Severinghaus Lecture: EEG a key to understanding anesthesia

Emery N. Brown, M.D., Ph.D.: ‘We use the EEG the least of probably all the specialties that can take advantage of it to take care of patients and make inferences about the state of the brain. But it should be just the opposite.

Many mysteries about the brain and its workings remain to be solved, and anesthesiologists play a big role in increasing that understanding. Emery N. Brown, M.D., Ph.D., is a leader in that effort, and one of his key tools is the EEG. Wednesday, he used the John W. Severinghaus Lecture on Translational Science to explain what we have learned through EEGs.

“The EEG signal we get from the brain under anesthesia is the strongest EEG signal for all of the things for which the EEG is used,” said Dr. Brown, the Warren M. Zapol Professor of Anesthesia at Harvard Medical School and an anesthesiologist at Massachusetts General Hospital. “We should take advantage of that to get as much information out of the brains in our anesthetized patients as possible.”

Dr. Brown traced the history of the EEG since the 1920s, including what he called “the era of brain state monitoring” in the 1990s. He also explored the present and future use of the EEG to advance knowledge of anesthesia and its effects on the brain.

The EEG was invented in 1924, and the first paper about it was published in 1929. A seminal paper published in 1937 evaluated the effects of specific drugs by using the EEG to record the brain’s oscillations. Neurological research from 1935 provided insight into separating the brain stem from the cerebrum in cats. Researchers found that this separation produces large, slow oscillations.

“All anesthetics produce slow oscillations. What that gives us insight into is that they have targets, which are in the brain stem, or major breaks in pathways that are projecting up to some of the arousal systems,” said Dr. Brown, who also is the Edward Hood Taplin Professor of Medical Engineering and Professor of Computational Neuroscience at the Massachusetts Institute of Technology.

A major advance using the EEG was to document different states of anesthetized patients. These states range from awake through isoelectric, which led into the era of brain state monitoring.

“They are taking EEG information and using it to help you infer anesthetic state,” Dr. Brown said of the work of researchers. “They are taking information and using it to compute an index.”

Using EEGs, researchers picked features to examine the power in certain frequency bands, the state of burst suppression and whether there is entropy. All of that information is correlated with observations of patients that match oscillations to judged states. A regression analysis of that information is then employed to get coefficients that are used in algorithms. Those algorithms help to develop indices for practices to track the anesthetic states.

“As you get into deeper states of anesthesia, things become more organized,” said Dr. Brown. “If you have any kind of crude measure of entropy, it is going to give you an indication of a person approximately moving to a deeper state of anesthesia. That is why entropy is one of the measures used in some of the (anesthesia) monitors.”

This information allowed systems neuroscience to be studied with molecular pharmacology. Researchers were able to determine that drugs were moving to specific targets in the brain and having specific effects, he said.

“That was a game changer,” Dr. Brown said. “If the drugs are going to a specific target, then you can look at where those targets lie and you can think about the neuroscience of what is transpiring. You have a way of thinking about the pharmaceutical agent acting on a target, which is going to influence the brain directly.

“Then, you look at that influence from a neuroscience perspective. That means that now you have a way to interpret how the drug generates the EEG you are seeing.”

Current research is examining how the oscillations measured by EEG are generated. It is known that oscillations change with the dose of a drug, the drug class and the patient’s age. That can be used in monitoring patients under anesthesia.

One study shows how the brain response to anesthesia changes with age. The large oscillations in younger patients under anesthesia decrease with age. The oscillations are so small in an 81-year-old patient that they can be interpreted as the patient being awake, Dr. Brown said.

Much has been learned from EEGs, and even more knowledge is anticipated.

“We use the EEG the least of probably all the specialties that can take advantage of it to take care of patients and make inferences about the state of the brain,” said Dr. Brown. “But it should be just the opposite.

“Because these oscillations change so systematically with drug class, with drug dose and with age, it defines a pretty clean paradigm to start thinking about how to use the EEG to track the anesthetic state. This is a very good starting point.”

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