Part 2: Intracranial pressure (ICP)
As mentioned in the previous tutorial, intracranial pressure is important as it affects
cerebral perfusion pressure and cerebral blood flow. Normal ICP is between 5 and
13mmHg. Because it is very dependant on posture, the external auditory meatus is
usually used as the zero point.
Some facts and figures:-
• Constituents within the skull include the brain (80%/1400ml), blood (10%/150ml)
and cerebrospinal fluid (CSF 10%/150ml)
• The skull is a rigid box so if one of the three components increases in volume, then
there must be compensation by a decrease in the volume of one or more of the
remaining components otherwise the ICP will increase (Monro-Kellie hypothesis).
The term compliance is often used to describe this relationship, but it is more
accurately elastance (change in pressure for unit change in volume)
• Compensatory mechanisms include movement of CSF into the spinal sac, increased
reuptake of CSF and compression of venous sinuses. These mechanisms reduce the
liquid volume of the intracranial contents
CSF is important for acid-base regulation for control of respiration
• CSF provides a medium for nutrients after they are transported actively across the
blood-brain-barrier
It is produced at a rate of 0.3-0.4ml/min (500ml/day) by the choroid plexus in the lateral,
third and fourth ventricles. CSF is produced by the filtration of plasma through
fenestrated capillaries followed by active transport of water and dissolved substances
through the epithelial cells of the blood-CSF barrier. This is distinct from the blood-
brain-barrier which consists of endothelial cells linked by tight junctions whose function
is to protect the brain from chemicals in the blood stream. CSF formation is dependent
on the CPP and when this falls below 70mmHg, CSF production also falls because of the
reduction in cerebral and choroid plexus blood flow. Following production, CSF then
circulates through the ventricular system and the subarachnoid spaces, aided by ciliary
movements of the ependymal cells. Resorption takes place mostly in the arachnoid villi
and granulations into the circulation: the mechanism behind the resorption is the
difference between the CSF pressure and the venous pressure. An obstruction in CSF
circulation, overproduction of CSF or inadequate resorption results in hydrocephalus.
Composition of Plasma and CSF
Plasma mmol/l CSF mmol/l
Urea 2.5-6.5 2.0-7.0
Glucose (fasting) 3.0-5.0 2.5-4.5
Sodium 136-148 144-152
Potassium 3.8-5.0 2.0-3.0
Calcium 2.2-2.6 1.1-1.3
Chloride 95-105 123-128
Bicarbonate 24-32 24-32
Protein 60-80g/l 200-400mg/l
Figure 6. Production, circulation and resorption of CSF. Production mostly takes place
in the choroid plexus of the lateral ventricles. CSF circulates to the subarachnoid spaces,
where resorption takes place via the arachnoid granulations and villi. When ICP is
raised, the pressure is transmitted along the optic nerve causing papilloedema. (Image
www.ihrfoundation.org/images/schematic_lg.gif)
Pathological Conditions Causing a Rise in Volume of Intracranial Constituents
Any of the three intracranial constituents (tissue, blood or CSF) can increase in size and
volume.
Brain Tissue Blood CSF
- Tumours
- Cerebral oedema secondary
to trauma, infection, infarction,
hyponatraemia, hypertensive
encephalopathy, acute liver
failure, Reye’s syndrome
- Cerebral abcesses
- Cerebral contusions
- Intracerebral, subarachnoid,
subdural, extradural haematomas
- Arteriolar dilatation secondary to
hypoxaemia, hypercarbia,
anaesthetic drugs, hyperthermia,
seizures, hypotension
- Venous dilatation secondary to
venous obstruction from high
PEEP, coughing, straining, heart
failure, venous sinus thrombosis,
head-down tilt, tight neck ties
- Hydrocephalus
- Meningeal diseases
- Choroid plexus tumours
Effects of a Raised ICP
As ICP rises, CPP falls eventually to a point when there is no cerebral blood flow, no
cerebral perfusion and brain death. Prior to this, brain structures begin to herniate
(protrude through an opening). Physiological compensatory mechanisms occur to try and
maintain cerebral blood flow:-
1. Temporal lobe herniation beneath tentorium cerebelli (uncal herniation) – causes
cranial nerve III palsy (dilatation of pupil followed by movement of eye down and out).
2. Herniation of cerebellar peduncles through foramen magnum (tonsillar herniation).
Pressure on the brainstem causes the Cushing reflex – hypertension, bradycardia and
Cheyne-Stokes respiration (periodic breathing).
3. Subfalcine herniation occurs when the cingulate gyrus on the medial aspect of the
frontal lobe is displaced across the midline under the free edge of the falx cerebri and
may compress the anterior cerebral artery.
