1. Cerebral Blood Flow Dr James F Peerless July 2015

2. The Brain Complex organ requiring continuous supply of O 2 and glucose Reduction in cerebral blood flow (CBF)  loss of consciousness within seconds Permanent damage occurs within 3-8 minutes

3. Cerebral Blood Flow 2% of body mass (1400g) 15% of C.O. CBF modelled by Hagen-Poiseuille equation for laminar flow

4. Arterial Supply to The Brain 7 8 9 10 11 1 2 3 4 5 6

5. Cerebral Perfusion Pressure The difference between the arterial and venous circulations. Also governed by ICP, due to the rigidity of the skull. Pathological conditions, leading to raised ICP, directly compromise CPP.

7. Control of Cerebral Blood Flow

8. Control of CBF Neurogenic control Flow-metabolism coupling Autoregulation CO 2 gas tension Temperature O 2 gas tension Rheology

9. Effect of PaO 2 on CBF

10. Arterial Oxygen Tension Hypoxia  vasodilation  increased CBF Clinically insignificant unless PaO 2 < 6.7 kPa

11. Effect of PaCO 2 on CBF

12. Arterial Carbon Dioxide Tension Sigmoid curve Linear between 2.7 – 10.5 kPa Rapid response

13. Autoregulation AKA myogenic regulation Ability of cerebral circulation to maintain a constant CBF, independent of BP Increase MAP  increase transmural tension  constriction of precapillary resistance vessels Outside these pressures, CBF becomes pressure-dependent

14. Effect of MAP on CBF

15. Autoregulation Almost instant process (1-10s) Occurs between MAP 50-150 mmHg Rightward shift for chronic hypertensives Autoregulation lost in head injury

17. Flow-Metabolism Coupling CBF variable across different areas of brain – Dependent upon neuronal activity Increase in CMR  proportional increases in CBF ?caused by chemical mediators – CO 2, H +, adenosine, K + – Most likely due to NO

18. Cerebral Metabolic Rate

20. Drugs, CMRO 2 & CBF CBF CMRO 2

21. Neurogenic Control Cerebral vasculature under autonomic neuronal control – Sympathetic activity  vasoconstriction (reduced CBF) – Parasympathetic activity  dilatation (increased CBF)

22. Temperature Reducing T  reduced CMRO 2 Per 1°C drop, CMR (and therefore CBF) is reduced by 7% T reduction post cardiac arrest and head injury has shown variable results; but hyperthermia is certainly bad.

23. Rheology Reduction in haematocrit  reduced viscosity Also reduces oxygen content Consensus: maintain Hct 30-35%

24. ICP-Volume Compliance Curve

25. Autoregulation

27. Reference Tameem A, Hari K (2013). Cerebral Physiology. Continuing Education in Anaesthesia, Critical Care & Pain. 13(4): 113-118