Fitbit devices will soon be able to tell users if they have an irregular heart rhythm. It’s designed more as an early warning than an emergency alert.

The feature will initially only be available in the United States, where it has been approved by the Food and Drug Administration. Unlike previous attempts that relied on an ECG sensor, the new “passive atrial fibrillation” monitoring uses the optical heart rate monitor, meaning it will work on most Fibit models from the past few years.

Rather than requiring the user to actively trigger a check (as happens with ECG sensors), the technology will work automatically while the user is still, include during sleep. It will build up a picture over several days to detect signs of atrial fibrillation.

If that happens, the device won’t sound an alert notification. Instead, a message will appear in the Fitbit app suggesting the user make an appointment to see a healthcare professional.

Atrial fibrillation involves the upper chambers of the heart beating irregularly and getting out of sync with the lower chambers. This doesn’t usually cause noticeable symptoms but can lead to blood clots that increase the risk of stroke.

Back in 2018 a British man was diagnosed with atrial fibrillation after his Apple watch alerted him to an unusually high heart rate whilst sitting down. Although the alert was simply triggered by the high reading, the man then kept an eye on the heart rate for several minutes and noted it swang dramatically in a range from 79 to 135 beats per minute.