Cerebral blood flow and intracranial pressure

The brain requires a stable environment for effective neuronal function and this is provided by the blood-brain barrier and the cerebrospinal fluid (CSF). Under normal circumstances cerebral blood flow (CBF) is constant at about 700 ml/min, but despite this high flow, cerebral blood volume (CBV) is only 150 ml. CBF is closely linked to brain metabolic activity and conditions reducing this (e.g. hypothermia, coma) reduce CBF. The partial pressure of carbon dioxide in arterial blood (PaCO2) also influences CBF, with an almost linear relationship between PaCO2 2.7 and 10.6 kPa, a result of progressive vasodilatation (and increase in CBV). During normal variations in systemic blood pressure, CBF remains relatively constant as a result of autoregulation, whereby vasodilatation occurs in response to falling cerebral perfusion pressure (CPP) and vasoconstriction to increasing perfusion pressure. These vascular changes also affect CBV. CPP is the difference between mean arterial pressure and intracranial pressure (ICP). ICP is the result of the relationship between the skull and the volumes of brain, CSF and blood within it. Any increase in the volume of one of the contents must be compensated for by a decrease in either or both of the others to prevent an increase in pressure. This is usually venous blood or CSF because brain is not easily or safely displaced. Once these compensatory mechanisms are exhausted, ICP rises rapidly, compromising perfusion. An understanding of the interrelationship between CBF, CBV and ICP is essential for the safe and effective management of the patient with a head injury.