Your Smartwatch Flagged an Irregular Heartbeat. Here’s What That Alert Can and Can’t Tell You

The notification arrived at 11:40 p.m., while Paul Harmon was brushing his teeth. His watch said it had detected signs of an irregular heart rhythm several times over the past few days and suggested he talk to a doctor. Harmon, a 58-year-old high school chemistry teacher in Sacramento, felt completely fine. He also did not sleep much that night. Two weeks and one cardiology appointment later, he learned he had atrial fibrillation, a common rhythm problem that raises the risk of stroke if it goes untreated.

Stories like Harmon’s are why heart-rhythm features have become a selling point on many smartwatches and fitness bands. They are also why cardiologists say they now spend a meaningful part of their week talking to patients about watch alerts, some of which turn out to be important and many of which do not. Byteline asked two cardiologists and a nurse practitioner how to understand what these features actually measure, and what to do when one goes off.

What your watch is actually measuring

Most wearables use an optical sensor on the back of the case. It shines green light into the skin and measures tiny changes in how much light is reflected as blood pulses through the wrist. From that, the watch estimates heart rate and the spacing between beats. When the spacing looks irregular in a particular pattern over several readings, some devices raise an irregular rhythm notification.

Some watches also include an electrical sensor that can record a single-lead electrocardiogram, or ECG. You hold a finger on the crown or a button for about 30 seconds, and the watch records the heart’s electrical signal. A hospital ECG uses 12 leads placed across the chest, so a watch reading is a much narrower view, but it can still show whether the rhythm looks like atrial fibrillation.

“Think of the optical sensor as a smoke detector and the ECG as someone walking into the room to look,” said Dr. Renee Calloway, a cardiologist at a hospital in Minneapolis. “Neither one is the fire department.”

What these alerts can catch

The features are designed mainly to spot possible atrial fibrillation, often called AFib. In AFib, the upper chambers of the heart beat chaotically, which can let blood pool and form clots. Many people with AFib have no symptoms at all, which is why a passive alert can be useful. Some watch features have been cleared by the FDA for this narrow purpose in adults, typically for people who have not already been diagnosed.

Doctors we spoke with said the most useful alerts tend to share a few traits. They repeat over days rather than appearing once. They show up during rest or sleep, when the sensor reading is cleanest. And a follow-up watch ECG, if the device has one, also suggests AFib.

What they can’t tell you

This is the part that worries clinicians most, because a quiet watch can feel like reassurance. Here is a short list of what heart-rhythm alerts are not built to do:

• They do not detect heart attacks. A heart attack is a blockage of blood flow, not mainly a rhythm problem. Chest pain, pressure, shortness of breath or pain spreading to the arm or jaw means calling 911, no matter what the watch says.
• They do not rule out AFib. Rhythm problems can come and go. A watch that never alerts may simply have missed an episode.
• They do not diagnose other conditions such as heart failure, valve disease or most other rhythm problems.
• They are less reliable during exercise, with a loose band, with tattoos on the wrist, or in very cold weather when blood flow to the skin drops.

A normal reading on a watch is not a checkup. I would rather patients treat these alerts as a reason to ask a question, not a reason to stop asking.

Dr. Renee Calloway, cardiologist

False alarms are real, too

Irregular beats are common and often harmless. Extra beats from the upper or lower chambers of the heart, called premature contractions, can make the rhythm look uneven to a sensor. So can a poorly fitting band. Studies of these features have found that a meaningful share of people who receive an alert do not turn out to have AFib when they are tested more thoroughly, especially younger users.

That has consequences beyond worry. Carmen Ortiz, a nurse practitioner at a cardiology clinic in Phoenix, said her office now books several appointments a week that begin with a watch screenshot. “Most are reasonable to check,” she said. “But I see younger, healthy people who have turned the sensor into a source of anxiety. Checking your ECG ten times a day is not making you healthier.”

What to do if you get an alert

Doctors offered a consistent set of steps. None of them replace professional care, and if you have symptoms, the order changes.

1. Check how you feel. Chest pain, fainting, severe shortness of breath, or sudden weakness on one side of the body are emergencies. Call 911.
2. If you feel fine, sit down and rest for a few minutes, then take an ECG reading if your watch can. Make sure the band is snug.
3. Save the history. Take screenshots of the alert, the dates and any ECG recordings. Most health apps let you export them as a PDF.
4. Book an appointment with your primary care doctor or a cardiologist, and mention the alert when you schedule. They may order a standard ECG or a wearable monitor you use for a week or two.
5. Do not start or stop medications based on a watch reading alone.

Who benefits most

Cardiologists generally see the most value for adults over 50, people with high blood pressure, diabetes or sleep apnea, and those with a family history of rhythm problems, since AFib becomes much more common with age. For younger people with no risk factors, the features are less likely to find something and more likely to produce a false alarm. Some doctors suggest that anxious users turn off routine notifications and rely on a scheduled checkup instead.

Questions worth bringing to your appointment

Calloway suggested patients go in with a short list rather than a vague worry. How often have the alerts appeared, and at what times of day? Were you drinking alcohol, short on sleep or sick when they happened? Do you have symptoms such as fluttering, fatigue or getting winded on stairs? Does anyone in your family have a rhythm problem? And, finally, should you keep the feature turned on at all? “That last question is completely fair,” she said. “For some patients the answer is yes, for some it is no, and that is a conversation to have with someone who knows your history.”

It is also worth checking your privacy settings. Heart data is health data, and many health apps let you control whether readings are shared with other apps, family members or insurers’ wellness programs.

Harmon, the chemistry teacher, now takes medication to lower his stroke risk and sees his cardiologist twice a year. He still wears the watch. “It did one job, and that job mattered,” he said. “But my doctor made the diagnosis. The watch just got me in the door.”

This article is for general information and is not medical advice. If you have questions about a heart-rhythm alert or any symptoms, talk to your doctor.