Question 14

College Answer

Cerebral Blood Flow is result of CPP/CVR. The brain defends a relatively constant high blood flow via multiple auto-regulatory processes that influence cerebral vascular resistance. This question required an in detail description of the physiological factors that alter CBF. Autoregulation via the myogenic and metabolic mechanisms, the difference between grey and white matter due to metabolic variation, the role of pCO2 and O2, sympathetic nervous system and temperature was expected. A "describe" question requires not only the factors that influence CBF but how and why, so detail is required. Whilst the monroe kellie doctrine does describe changes in blood flow this is only important at extremes of intracranial pressure when these normal autoregulatory mechanisms are exceeded, as such it was not part of the answer to this question.

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 
    • At a PaCO₂ of around 70 mmHg, the CBF is basically doubled (Kety & Schmidt, 1947) 
  • PaO₂:  PaO₂ falling below 50 mmHg leads to exponentially increased CBF.
    • At a PaO₂ of around 20 mmHg, CBF may increase up to 700% (Jóhannsson and Siesjö, 1975),
  • MAP: CBF is stable over a range of MAP between 50 and 150 mmHg.
    • As MAP decreases below around 60 mmHg, CBF cannot be maintained
    • As MAP increases beyond around 150 mmHg, cerebral autoregulation fails and CBF increases in proportion to the increase in pressure 
• These relationships can be described graphically:
  
• The mechanisms involved in auoregulation are:
  • Autonomic (cerebral vessels have rich autonomic innervation)
  • Endothelial mechanisms in response to mechanical stress
  • Myogenic autoregulation in response to stretch
  • Metabolic autoregulation in response to byproducts of metabolism ( hydrogen ions, CO₂, lactate, adenosine, potassium and calcium)

Harper, A. MURRAY. "Autoregulation of cerebral blood flow: influence of the arterial blood pressure on the blood flow through the cerebral cortex." Journal of neurology, neurosurgery, and psychiatry 29.5 (1966): 398.

Phillips, Stephen J., and Jack P. Whisnant. "Hypertension and the brain."Archives of internal medicine 152.5 (1992): 938-945.

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.