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Use of EEG Patterns to Track Consciousness Levels When Under Propofol Anesthesia

8 May, 2013 | EEG

Brain waves

Brain waves

A study conducted at Massachusetts General Hospital used Electroencephalography (EEG) patterns to identify a patient’s consciousness level when under general propofol anesthesia. Highly structured EEG patterns enabled identifying when the patient was sedated, unconscious and regaining consciousness.

This is the first time that specific patterns indicating the state of consciousness were identified. Currently the monitoring of anesthesia patients focuses on the vital signs and the dose is based on the concentration of the drug in the blood or lungs. This is despite the fact that the drug’s primary effect is actually on the brain and central nervous system.

EEG raw signals and frequency components can provide specific information on the state of the brain, but most EEG units reduce the signal to a single number, which does not enable identifying the physiological processes that actually create the pattern.

The experiment

In the study, ten healthy volunteers were placed under general propofol anesthesia. The propofol dosage was gradually increased to a target level over a period of an hour and then was gradually decreased during a second hour. This is in contrast to the clinical use of propofol, where the unconscious level is induced within less than a minute, making the tracking that was possible in the study, impossible.

During the experiment, certain sounds were operated every four seconds, to which the participants responded. When they stopped responding it indicated sedation, and when this persisted it indicated loss of consciousness. This enabled identifying characteristics in the EEG patterns that could be tagged as sedation, loss of consciousness, deep unconsciousness and return to consciousness.

Interesting patterns were observed during the experiment; a pattern observed during transitions in consciousness was named trough-max, and one observed during the highest dosage of propofol was named peak-max. The trough-max was especially observed right before returning to consciousness and the peak-max was observed in the deepest unconsciousness levels.

The implications of the study

 The implications of this study mean that EEG systems, which show the raw detailed signals, could be used in the operating room to know when the patient is unconscious and when the patient is regaining consciousness.

Patrick Purdon, PhD, an instructor in anesthesia at the Harvard Medical School, said:  ”We are working on new monitoring technologies to develop EEG monitors that will display the peak-max and trough-max patterns, helping us ensure when patients are unconscious and predict when they might be regaining consciousness. We also are working to characterize similar EEG patterns for other anesthetic agents and to develop a program to train anesthesiologists how to interpret the EEG in terms of the neurophysiology and mechanisms for different drugs.”

Emery N. Brown, MD, PhD MGH anesthetist said: “This work helps establish the basic and clinical science needed to set standards for use of the EEG to track the brain states of patients under general anesthesia. 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.”


With new discoveries on brain waves under anesthesia using EEG patterns, more precise doses of propofol can be administered and the patient’s consciousness levels can be monitored much more precisely. This paves the way to tailored EEG monitors that specifically indicate trough-max and peak-max, and perhaps additional states and transitions in brain waves which may be discovered over time.