Cerebral blood flow is depend on
i.e. (CO2 concentration): (441- Lee's) (376-77-AK-Jain 4th/anaesthesia 12th)Cerebral blood flow increases in linearity with rise in arterial PC02 (Direct effect)Cerebral blood flow (CBF) is the difference between mean arterial pressure and intracranial pressure or central venous pressure which ever is higher.It is 50 mi/100 gm of brain tissueSince ions are not able to cross the blood brain barrier as easily as O2, O2 and lipid soluble substances (such as most anaesthetics) the ions have little effects on cerebral blood flowFactors affecting cerebral blood flow (CBF)A. Cerebral metabolism - cerebral blood flow varies directly with metabolic activity, various studies have shown that regional CBF parallels metabolic activity for eg motor activity of a limb is associated with an increased in regional CBF of the corresponding motor cortex. Similarily, visual activity is associated with an increase in regional CBF of the corresponding occipital cortex.B. Cerebral Perfusion pressure: is the difference between mean arterial pressure (MAP) and intracranial pressure (ICP) or central venous pressure (CVP) which ever is greater CPP is normally 80-100 mm Hg. Because ICP is normally less than 10 mm Hg. CPP is primarily dependent on MAP. Autoregulation maintains a steady CBF as perfusion pressure varies between 60 and 160 mm Hg. Decrease in CPP results in cerebral vasodilation, where as elevations induce vasoconstriction.C. Arterial PCO2 - The most important extrinsic influences on CBF are respiratory gas tensions particularly arterial PCO2. CBF is directly proportional to arterial PCO2 between tensions of 20 and 80 mm of Hg. The effect is almost immediates and is thought to be secondary to changes in the PH of CSF and cerebral tissueD. Arterial PO2: only marked changes in arterial PO2 alter CBF whereas hypoxia may be associated with only minimal decrease (-10%) in CBF severe hypoxemia (arterial PO2 less than 50 mm Hg) profoundly increases CBFE. Temperature - CBF changes 5 - 7% 1degC change in temperature. Hypothermia decreases both cerebral metabolic rate and CBF. Whereas pyrexia has the reverse effect.F. Blood viscosity - CBF rises as viscosity falls and vise versaG. Autonomic system* Cerebral blood flow is maintained at constant levels (does not change) in response to moderate exercise)**** Mild to moderate decrease in perfusion pressure decreases driving forces for flow, resulting in CO2 accumulation, which by release of H+ produces cerebral vasodilation.* Most important factor for regulating cerebral blood flow under normal conditions is the rate of cerebral CO2formation.
O2 concentration
CO2 concentration
K+ concentration
Cl- concentration
Nervous System