EEG Helps Researchers Understand Sedation And Consciousness
March 5, 2013

EEG Helps Researchers Understand Sedation And Consciousness

Brett Smith for — Your Universe Online

Doctors and surgeons have been using general anesthetic to induce unconsciousness for over one hundred years, but little work has been done to define what unconsciousness actually is.

A new study in the Proceedings of the National Academy of Sciences from a group of Massachusetts General Hospital (MGH) researchers has defined consciousness more thoroughly by using a high quality electroencephalogram (EEG) to identify the brain waves that mark the transition in and out of consciousness.

"We have discovered highly structured EEG patterns that indicate when people are sedated during administration of propofol, when they are unconscious and when they are able to regain consciousness," said lead author Patrick Purdon, PhD, from MGH´s Department of Anesthesia, Critical Care and Pain Medicine. "These findings provide precise, neurophysiologically principled markers that can be used to monitor the state of a patient's unconsciousness under propofol general anesthesia."

The latest study is a continuation of a November 2012 report from a team led by Purdon that found a pattern of brain activity correlated to the loss of consciousness under propofol, but only three patients were involved in that study and brain activity was measured through electrodes implanted into participants' brains as part of a treatment for epilepsy. The new study differs by looking at detailed EEG readings of ten healthy volunteers after being administered the same general anesthesia.

Propofol is typically used to induce an unconscious state within less than a minute, making detailed EEG records changes virtually impossible. However, Purdon and his team administered a propofol dosage that was slowly increased over a period of one hour, and slowly decreased over a second hour.

During the experiment, volunteers listened to prerecorded sounds every four seconds and were asked to identify the type of sound by pressing a specific button. The researchers took an initial lack of response as a sign of the onset of sedation, and a persistent loss of response was considered loss of consciousness. The team also took readings throughout the two-hour study period to identify marked changes in EEG patterns.

The team found distinct brain patterns that correlated to a lack of response. When the subjects were beginning to lose consciousness, the scientists found a signature brain activity in the frontal cortex. When the subjects descended into a lower level of anesthesia, the EEG showed marked transition in brain activity.

The signature activity was marked by certain alpha and low-frequency oscillations, which they also detected in last year´s study. This activity produces unconsciousness by interrupting normal signaling between different brain regions, the authors said in their report. They noted the oscillations appear to reduce the amount of information passing between the frontal cortex and the thalamus.

"The consequences of this could be huge, because it would mean we have found a brain state where we know patients will be unconscious and could monitor that brain state in the operating room using EEG," Purdon said.

“Reading the EEG will allow anesthesia caregivers to adjust dosage more precisely and thereby help reduce the incidence of cognitive dysfunction and delirium following anesthesia as well as making unintended regaining of awareness while under anesthesia a phenomenon of the past,” added Dr. Emery N. Brown, a professor of anesthesia at Harvard Medical School.