Oxygen and blood in Brain-Brain Protection Part 1

Brain is metabolically very active and its oxygen consumption is about 3.5 - 4.0 ml/100 gm/min.

Electrical activity of neurons (transient depolarization and repolarization with their attendant ionic shifts) consumes about 50% of the total energy production of neurons.

Thus, energy consumption can be reduced significantly by agents that can render the EEG isoelectric (e.g., barbiturates).

The remaining 50% is used to maintain basal cellular homeostasis.Although this portion of the total energy consumption is not amenable to reduction by anesthetic agents, hypothermia can reduce it substantially.

The normal CBF in humans is about 50 ml/100 gm/min.

The response of the brain to ischemia has been well characterized.

When CBF reaches about 20 ml/100 gm/min, EEG isoelectricity occurs.

At a flow of about 15 ml/100 gm/min, evoked responses can no longer be obtained. Although neurons do not immediately die at this flow rate, death will eventually occur if flow is not restored.

Below a flow of 10 ml/100 gm/min, ATP levels decline rapidly (within 5 minutes) and the neuron is

unable to maintain ionic homeostasis. At this point, the neuron undergoes depolarization (anoxic depolarization). By activating several biochemical cascades in a haphazard manner, calcium ultimately leads to neuronal death.

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