New research suggests that A-fib may be more prevalent, and more dangerous, in people under 65 than previously thought.

Atrial fibrillation, a common cardiac condition that raises the risk of stroke, is increasingly affecting the health of people under the age of 65.

For years, experts thought that A-fib, a type of irregular heartbeat, primarily occurred in people age 65 and older, and that younger people with the condition most likely wouldn’t develop other cardiac issues, said Dr. Aditya Bhonsale, a cardiac electrophysiologist at the University of Pittsburgh Medical Center. But in new research published Monday, Dr. Bhonsale and his colleagues examined data on more than 67,000 patients with A-fib who were treated at the center, nearly one-quarter of whom were under the age of 65. Those patients had an increased risk of death, compared to people without the condition. They also often had risk factors for A-fib like high blood pressure, obesity and sleep apnea, which could make cardiovascular health issues worse.

In A-fib, the upper and lower chambers of the heart are not coordinated as they should be, which makes the heart beat chaotically — sometimes too slow, sometimes too fast, often just irregularly. People with the condition can experience a fluttering heartbeat and palpitations.

Dr. Geoffrey Barnes, a cardiologist and vascular medicine specialist at University of Michigan Health, said that he frequently saw patients who say “I felt like my heart was racing away” or “I felt like it was skipping beats.” People with A-fib can also experience shortness of breath and chest discomfort. Some people have occasional episodes; others will continuously experience an irregular heartbeat. People often have no symptoms and don’t even realize they have the condition, Dr. Barnes said.

Increasingly, doctors said, patients are seeking care because their smart watches have detected an irregular heartbeat. That might help explain why more younger people are being diagnosed, said Dr. Hugh Calkins, a professor of cardiology at Johns Hopkins Medicine. “The more you screen, the more you’re going to find,” he said.

Doctors typically diagnose atrial fibrillation with an electrocardiogram while factoring in your medical and family history. They will sometimes give patients a portable monitor to wear for up to a month to look for an irregular heartbeat.

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