Are Smartwatch ECG and AFib Alerts Accurate? What Patients Should Know

You’re sitting on the couch when your watch buzzes: “Irregular rhythm detected.” Or you take an ECG reading because you feel fluttery and it says “Atrial fibrillation.” Now what? Is your watch right, and do you need to go to the ER?

Smartwatch heart features have become common, and several have FDA clearance. They really do help some people find atrial fibrillation (AFib) they didn’t know they had. But they’re also easy to over-trust or to panic about. This guide explains what these features measure, what the research says about accuracy, what they can’t detect, and what to do when you get an alert.

This is general information, not medical advice. If you have chest pain, trouble breathing, fainting or signs of stroke, call 911 right away.

The 2 kinds of smartwatch heart features

Most people lump these together, but they work differently.

1. Passive irregular rhythm notifications. The watch uses its optical sensor, the green light on the back, to measure your pulse in the background. If it sees a pattern of irregular rhythm across several checks, it sends a notification suggesting possible AFib. You don’t have to do anything for this to run.

2. On-demand ECG recordings. You hold a finger on the watch’s crown or a sensor for about 30 seconds. The watch records a single-lead electrocardiogram, similar to one lead of the 12-lead ECG used in a clinic, and software classifies the result.

Apple received FDA De Novo authorization for its ECG app in September 2018 as an over-the-counter, Class II device. The FDA’s description says the app records a single-channel ECG similar to Lead I and determines whether it shows AFib or normal sinus rhythm on a classifiable recording. Samsung and Fitbit announced FDA clearances for their own ECG apps in 2020, and other brands offer ECG features on some models.

What a watch ECG can and can’t detect

This is the part most people miss. A watch ECG answers a narrow question.

Apple’s own support documentation is clear: the ECG app can’t detect a heart attack, blood clots or a stroke, or other heart conditions such as high blood pressure, congestive heart failure, high cholesterol or other arrhythmias. If you have chest pain, pressure or tightness, Apple tells users to call emergency services.

Key takeaway: A normal watch ECG doesn’t mean your heart is fine, and an AFib reading doesn’t mean you have AFib. The watch is a screening tool that flags a possible problem. Symptoms always outrank the screen.

What the results mean

Results vary by brand, but Apple’s are a typical example:

• Sinus rhythm means a uniform heartbeat between 50 and 100 beats per minute during that recording.
• Atrial fibrillation means the app saw an irregular rhythm consistent with AFib. Apple’s ECG app version 2 checks for AFib between 50 and 150 beats per minute.
• Low or high heart rate means the rate was outside the range where the app can check for AFib.
• Inconclusive means the recording couldn’t be classified. Causes include a pacemaker or implanted defibrillator, other arrhythmias, and a weak signal.
• Poor recording often comes from movement, a loose band, moisture or electrical interference.

Apple’s ECG app isn’t intended for people under 22, and it isn’t recommended for people with other known arrhythmias. Its irregular rhythm notifications also aren’t intended for people already diagnosed with AFib.

How accurate are smartwatch AFib alerts?

The largest early study was the Apple Heart Study, published in the New England Journal of Medicine in 2019 and funded by Apple. It enrolled 419,297 people who didn’t report a history of AFib. Key findings:

• About 0.52% of participants received an irregular pulse notification over a median of 117 days.
• Among notified participants who later wore an ECG patch and returned usable data, 34% had AFib on the patch. That doesn’t mean the rest were false alarms. AFib often comes and goes, so it may not have appeared during the patch period.
• When an irregular pulse notification happened while the patch was being worn, the positive predictive value was 0.84. In other words, AFib was confirmed on the patch for most of those simultaneous alerts.

What this means for you:

• Alerts are uncommon, especially in younger people.
• An alert is meaningful, but it isn’t proof. You need clinical confirmation.
• The absence of an alert isn’t a guarantee. Watches check intermittently, and the notification feature isn’t designed to catch every episode.

Accuracy also depends on how you use the watch. A snug fit, a still arm resting on a table, and dry skin all help. Tattoos, poor circulation and movement can interfere with optical readings.

