EEG has been widely used in research and medicine as a handy tool to study the brain - it is used to measure electric activity and deduce neural activity in the brain.
The research, which was carried out at Massachusetts General Hospital (MGH), is part of a large investigation into the effects of general anesthesia.
According to the lead author of the report, Patrick Purdon, PhD, of the MGH Department of Anesthesia, Critical Care and Pain Medicine:
"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.
These findings provide precise, neurophysiologically principled markers that can be used to monitor the state of a patient's unconsciousness under propofol general anesthesia."
Two in every 1,000 patients wake up during their operation while they are supposed to be unconscious, researchers revealed in the German medical journal Deutsches Ärzteblatt International (January 2011 issue) in a separate study. If a patient wakes up in the middle of an operation, is aware of what is happening and can recall details later, there is a serious risk of long-term psychological trauma.
For decades it's been known that some general anesthetics can produce changes in EEG patterns, yet until now there was no evidence to suggest that these changes can cause loss and recovery of consciousness. Even though the primary effects of anesthetic drugs are in the brain and central nervous system, dosing of the drugs focuses on target concentrations in the blood or lungs.
Propofol is commonly used for the induction and maintenance of general anesthesia in humans and animals
The current study included 10 healthy volunteers who underwent propofol general anesthetic treatment, the authors studied their EEG readings.
Propofol induces unconsciousness in less than a minute when used clinically, making it very hard to track EEG changes. This study involved slowly increasing the propofol dosage over the first hour and then decreasing it during the second hour, making the tracking of any EGG changes much easier to do.
Throughout the two hours of propofol administration, the participants were exposed to sounds in the form of words or clicks, they had to identify the type of sound by pressing a button.
When the patients began to stop responding to clicks they were at the onset of sedation, when they didn't respond to words it indicated loss of consciousness. The readings showed the characteristic changes in EEG patterns to indicate changes in consciousness.
Trough-Max and Peak-Max patternsThe signaling frequencies measured by EGG had important meanings. One pattern known as "trough-max" occurred right before consciousness returned. The "peak-max" pattern was observed during the deepest levels of consciousness, when propofol dosage was at its highest.
"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.
Appearance of the trough-max pattern would provide a way to predict when patients might be regaining consciousness. With these fundamental neurophysiological markers of the sedative and unconscious states under propofol and with some existing EEG equipment, we can start monitoring these EEG signatures when administering propofol immediately."
Purdon said that they are working on developing new monitoring technologies that will enable them to see when peak-max and trough-max patterns occur, which will help in predicting when patients will lose or regain consciousness.
The study's senior author, Emery N. Brown, MD, PhD, concluded: "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."
In a previous study, researchers from Harvard Medical School revealed in PNAS (Proceeding of the National Academy of Sciences) (November 2012 issue) that the brain has a distinct pattern of electrical activity when a patient loses consciousness during anesthesia. The pattern displays very slow oscillations, reflecting a communication breakdown between various brain regions, where there are shorts bursts of activity alternating with longer inactive periods. They believe that when we have a better understand of what occurs in the brain as it loses consciousness, anesthesiologists will be able to maintain a more accurate balance between too much and too little anesthetic.