Cerebral blood flow

Cerebral blood flow (CBF) is around 15% cardiac output and is affected by various factors (Figure 9.1).

• A high PaCO2 increases CBF by vasodilatation of blood vessels. Conversely, a low PaCO2 causes vasoconstriction -reducing the PaCO2 from 5 to 4 kPA (38-5-30-7 mmHg) reduces CBF by almost 30%.

• Hypoxaemia < 7 5 kPA (515 mmHg) has the effect of increasing CBF.

• Certain drugs affect CBF.

CBF is controlled by alterations in cerebral perfusion pressure (CPP) and cerebral vascular resistance (R). CBF = CPP/R. Cerebral perfusion pressure is the pressure gradient in the brain or the difference between the incoming arteries and the outgoing veins.

CPP = MAP - venous pressure

Venous pressure is equal to intracranial pressure (ICP), so CPP is usually expressed as:

Like the kidneys, the brain autoregulates blood flow so that it is constant between a MAP of 50 and 150 mmHg there is autoregulation (see Figure 9.1). CBF is regulated by changes in resistance of cerebral arteries. Unlike the rest of the body, the larger arteries play a main role in this. Local chemicals, endothelial mediators, and neurogenic factors are thought to be responsible. However, blood vessels in injured areas of the brain do not respond to normal regulation. Hypercapnia causes vasodilatation in normal tissue but diverts blood away from injured areas (the steal phenomenon). Hypocapnia may

MAP (mmHg)

Figure 9.1 Cerebral blood flow (CBF), arterial blood gases and mean arterial pressure (MAP). (a) Between 3-5 and 10-6 kPa CO2 there is an almost linear increase in CBF. There is little change in CBF until below 7-5 kPa O2. (b) Autoregulation of cerebral perfusion occurs between a MAP of 60 and 150. Beyond these limits, CBF is dramatically affected

MAP (mmHg)

Figure 9.1 Cerebral blood flow (CBF), arterial blood gases and mean arterial pressure (MAP). (a) Between 3-5 and 10-6 kPa CO2 there is an almost linear increase in CBF. There is little change in CBF until below 7-5 kPa O2. (b) Autoregulation of cerebral perfusion occurs between a MAP of 60 and 150. Beyond these limits, CBF is dramatically affected increase blood flow through damaged vessels leading to oedema (inverse steal).

The brain is uniquely vulnerable to secondary insults and less capable of maintaining an adequate blood flow and metabolic balance. A decrease in CBF in an injured brain may induce ischaemia; an increase in CBF may induce oedema.

Reduced intracranial CSF volume

Reduced intracranial CSF volume

Figure 9.2 The Monro-Kellie doctrine. The CT scan shows a large extradural haematoma. Brainstem herniation eventually occurs

The skull is a rigid box

Increased intracranial blood volume

Figure 9.2 The Monro-Kellie doctrine. The CT scan shows a large extradural haematoma. Brainstem herniation eventually occurs

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