Study finds tracking brain waves could reduce post-op complications

Distinctive EEG patterns indicate when a patient’s state of unconsciousness under general anesthesia is more profound than necessary.

By Anne Trafton

When patients undergo general anesthesia, their brain activity often slows down as they sink into unconsciousness. Higher doses of anesthetic drugs can induce an even deeper state of unconsciousness known as burst suppression, which is associated with cognitive impairments after the patient wakes up.

A new study from MIT, in which the researchers analyzed the EEG patterns of patients under anesthesia, has revealed brain wave signatures that could help anesthesiologists determine when patients are transitioning into that deeper state of unconsciousness. This could enable them to prevent patients from falling into that state, reducing the risk of postoperative brain dysfunction.

MIT researchers analyzed the EEG patterns of patients under general anesthesia and found brain wave signatures that could help doctors determine when patients are transitioning into a deep state of unconsciousness known as burst suppression, which is associated with cognitive impairments after patients wake up. Credits: Jose-Luis Olivares/MIT with figures from iStock

One of these distinctive patterns emerged in the brain’s alpha waves (which have a frequency of eight to 14 cycles per second). Once patients became unconscious, these waves started to wax and wane in amplitude. As patients went deeper into unconsciousness, the pattern of this waxing and waning in amplitude, or amplitude modulation, continually changed.

“If you track this modulation as it gets deeper or shallower, you have a very principled way to track level of unconsciousness under anesthesia,” says Emery Brown, the Edward Hood Taplin Professor of Medical Engineering and Computational Neuroscience and a member of MIT’s Picower Institute for Learning and Memory and the Institute for Medical Engineering and Science.

Brown is the senior author of the new study, which appears this week in the Proceedings of the National Academy of Sciences. The lead authors of the paper are Picower Institute research scientist Elie Adam, Ohyoon Kwon ’20, and graduate student Karla Montejo.

Measuring brain waves

Brain waves, which are generated by synchronized neuronal activity, oscillate at different frequencies depending on what kind of task the brain is performing. When the brain is strongly engaged in mental activity, it produces higher-frequency beta (15-30 hertz) and gamma (greater than 30 hertz) oscillations, which are believed to help organize information and enhance communication between different brain regions.

Commonly used anesthesia drugs such as propofol have a significant effect on these oscillations. During anesthesia induced by propofol or other anesthetics that increase the effectiveness 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 (0.1-1 hertz), delta (1-4 hertz) and alpha (8-14 hertz) oscillations.

Higher doses of anesthetic drugs can induce an even deeper state of unconsciousness known as burst suppression, which is associated with cognitive impairments after the patient wakes up. Credit: Stefan_Schranz

With higher doses of these anesthetic drugs, the brain can fall into an even deeper state of unconsciousness. When in this state, known as burst suppression, EEG recordings from the brain show long periods of inactivity, punctuated by brief bursts of low-amplitude oscillations. When patients enter this state, they are more likely to experience postoperative confusion, delirium, and memory loss. These effects, which can last for hours, days, weeks, or months, are more common in elderly patients.

SDA and burst suppression produce distinctive EEG patterns that have been well-studied. However, they have been studied as separate brain states; what happens during the transition between the two states is less clear. That transition is what the MIT team set out to analyze in this study.

As the dosage of propofol was increased, patients showed two distinctive patterns of change in their EEGs.

To do that, the researchers studied 10 healthy volunteers and 30 patients who were undergoing surgery. Most of the patients received propofol intravenously, and the rest received sevoflurane, a commonly used anesthetic gas. Both of these drugs act on GABA receptors in the brain, which reduce neuron excitability.

As the dosage of propofol was increased, patients showed two distinctive patterns of change in their EEGs. The first pattern was seen in the alpha waves, which started to wax and wane. As the dose increased, waxing was shortened and waning was prolonged, until the patient reached the state of burst suppression.

“You can see a very strong modulation, which is always there. As the modulation gets to be more profound, it eventually flattens out, and that's when the brain reaches the deeper state,” Brown says.

• Provided by MIT