Cerebral Blood Flow Dr James F Peerless July 2015

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

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

Arterial Supply to The Brain

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.

Control of Cerebral Blood Flow

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

Effect of PaO 2 on CBF

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

Effect of PaCO 2 on CBF

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

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

Effect of MAP on CBF

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

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

Cerebral Metabolic Rate

Drugs, CMRO 2 & CBF CBF CMRO 2

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

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.

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

ICP-Volume Compliance Curve

Autoregulation

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