Intracranial Pressure and Cerebral Blood Flow

The brain is only able to withstand very short periods of ischaemia, unlike the kidney, liver or muscle

Maintenance of cerebral blood flow depends on a balance between the pressure within the skull, intracranial pressure (ICP) and the arterial pressure of the blood, mean arterial pressure (MAP).

Thus when blood pressure falls, physiological mechanisms attempt to maintain flow to prevent ischaemia. This process is autoregulation

If there is an increase in the volume of either the brain or blood the normal initial response is a reduction in CSF volume within the skull. CSF is forced out into the spinal sac

It is believed that a MAP that is greater than 60 mmHg is enough to sustain the organs of the average person.

CPP = MAP - ICP (normal 10 mmHg)
Normal cerebral perfusion pressure is 80 mmHg

but when reduced to less than 50 mmHg there is metabolic evidence of ischaemia and reduced electrical activity

In the head injured patient, CPP should not fall below 70 mmHg.

Drowsy and confused: (GCS 13-15)ICP=20 mmHg,Severe brain swelling (GCS <8) ICP=30 mmHg

The normal cerebral blood flow is 45-50ml 100g-1 min-1, ranging from 20ml 100g-1 min-1 in white matter to 70ml 100g-1 min-1 in grey matter. There are two essential facts to understand about cerebral blood flow. Firstly, in normal circumstances when the flow falls to less than 18-20ml 100g-1 min-1, physiological electrical function of the cell begins to fail

Normally autoregulation maintains a constant blood flow between MAP 50 mmHg and 150 mmHg

Carbon dioxide causes cerebral vasodilation. As the arterial tension of CO2 rises, CBF increases and when it is reduced vasoconstriction is induced.

Full article here http://www.nda.ox.ac.uk/wfsa/html/u08/u08_013.htm