Cerebral Blood Flow (CBF)
The normal cerebral blood flow is approximately 50ml/100g/min or 700 ml/min, which is roughly 14 % of the cardiac output. This ranges from 20ml/100g/min in white matter to 70ml/100g/min in grey matter.
The Kety-Schmidt technique applies the Fick principal using N2O to calculate whole brain CBF.
Cerebral Blood Flow = Cerebral Perfusion Pressure / Cerebrovascular resistance
Cerebral Perfusion Pressure
CPP = CBF X CVR (Compare BP = CO X SVR)
Cerebral perfusion pressure (CPP) is defined as the difference between mean arterial pressure (MAP) and intracranial (ICP) or central venous pressures (CVP), whichever is the highest. MAP is the diastolic pressure plus one third of the pulse pressure (difference between the systolic and diastolic).
CPP = MAP – ICP (or CVP, whichever is the highest)
A CPP less than 70 mmHg can lead to a rapid decrease in jugular venous bulb saturation (normal range 65%-75%) because of increased oxygen extraction
A number of studies on patients with severe head injuries have shown an increase in mortality and poor outcome when CPP falls to less than 70 mmHg for a sustained period.
Pressure autoregulation maintains CBF at a constant level in normal brain in the face of the usual fluctuations in blood pressure. It is a poorly understood local vascular mechanism. Normally autoregulation maintains a constant blood flow between MAP 50 mmHg and 150 mmHg. However in traumatised or ischaemic brain, CBF may become blood pressure dependent.
Regional blood flow is tightly coupled to brain metabolism. Energy metabolites cause local vasodilatation, assisting with rapid regional control of CBF.
Carbon dioxide can have a significant influence on CBF. As the arterial pCO2 rises, vasodilatation increases CBF and when CO2 is reduced vasoconstriction occurs. When PaCO2 is less than 3.3kPa (25 mmHg) there is no further reduction in CBF. Therefore there is no advantage in inducing further hypocapnia as this will only shift the oxygen dissociation curve further to the left, making oxygen less available to the tissues.
Arterial pO2 has a minimal effect until pO2 drops below 6.7 kPa (50 mmHg), when CBF increases significantly.
SNS not too important (CVR 5-10%)
The autonomic nervous system is extremely important in the control of MAP and therefore CPP, but has a small contribution to CVR (5 – 10%).
Cerebral metabolic rate for oxygen (CMRO2)
This is the volume of O2 metabolised by the brain and is normally around 3.5 ml/100g/min or roughly 50 ml/min (20% of total basal requirements).
It can be calculated using the Fick principal and equals CBF X arteriovenous O2 content difference.