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Number Conscious

The bispectral index uses EEG readings to assign a number to a patient's level of consciousness, allowing doctors to administer more precise doses of anesthesia.

By Charles Slack // Summer 2006
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EEG chart

EEG chart

Courtesy of Aspect Medical

MID TO HIGH 90s:

Patient is fully conscious; responds to normal-volume speech

80 TO LOW 90s:

Lightly to moderately sedated; awake but drowsy

70–80:

Able to respond to loud commands or mild prodding or shaking

60–70:

Deeply sedated; probably unresponsive to speech or touch; low probability of explicit recall of surgical events

40-60:

Under general anesthesia; low probability of consciousness; level considered low enough for most patients to be unaware

20–40:

Deep hypnotic state; extremely low probability of consciousness

1–20:

Burst suppression (a flat EEG line with intermittent bursts of activity)—dangerously low level

0:

Absence of brain activity

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Understanding Anesthesia

The waning of consciousness during surgery is as mysterious as it is routine. Finally, the curtain may be about to part.

Consciousness Unbound

Some researchers say the key to learning how anesthetics work is to examine a much tougher subject: consciousness.

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