CEREBRAL BLOOD FLOW (contd)
Autoregulation is a compensatory mechanism which permits fluctuation in the cerebral perfusion pressure within certain limits without significantly altering cerebral blood flow.
A drop in cerebral perfusion pressure produces vasodilation (probably due to a direct 'myogenic' effect on the vascular smooth muscle) thereby maintaining flow; a rise in the cerebral perfusion pressure causes vasoconstriction.
Neurogenic influences appear to have little direct effect on the cerebral vessels but they may alter the range of pressure changes over which autoregulation acts.
Autoregulation fails when the cerebral perfusion pressure falls below 60 mmHg or rises above 160 mmHg. At these extremes, cerebral blood flow is more directly related to the perfusion pressure.
In damaged brain (e.g. after head injury or subarachnoid haemorrhage), autoregulation is impaired; a drop in cerebral perfusion pressure is more likely to reduce cerebral blood flow and cause ischaemia. Conversely, a high cerebral perfusion may increase the cerebral blood flow, break down the blood-brain barrier and produce cerebral oedema as in hypertensive encephalopathy.
INTRACRANIAL PRESSURE (ICP)
Intracranial pressure, measured relative to the foramen of Monro, under normal conditions ranges from 0-135 mm CSF (0-10 mmHg) although very high pressures, e.g., 1000 mm CSF may occur transiently during coughing or straining.
AUTOREGULATION: CBF maintained despite change in CPP
Cerebral perfusion pressure (BP-ICP)
When a mass expands within the skull compensatory mechanisms initially maintain a normal intracranial pressure
100 80 60 40 20
100 80 60 40 20
Eventually further small increments in volume produce larger and larger increments in intracranial pressure
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