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What Happens to Your Brain When You’re Under Anesthesia?

How does anesthesia work? You might be out for the count, but scientists are learning a lot from the anesthetized brain.

By Avery Hurt
Sep 22, 2023 2:00 PM
Doctor putting a female surgical patient under anesthesia with inhalation


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Until the middle of the 19th century, surgery was performed with no anesthesia. You don’t need a fertile imagination to realize how excruciating the experience was for patients. Nor did the surgeons who administered this particular type of torment take it lightly. 

In The Worst of Evils: The Fight Against Pain, Thomas Dormandy tells how the 19th-century surgeon and medical pioneer Sir James Paget recalled those gruesome days before anesthesia in his memoirs: “They had been the worst nightmares. I can remember them still. Even now, they sometimes rouse me from my sleep. I wake drenched in sweat.” 

Paget wasn’t the only one who was disturbed by the experience. Charles Darwin dropped out of medical school, at least partly because he didn’t have the stomach for pre-anesthesia surgery.

These days, surgery is much less traumatic. You start counting backward from 100, and before you get to 95, you find yourself waking up in recovery. Modern anesthesia is genuinely one of the miracles of modern medicine. While the surgical team takes care of business, you’re checked out. But what happens to your brain during the time you aren’t there? 

How Does Anesthesia Work?

Anesthesia doesn’t shut down the brain. Instead, it interferes with the communication of neurons across different parts of the brain. The ability to experience anything, from thinking to processing sensory experiences — such as what it feels like to have your body sliced open and your innards tinkered with — depends on this more or less constant neuronal chatter.

Research by scientists, such as Emery Brown, anesthesiologist and neuroscientist at Harvard Medical School, has found that anesthesia dramatically alters this chatter. The result? “Essentially a drug-induced coma,” explains Miles Berger, anesthesiologist, and neuroscientist at the Duke University Institute for Brain Sciences.

He compares it to an old television that goes from a normal picture to big, rolling static waves. “There may actually be more total signal in the case of the static waves,” he says, “but the information transfer is less.” The neurons are still active under anesthesia, but it’s as if all the information is lost and replaced by large static rolling waves.

Read More: Your Native Language May Wire the Brain in Unique Ways

Stress Test for the Brain

Being anesthetized is very different from being asleep, but in both cases, the brain is active, even if we’re unaware of it. Using electroencephalography (EEG) to track this activity while we’re anesthetized can help neuroscientists better understand the workings of the brain — and even potentially screen for brain illnesses. Berger and colleagues have found that EEG patterns while under anesthesia may help identify which patients are most likely to have delirium post-surgery, a common problem in older patients.

In addition, studying how brains respond to anesthesia might one day help to screen patients for cognitive decline. Berger uses the analogy of a stress test for heart disease. Cardiologists put people on treadmills to see how their hearts respond to exercise stress. That can help determine how likely a patient is to have a heart attack. Berger and his colleagues see anesthesia and surgery as something similar — a stressor for the brain.

Read More: What Happens When Hearts Attack

“If we can monitor the brain in real-time and see how it’s responding to the stress of surgery and anesthetic drugs,” he says, “that could allow us to predict who might potentially have a brain problem in the future.” This wouldn’t diagnose dementia or even early cognitive decline, but it might one day be a useful screening tool. “Our hope,” says Berger, “is that we can identify people who are at risk for cognitive decline just by looking at how their brains respond to anesthesia, and then those patients can be referred for further workup.”

And that’s just one area of research. It turns out that studying how the brain responds to anesthesia offers a great opportunity to study the brain and perhaps consciousness as well. When you’re under anesthesia, your neurons might not be talking to each other very much, but to scientists like Berger, who are learning how to read these signals, they might have a lot to say.

Read More: Analyzing Brain Waves for Near-Death Experiences

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