General anesthesia is essential to modern medicine. It allows physicians to temporarily keep people in an unconscious state. When infusions of the anesthetic drug stop, patients gradually recover consciousness and awaken, a process called emergence.

Previous studies using recordings of electrical activity in the brain have documented spontaneous changes during anesthesia. In addition, the way the brain responds to sounds or other stimulation is altered. How the brain switches between the anesthetized and awake states is not well understood.

Studying the changes that happen during emergence may help scientists learn how the brain awakens after anesthesia. A key question is whether the changes that occur during emergence are the reverse of what happens when someone is anesthetized, or whether it is a completely different process. Knowing this could help clinicians monitoring patients under anesthesia, and help scientists understand more about how the brain transitions into the awake state.

Electrical Activity Changes

Now, Laura D Lewis, of Harvard University, and colleageus show that people go through a sleep-like state right before awakening from anesthesia-induced unconsciousness. In the experiments, recordings were made of the electrical activity in the brains of people emerging from anesthesia.

One set of recordings was taken in people with epilepsy, who had electrodes implanted in their brains as part of their treatment. Similar recordings of brain electrical activity during emergence were also made on healthy volunteers using electrodes placed on their scalps.

[caption id=“attachment_96991” align=“aligncenter” width=“680”]Auditory stimuli can evoke large, slow, asymmetric events during emergence from general anesthesia. Auditory stimuli can evoke large, slow, asymmetric events during emergence from general anesthesia.
Credit: Lewis et al. CC-BY[/caption]

In both groups of people, Lewis et al. documented large changes in electrical activity in the brain’s response to sound in the minutes before emergence.

Sleep-like State

These patterns of electrical activity during emergence were similar to those seen in patients during a normal stage of sleep (stage 2). Patients who were about to wake up from general anesthesia had suppressed brain activity in response to sounds, such as their name.

Moreover, this sleep-like state happened only during emergence, indicating it is a distinct process from going under anesthesia. The experiments also suggest that the brain may use a common process to wake up after sleep or anesthesia.

More studies may help scientists understand this process and how to better care for patients who need anesthesia.

The research received support from National Institutes of Health and Harvard University.

Laura D Lewis, Giovanni Piantoni, Robert A Peterfreund, Emad N Eskandar, Priscilla Grace Harrell, Oluwaseun Akeju, Linda S Aglio, Sydney S Cash, Emery N Brown, Eran A Mukamel, Patrick L Purdon A transient cortical state with sleep-like sensory responses precedes emergence from general anesthesia in humans eLife 2018;7:e33250 DOI: 10.7554/eLife.33250

Top Image: Lewis et al. CC-BY. Spatial distribution of the large evoked potential, where color indicates absolute value of the amplitude of the mean potential over all ‘event trials’.

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