Research finds that tracking EEG may reduce postoperative complications | Massachusetts Institute of Technology News

When patients receive general anesthesia, brain activity often decreases as they lose consciousness. Higher doses of anesthetics can induce an even deeper state of unconsciousness known as burst suppression, which is associated with cognitive deficits after patients wake up.

In a new study from the Massachusetts Institute of Technology (MIT), researchers analyzed brain wave patterns in patients under anesthesia, which could help anesthesiologists determine when patients transition into deeper unconsciousness. revealed some EEG signatures. This may prevent patients from falling into such a state and reduce the risk of postoperative brain dysfunction.

One of these characteristic patterns appeared in brain alpha waves (frequency between 8 and 14 cycles per second). As the patient lost consciousness, the amplitude of these waves began to increase and decrease. This pattern of increasing and decreasing amplitude, or amplitude modulation, changed continuously as the patient descended deeper into unconsciousness.

“Following the deepening and shallowing of this modulation gives us a very principled way of tracking the level of consciousness under anesthesia,” says Edward Hood Taplin, Professor of Medical Engineering and Computational Neuroscience. said Emery Brown, a member of the Massachusetts Institute of Technology’s Picower Institute. Institute of Learning and Memory and Biomedical Engineering Sciences.

Brown is the lead author of the new study, which will be published this week. Proceedings of the National Academy of Sciences. The lead authors of the paper are research scientist Ellie Adam, Oyun Kwon ’20 and graduate student Carla Montejo of the Picower Institute.

EEG measurement

The brain waves produced by synchronized neural activity oscillate at different frequencies depending on the type of task the brain is performing. When the brain is heavily involved in mental activity, it produces higher frequency beta (15 to 30 hertz) and gamma (above 30 hertz) oscillations. This is thought to help organize information and enhance communication between different brain regions.

Commonly used anesthetics such as propofol have a significant effect on these oscillations. During anesthesia induced by propofol or other anesthetics that increase the availability of GABAergic inhibitory receptors in the brain, the brain enters a state of unconsciousness known as slow delta alpha (SDA). This state is characterized by slow oscillations (0.1 to 1 hertz), delta oscillations (1 to 4 hertz), and alpha oscillations (8 to 14 hertz).

With increasing doses of these anesthetics, the brain can go into a deeper state of unconsciousness. In this condition, known as burst suppression, EEG recordings from the brain show long periods of inactivity punctuated by brief bursts of low-amplitude oscillations. Patients with this condition are more likely to experience postoperative confusion, delirium, and memory loss. These effects may last hours, days, weeks, or months and are more common in older patients.

SDA and burst suppression produce distinctive EEG patterns that have been well studied. However, they have been studied as separate brain states. It’s less obvious what happens during the transition between the two states. It is that transition that the MIT team sought to analyze in this study.

To do so, researchers studied 10 healthy volunteers and 30 patients undergoing surgery. Most patients received intravenous propofol and the remaining patients received sevoflurane, a commonly used anesthetic gas. Both of these drugs act on her GABA receptors in the brain, making neurons less excitable.

As the dose of propofol increased, the patients’ electroencephalograms showed two distinct patterns of change. The first pattern was found in alpha waves, which began to rise and fall. As the dose increased, the increase was shorter and the decrease was prolonged until the patient reached a state of burst suppression.

“You can see that there’s always a very strong modulation, and as the modulation gets deeper, it eventually flattens out, and that’s when the brain reaches a deeper state,” Brown says.

When the dose of the drug was reduced, the amplitude of alpha waves began to increase again.

The researchers also found a characteristic pattern of slow and delta waves in the patient’s EEG readings. Slow and delta oscillations are the slowest brain waves, and as the amount of drug increases, the frequencies of these waves become slower and slower, reflecting a decline in brain activity.

Metabolic disruption

Researchers hypothesize that propofol exerts these effects through its effects on neuronal metabolism. The drug is supposed to interfere with the production of ATP, a molecule that cells use to store energy. Decreased ATP production ultimately prevents neurons from firing, leading to suppression of bursts.

“This is consistent with the observation that burst suppression is much more frequent in older patients because their metabolic state may be less regulated than in younger patients,” says Professor Brown.

The findings could give anesthesiologists more precise control over a patient’s unconscious state during surgery, Brown said. He now wants to develop an algorithm that can generate a warning that a patient is nearing burst suppression and display it on an operating room monitor. Anesthesiologists could also learn how to make that decision by looking for these patterns in their patients’ brain waves, he said.

“One of the reasons we’re excited about this is because it’s something you can actually see in raw EEG,” says Brown. “Having pointed out these patterns, they are very easy to spot.”

The researchers now plan to use animal models to further investigate what happens to brain metabolism during the transition to burst suppression.

This study was partially funded by the Picower Institute Innovation Fund and the National Institutes of Health.