The American Heart Association advises that if your watch says you have AFib, you should see a doctor to confirm it. AHA experts have also noted that in low-risk people, false alarms can cause anxiety, so they don’t recommend watches for heart screening across the board.

Why AFib alerts are worth taking seriously

AFib is an irregular heart rhythm in which the upper chambers of the heart quiver instead of beating effectively. The American Heart Association says the risk of stroke is up to 5 times higher in people with AFib. Many people have no symptoms, which is why a watch alert sometimes leads to a diagnosis that would otherwise be missed.

Confirmed AFib is treatable. Depending on your stroke risk, a clinician may recommend blood thinners, medications to control heart rate or rhythm, or procedures. That’s why the right response to an alert is calm follow-up, not either panic or dismissal.

What to do when you get an alert

Call 911 if you have:

• Chest pain, pressure or tightness
• Shortness of breath, fainting or near-fainting
• Stroke signs: face drooping, arm weakness, speech difficulty

If you feel fine:

1. Take an ECG reading on the watch if it has one, sitting still with your arm resting on a table.
2. Save or export the reading. Most apps let you create a PDF of the tracing.
3. Note what you were doing, how you felt, and any caffeine, alcohol or sleep loss.
4. Contact your clinician within a few days, sooner if alerts repeat. Send the PDF through your patient portal if you can. Our guide on how to use a patient portal shows how.
5. Expect confirmation testing, such as a 12-lead ECG in the office or a wearable patch monitor for several days or weeks.

Don’t start, stop or change any medication based on a watch reading alone.

Who benefits most, and who should be cautious

Smartwatch heart features may be most useful for:

• Adults over 65 or with AFib risk factors such as high blood pressure, diabetes, obesity, sleep apnea or heart disease, if they discuss it with their clinician.
• People with occasional palpitations who want to capture a tracing during an episode.
• People with diagnosed AFib whose clinician wants help tracking episodes. Some features, such as AFib history tracking, are designed for that.

Be cautious if you:

• Are under 22, since key features aren’t intended for that age group.
• Have a pacemaker, implanted defibrillator or other known arrhythmia.
• Tend to feel anxious about health data. Turning off some notifications is a reasonable choice.

If you’re still choosing a device, our comparison of smartwatches vs. fitness trackers explains which models include medical-grade features. For other home monitoring, see our guide to over-the-counter glucose monitors and our overview of telehealth and remote patient monitoring.

Where to start

If you have a watch with heart features:

1. Check that features are set up correctly, with your correct age, in your watch’s health app.
2. Learn how to take a clean ECG before you need one.
3. Know your own normal. Look at your resting heart rate trend.
4. Tell your clinician you use the features, and ask whether they want you to send readings.
5. Keep your health data secure. Lock your phone and protect your account with two-factor authentication.

The bottom line

Smartwatch ECG and AFib alerts are real, FDA-cleared screening tools that have helped people find AFib early. In the large Apple Heart Study, simultaneous alerts matched patch-confirmed AFib most of the time. But a watch records 1 lead for 30 seconds, checks for only a few rhythms, and can’t detect a heart attack or stroke. Use an alert as a prompt to see a clinician, and never let a normal watch reading talk you out of getting help for symptoms.

This is general information, not medical advice.

Sources

• FDA De Novo decision summary for Apple’s ECG app (DEN180044) — Authorization, single-channel Lead I-like ECG, AFib and sinus rhythm classification, age and arrhythmia limitations
• Apple Support: Take an ECG with the ECG app on Apple Watch — Result types, heart rate ranges, and what the app can’t detect
• Apple Heart Study results (NEJM 2019), via eScholarship — Enrollment, notification rate, 34% AFib on patch and 0.84 positive predictive value
• American Heart Association: What is atrial fibrillation? — AFib definition and stroke risk
• American Heart Association News: New AFib diagnosis? Here’s some help to clear any confusion — Advice to confirm smartwatch AFib alerts with a doctor