4. Upward, or cerebellar herniation occurs when either a large mass or increased
pressure in the posterior fossa occurs. The cerebellum is displaced in an upward
direction through the tentorial opening and causes significant upper brainstem
compression.
How can ICP be influenced?
Primary brain damage occurs at the time of a head injury and is unavoidable except
through preventative measures. The aim of management following this is to reduce
secondary brain damage which is caused by a reduction in oxygen delivery due to
hypoxaemia (low arterial PaO2) or anaemia, a reduction in cerebral blood flow due to
hypotension or reduced cardiac output, and factors which cause a raised ICP and reduced
CPP.
The most important management strategy ensures A (Airway and C spine protection), B
(Breathing and adequate oxygenation) and C (blood pressure and CPP). Following this,
further strategies to reduce ICP and preserve cerebral perfusion are required. Techniques
that can be employed to reduce ICP are aimed at reducing the volume of one or more of
the contents of the skull.
Reduce brain tissue volume Reduce blood volume Reduce CSF volume
-Tumour resection, abcess
removal
-Steroids (especially
dexamethasone) to reduce
cerebral oedema
-Mannitol/furosemide to reduce
intracellular volume
-Hypertonic saline to reduce
intracellular volume
- Decompressive craniectomy
to increase intracranial volume
- Evacuation of haematomas
- Arterial: avoiding hypoxaemia,
hypercarbia, hyperthermia,
vasodilatory drugs, hypotension
- Barbiturate coma to reduce CMRO2
and cerebral blood volume
- Venous: patient positioning with 30°
head up, avoiding neck compression
with ties/excessive rotation, avoiding
PEEP/airway obstruction/CVP lines
in neck
- Insertion of external
ventricular drain or
ventriculoperitoneal
shunt to reduce CSF
volume (more long term
measure)
If ICP is not measured directly, we can estimate it and therefore make changes in MAP to
maintain CPP-
o Patient drowsy and confused (GCS 9-13) ICP ∼ 20mmHg
o GCS ≤ 8 ICP ∼ 30mmHg
Often, blood pressure needs to be augmented with drugs that produce arterial
vasoconstriction such as metaraminol or noradrenaline (which requires central venous
access). Following a head injury when autoregulation is impaired, if there is a drop in
MAP from drugs or blood loss, the resulting cerebral vasodilatation increases cerebral
blood volume which in turn raises ICP and further drops CPP. This starts a vicious cycle.
So by raising MAP, ICP can often be reduced.
Measuring ICP
ICP is traditionally measured by use of a ventriculostomy, which involves a catheter that
is placed through a small hole in the skull (burr hole) into the lateral ventricle. ICP is
then measured by transducing the pressure in a fluid column. Ventriculostomies also
allow for drainage of CSF, which can be effective in decreasing the ICP. More
commonly ICP is now measured by placing some form of measuring device (for example
a minature transducer) within the brain tissue (intraparenchymal monitor). An epidural
monitor can also be used but becomes increasingly unreliable at extremes of pressure.
The normal ICP waveform is a triphasic wave, in which the first peak is the largest peak
and the second and third peaks are progressively smaller. When intracranial compliance
is abnormal, the second and third peaks are usually larger than the first peak. In addition,
when intracranial compliance is abnormal and ICP is elevated, pathologic waves may
appear. Lundberg described 3 types of abnormal ICP waves in 1960, that he named A, B,
and C waves. Although these can be identified, it is more common nowadays to measure
the mean ICP and use this to calculate CPP.
Measuring the Adequacy of Cerebral Perfusion
This is difficult as ideally adequacy of cerebral perfusion would be determined at a
cellular level to determine whether neurones are receiving adequate oxygen and nutrients.
Inferences about cerebral perfusion can be made by looking at a variety of measured
variables. The first five techniques can be used at the bedside and are often part of
multimodal monitoring of head injured patients. The latter techniques are more invasive
and generally restricted to research programs.
o Measuring ICP and calculating CPP (most common method)
o Jugular venous bulb oxygen saturations (Sjv02, usually 65-75%). Reflects the
balance between cerebral oxygen delivery and CMR02. Low Sjv02 reliably
indicates cerebral hypoperfusion
o Transcranial Doppler to measure blood velocity and estimate CBF
o Microdialysis catheters to measure glucose, pyruvate, lactate, glycerol, glutamate
(metabolic variables)
o Positron Emission Tomography – the distribution of radiolabelled water in the
brain is monitored to indicate metabolic activity
Functional MR imaging techniques
o Kety-Schmidt equation to determine CBF by using an inert carrier gas (133Xe)
o Near infrared spectroscopy (NIRS) to measure oxygenation in a localised cerebral
field