Question 5

College Answer

The main points expected for a pass were:

• Description of the relationship of CO2; O2; MAP and Cerebral metabolism with cerebral blood flow. The use of graphs, correctly labelled, and associated free text would be an effective means of portraying this information.

• The effect of other factors such as intracranial pressure, cerebral venous pressure, vascular calibre, blood viscosity and regional blood flow differences.

Syllabus C1f2c

Discussion

• 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 = (P_a- P_v) / R, where
  • (P_a- P_v)  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
• These relationships can be described graphically:
  

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.