more of the following factors
Age >40
Hypotension
Unilateral posturing
Bilateral posturing
How can you measure intracranial pressure clinically
There are four methods commonly used via the skull and a lumbar approach
via a CSF catheter.
- Epidural catheter
Strain gauge transducer at tip or
fibre-optically supplied light reflecting off a
pressure-sensitive membrane - Subdural bolt or catheter
Prone to blocking and leak but less risk of
infection than ventricular catheter - Ventricular catheter
Gold standard, accurate, CSF can be drained
but risk of infection - Intraparenchymal catheter
Light reflecting pressure-sensitive membrane
- Intraparenchymal catheter
The appropriate monitor will display the ICP and a waveform.
Lundberg waves:
A-waves
Sustained pressure waves (60–80mmHg) every 5–20 minutes
Life-threatening and represent cerebral vasodilatation in
response to ↓ CPP. Need urgent treatment.
B-waves
Small and short lasting waves (10–20mmHg) every 30–120
seconds. Caused by fluctuations in CBV.
C-waves Small oscillations (0–10 mmHg),
reflect changes in systemic
arterial pressure.
The Management of Raised ICP
Moderate head-up position will reduce venous pressure without unduly affecting the
MAP (provided there is no physical constriction to drainage by artefacts such as
tracheal tube tapes). Moderate hypocapnia will reduce ICP, but the benefit is
short-lived, and there is a risk of rebound hyperaemia.
Mannitol 20% in a dose of 0.5 g kg1 has a marked but transient effect. It may shift the patient back down the
intracranial compliance curve and gain sufficient time for definitive treatment before
a catastrophic rise in ICP
High-dose dexamethasone
reduces oedema secondary to intracranial tumours but has no effect on raised ICP
following trauma. Hypertonic saline has a well-established role in the management of
raised ICP secondary to brain injury and neoplasm, although not in other conditions.
It also has a role in limiting secondary brain injury due to neurochemical changes.
Hyperthermia will increase the cerebral metabolic rate for oxygen (CMRO2) and
cerebral blood flow and must be avoided. Hypothermia has the opposite effect and
may confer some benefit. Intermittent cerebrospinal fluid drainage via an intraventricular
catheter can be very effective. (Continuous drainage risks emptying the
ventricles completely, which will then make ICP monitoring impossible.)