What Can a Smartwatch ECG Tell You, and What Can't It?

An electrocardiogram (ECG) records the electrical activity of your heart. A watch ECG, recorded by touching the crown with a finger of the other hand, is a single-lead ECG: one view of the heart’s electrical activity, from arm to arm. That one view is good at telling whether your rhythm is regular, and in particular at spotting atrial fibrillation (AFib). It can’t do most of what a hospital’s 12-lead ECG does.

This article is general information, not medical advice. If you have chest pain, pressure, or tightness, call emergency services; don’t take a watch ECG first.

What the Results Mean

Apple’s ECG app, as an example, classifies each 30-second recording as one of:

• Sinus rhythm: a regular rhythm, with a heart rate between 50 and 100 beats per minute (bpm) in Apple’s definition. It applies to that recording only.
• Atrial fibrillation: an irregular rhythm with the pattern of AFib.
• Low or high heart rate: too slow or fast for the app to classify the rhythm reliably.
• Inconclusive or poor recording: the recording can’t be classified, such as from movement, a loose watch, or a pacemaker.

Each result applies only to the 30 seconds it recorded. A normal recording doesn’t rule out a rhythm problem that comes and goes.

What It Can’t Detect

Apple’s guidance is explicit that its ECG app can’t detect a heart attack, blood clots, a stroke, or other heart conditions such as heart failure, high blood pressure, or high cholesterol. A heart attack often needs several leads to see, and many cause no change in a single lead at all.

How Accurate the Alerts Are

Watches also check your pulse rhythm in the background with their optical sensor and can notify you of an irregular rhythm. In the Apple Heart Study of 419,297 people, Perez and colleagues found:

• Only 0.52% received an irregular pulse notification over a median of about four months.
• Of those who then wore an ECG patch, 34% had AFib on the patch, often because AFib comes and goes and the patch went on about 13 days later.
• When a notification and the patch recording overlapped, 84% of notifications matched AFib on the patch.

The study was funded by Apple. Whether screening people without symptoms for AFib improves their health is not settled: the US Preventive Services Task Force found the evidence insufficient to recommend for or against screening adults aged 50 or older.

Making Recordings Useful

1. Record when you feel symptoms, such as palpitations, a racing or skipping heartbeat, or dizziness, and note what you felt.
2. Sit still with your arm resting, and keep the watch snug.
3. Take the recording’s PDF or result to your doctor; a clinician can read the trace itself, beyond the label.

Following It in Kaizora

Each ECG recording comes in with your Apple Health export, or can be logged by hand, as an entry on the Vitals page with its classification, its average heart rate, and the device. The classification shows as a badge on each recording in History and on the Timeline, so you can see when an AFib result happened beside the night before or a stressful day. ECGs Logged counts your recordings. For people with AFib, see What Is AFib Burden?

The Short Version

• A watch ECG is a single lead: good for rhythm, especially AFib.
• It can’t detect a heart attack, stroke, or most other heart conditions.
• Most irregular rhythm alerts that overlapped a patch recording matched AFib, but whether screening helps is unproven.
• Record when you have symptoms, and share recordings with your doctor.

Sources

• Apple, Take an ECG with the ECG app on Apple Watch.
• Perez MV et al., “Large-scale assessment of a smartwatch to identify atrial fibrillation,” New England Journal of Medicine 381, no. 20 (2019): 1909–1917. doi:10.1056/NEJMoa1901183
• US Preventive Services Task Force, Atrial Fibrillation: Screening (2022).