Research ArticleConsciousness

Slow-Wave Activity Saturation and Thalamocortical Isolation During Propofol Anesthesia in Humans

Science Translational Medicine  23 Oct 2013:
Vol. 5, Issue 208, pp. 208ra148
DOI: 10.1126/scitranslmed.3006007

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Can You Hear Me Now?

What actually happens in your brain when you lose awareness of what is going on around you? Clinicians would like to know so they can monitor awareness in the operating room without requiring the person to respond to a sound or a touch. Ní Mhuircheartaigh and colleagues have found a signal in the EEG (electrical brain waves recorded from outside the head) that may reflect this transition.

The authors gave the drug propofol to 16 individuals and monitored their EEGs. In each patient, as the drug concentrations in their system rose, the amplitude of electrical waves of a certain frequency (slow-wave activity) grew. Eventually, the slow-wave amplitude reached a plateau and, even though drug concentrations continued to increase, the amplitude stayed at this plateau value. To test whether the transition to the plateau represented the point at which the subjects lost awareness, they performed simultaneous functional magnetic resonance imaging and sensory stimulation. The beginning of the plateau coincided with the point at which sensory signals no longer reached the thalamocortical portion of the brain, rendering it an “island” of activity. Unexpectedly, other brain regions still responded to stimuli, including posterior parietal and prefrontal cortices.

Although more studies of slow-wave activity and its relation to perceptual awareness are needed, the transition of slow-wave activity to saturation is a potential individualized index of sensory isolation that may prove useful in the operating room.