What is the normal blood flow to the brain?
- Global CBF is ~50ml / 100g brain tissue / minute
- White matter: 20ml/100g/min
- Grey matter: 70ml/100g/min
- Receives ~15% of cardiac output at rest
Which factors affect cerebral blood flow?
Affecting Cerebral Perfusion Pressure
- Changes in mean arterial pressure (MAP)
- Changes in intracranial pressure (ICP)
- Changes in central venous pressure (CVP)
Affecting Radius of Cerebral Vessels
- Myogenic control
- Metabolic mediators:
- pCO2
- pO2
- Cerebral metabolic rate
- Temperature
- Neurogenic mediators
- Endothelial mediators
- Chemical mediators:
- Volatile anaesthetics
- Intravenous anaesthetics
Affecting Blood Rheology
- Haematocrit
What effect does PaCO2 have on cerebral blood flow?
- Increases linearly between a PaCO2 range of 3-10 kPa:
- Due to CO2 mediated vasodilatation
- Blood flow increases by 3-4% for each 0.13kPa (1mmHg) increase in partial pressure of carbon dioxide (PaCO2)
- Below 3.5 kPa, cerebral vasoconstriction leads to tissue hypoxia with subsequent reflex vasodilatation and maintenance of blood flow
- Above 10 kPa vasodilatation is maximal with no further increase in flow
- In chronic hypercapnia, the curve is shifted to the right, with a higher kPa over which linear vasodilatation occurs
