Cerebral Blood Flow and Intracranial Pressure
Dr Lisa Hill, SpR Anaesthesia, Royal Oldham Hospital, UK.
Email – lambpie10@hotmail.com
Dr Carl Gwinnutt, Consultant Neuroanaesthetist, Hope Hospital, UK.
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
Figure 5. ICP elastance curve (change in pressure per unit change in volume)
Stage 1/2 = compensation phase. As one of the intracranial constituents increases in
volume, the other two constituents decrease in volume in order to keep the intracranial
pressure constant.
Stage 3/4 = decompensated phase. When compensatory mechanisms are exhausted,
small increases in the volumes of intracranial constituents cause large increases in ICP.
The slope of the curve is dependent on which intracranial constituent is increasing. If it
is blood or CSF, both of which are poorly compressible, then the slope is steeper. If it is
brain tissue, such as from a tumour, the curve is less steep as the tissue is compressible.
Cerebrospinal Fluid (CSF)
CSF is a specialised extracellular fluid i