MICHEL MARTIN, HOST:
And now an update on the science of swooning. NPR's Jon Hamilton reports on evidence that fainting can be caused by a newly discovered pathway between the heart and the brain.
JON HAMILTON, BYLINE: About 40% of people pass out at some point in their lives. Vineet Augustine of the University of California, San Diego, says often there's no medical reason.
VINEET AUGUSTINE: A lot of people faint at the sight of blood, or, like, when you're exposed to a very intense emotional stimulus, you would faint.
HAMILTON: Doctors call this sort of fainting vasovagal syncope. It occurs when there's a sudden drop in heart rate and blood pressure. That reduces circulation to the brain, which shuts down the circuits that keep us conscious. Augustine says research dating back to the 19th century links this type of fainting to the vagus nerve, which connects the brain to internal organs including the heart, lung and gut.
AUGUSTINE: But what was not clear was which part of the vagus nerve. The vagus nerve is big. It's a major highway between the body and the brain.
HAMILTON: Scientists once thought the vagus nerve was merely a way for the brain to control internal organs, but studies show it's a two-way street. The gut, for example, can also affect the brain. Augustine's team figured that might be true of the heart as well.
AUGUSTINE: What we were trying to argue - well, the heart also sends signals back to the brain, which can influence its function and behavior.
HAMILTON: The team used genetic tools to study the vagus nerve in mice, and they found a group of nerve cells that connect the heart's ventricles, which pump blood, with a small region of the brainstem, which regulates breathing and heart rate. To see whether this pathway could cause fainting, they used a pulse of laser light to stimulate those nerve cells in mice.
AUGUSTINE: When the pulse hits them, the heart rate immediately dips. They wobble around a little bit, and then around seven seconds, they fall over.
HAMILTON: And like people who faint, their pupils dilate; their eyes roll back; their breathing slows, and their blood pressure plummets. The finding, which appears in the journal Nature, seems to confirm that fainting can be triggered by this pathway between the heart and brain. Dr. Rob Wilson, a neurologist at the Cleveland Clinic, says the study also provides a clearer picture of how the brain and body usually work together to keep us from passing out.
ROB WILSON: There's this whole orchestra that responds to how the blood's flowing to the body. It tells the heart how to speed up, how much to pump, how much to move as a response.
HAMILTON: Wilson says scientists are just beginning to understand how that works. For example, it's only been a few years since a team explained the reflex that keeps blood pressure constant whether we're sitting or standing. That research helped win a Nobel Prize in 2021. Wilson says the discovery of a fainting reflex could eventually help patients with disorders that affect blood flow to the brain.
WILSON: This is probably a new door to go through for treatments and understanding.
HAMILTON: Wilson says autonomic disorders, which affect the brain's regulation of internal organs, didn't used to get much attention.
WILSON: Then COVID occurred, and a lot of the long COVID patients have autonomic dysfunction - dizziness, fainting - and it's a big deal.
HAMILTON: Suddenly, there are a lot more people passing out, apparently because COVID affects some of the signals that pass through the vagus nerve. Wilson says he has limited options for patients who faint frequently for no obvious reason.
WILSON: Sometimes people just need to avoid triggers, and sometimes people might need an actual medication to sort of prevent this from happening at times.
HAMILTON: But those medications may simply raise a person's blood pressure. The new study could help find a treatment that addresses the underlying problem.
Jon Hamilton, NPR News.
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