Figure 21-13 Relationships between cerebral perfusion, cerebral blood flow (CBF), the electroencephalogram (EEG), and the functional status/viability of neurons. Note that in the approximate CBF range of 6 to 12 mL/kg/min, the energy supply is insufficient to support electrophysiologic activity (i.e., EEG flat), but it can prevent complete membrane failure and neuronal death for extended periods. These areas are referred to as the ischemic penumbra.^[437] The data are derived from studies in the cerebral cortex of barbiturate-anesthetized baboons^{[437] [438]} and unanesthetized monkeys.^[439] The CBF and mean arterial pressure thresholds may vary with the anesthetic and species.^[440]