Cerebral circulation

The brain accounts for only 2% of the total body weight yet its blood flow represents 15% of the resting cardiac output and it uses 20% of the oxygen used by the whole body at rest. Cerebral blood flow (CBF) has been studied by the passage of biochemically inert gases (e.g. nitrous oxide, krypton and xenon) through the brain. The value for a normal conscious human at rest is 50-60 ml of blood/100g brain tissue/min. Each day the brain requires about 1000 litres of blood in order to obtain 71 litres of oxygen. Cessation of blood flow causes unconsciousness within 5-10 s.

The brain can regulate the blood flow in accordance to its metabolic need. The cerebral vasculature can adjust to changes in arterial blood pressure keeping the flow constant within certain limits, this is called cerebrovascular autoregulation. The CBF is autoregulated within certain parameters:

• CBF is maintained at systemic mean arterial pressures between 60 and 150mmHg. Below systemic blood pressures of 60 mmHg, CBF lessens such that if the pressure falls rapidly to 20 mmHg, CBF virtually ceases. Slower falls under controlled conditions, such as hypotensive anaesthesia, can be tolerated to 40 mmHg before CBF starts to lessen. When the systemic arterial pressure is raised, CBF remains constant until 150 mmHg when an increase occurs, often described as breakthrough of autoregulation. In chronically hypertensive patients, this limit may be higher at around 170 mmHg.

• Arterial blood gases have a major influence on CBF. CBF and cerebral blood volume increases when the arterial PaCO2 is raised due to dilatation of pial arterioles. When the arterial PaO2 falls, CBF starts to rise.

When CBF falls to 25-30 ml/100g/min, neurological dysfunction occurs leading to cellular chemical events culminating in neuronal death. Some areas are particularly sensitive including not only the deep nuclei but also the boundary areas of the cortex where the distal branches of the major vessels of the circle of Willis meet and as a consequence the circulation and perfusion is less resistant to falling CBF (these areas may infarct giving rise to the so-called boundary zone lesions/infarcts). These changes have major implications not only for intracranial pathological processes such as stroke, head injury and neoplasia, but also for systemic processes such as shock and severe respiratory dysfunction. Therefore, it is imperative to attempt to normalize blood pressure and arterial blood gases and reduce raised intracra-nial pressure (ICP) when it occurs.

The difference between the arterial pressure and the intra-cranial pressure is the cerebral perfusion pressure (CPP). In patients where autoregulation is impaired such as after head injury falls in the CPP will lead to cerebral ischaemia and infarction. It is currently recommended to keep the CPP above 70 mmHg following severe head injury. Often the intracranial pressure after head injury can be in the region of 30 mmHg, it therefore follows that the mean arterial blood pressure (MABP) should be maintained at around 100 mmHg until the intracranial pressure can be directly measured.

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