Describe the control of cerebral blood flow.
Good answers included an equation and then explored the various components of the
equation. Main points for a pass included pressure and metabolic autoregulation and the
various factors that affect cerebral vascular resistance. Graphs were a useful way to answer
this question but were generally underutilised. Several candidates wrote about the Monroe-
Kellie doctrine which was not directly relevant to the question.
Reference: Power and Kam 1st edition p 42-43
Guyton and Hall 11th edition p 761-3
The most economical way to answer this question would probably look something like this:
- Cerebral blood flow is supplied by the carotid (70% and vertebral (30% arteries)
- It is usually 50ml/100g/min, or 14% on normal cardiac output
- It is described by the Ohm equation, Q = (Pa- Pv) / R, where
- (Pa- Pv) is the cerebral perfusion pressure (CPP)
- R is the cerbral vascular resistance
- Cerebral perfusion pressure = MAP - (ICP or CVP, whichever is higher)
- The higher the ICP (or CVP), the lower the CPP, if the MAP remains stable
- Cerebral resistance (R) = (8 l η) / πr4, where
- l = length of the vessel
- η = viscosity of the blood
- r = radius of the cerebral vessels, which is the main variable susceptible to regulation
- Cerebral autoregulation is a homeostatic process that regulates and maintains cerebral blood flow (CBF) constant and matched to cerebral metabolic demand across a range of blood pressures.
- It is affected by:
- PaCO2: increased PaCO2 leads to increased CBF
- PaO2: PaO2 falling below 50 mmHg leads to exponentially increased CBF
- MAP: CBF is stable over a range of MAP between 50 and 150 mmHg
Paulson, O. B., S. Strandgaard, and L. Edvinsson. "Cerebral autoregulation." Cerebrovascular and brain metabolism reviews 2.2 (1989): 161-192.
Busija, David W., and Donald D. Heistad. Factors involved in the physiological regulation of the cerebral circulation. Springer Berlin Heidelberg, 1984.
Mchedlishvili, George. "Physiological mechanisms controlling cerebral blood flow." Stroke 11.3 (1980): 240-248.