The Apple Heart Study followed more than 400,000 users; the device flagged irregular rhythms in roughly 0.5% of them. Of those who completed clinical follow-up, about 84% had confirmed atrial fibrillation. That sounds excellent. It is also more complicated than the marketing suggests — the false-positive rate in low-risk populations means a watch alert in a healthy 32-year-old is statistically more likely to be wrong than right. Here is how to read these devices honestly.
What single-lead wrist ECG actually measures
A clinical 12-lead ECG captures the heart’s electrical activity from twelve angles. It can diagnose a wide range of arrhythmias and structural heart conditions.
A wrist ECG is a single lead, roughly comparable to Lead I of a standard ECG.
What that single lead can reliably detect
- Atrial fibrillation
- Sinus tachycardia (fast heart rate)
- Sinus bradycardia (slow heart rate)
- General rhythm variability
What it cannot reliably detect
- Most other arrhythmias
- ST-segment changes (heart attack indicators)
- Bundle branch blocks
- Most specific cardiac pathologies
Saying a watch “does ECG” is technically true. It understates the difference.
The atrial fibrillation case
AFib is the specific indication these devices are cleared for. Validation studies show high sensitivity and specificity for AFib in inactive users. This is where wearable ECG has actually changed medicine — by detecting AFib in users who would not otherwise have been screened.
The catch: incidental findings
Screening healthy populations for rare conditions creates a math problem. Even a highly accurate test produces false positives when the base rate is low. In someone in their 30s with no risk factors, a positive AFib alert from a watch is statistically more likely to be a false alarm than the real thing. This drives unnecessary ER visits, anxiety, and workups.
The FDA’s clearance acknowledges this. The devices are explicitly marketed as alerts that warrant clinical follow-up, not diagnoses.
What to do if your watch flags an AFib alert
- Do not ignore it.
- Do not panic.
- Schedule an appointment with your doctor and bring the data. Most platforms let you export the ECG strip as a PDF — that PDF is genuinely useful in the clinic.
Specific devices
Apple Watch
First to clear, the most studied, the best ecosystem integration. The AFib History feature on newer models tracks how much time you spend in AFib over weeks — useful data for managing diagnosed patients.
Samsung Galaxy Watch
Solid ECG implementation, cleared in major markets. Some features tied to Samsung phones.
Withings ScanWatch
Combines ECG with traditional watch design. In some regions, also includes sleep apnea detection.
Fitbit Sense / Charge
Cleared, but historically slower to gain new cardiac features.
Blood pressure: separate question
Several watches now claim blood pressure measurement (Samsung, Huawei, some smaller brands). Accuracy is more variable than ECG, and the FDA has been more cautious about clearance. If you have hypertension, a validated cuff at home is still the standard.
What to expect next
- Continuous AFib monitoring rather than on-demand checks
- Sleep apnea screening from wearables — already arriving in some markets
- Blood pressure validation that eventually meets clinical standards
Bottom line
A wrist ECG is a useful, FDA-cleared tool for detecting AFib — particularly in older adults at elevated risk. It does not replace a clinical ECG, does not diagnose heart attacks, and will produce false alarms in low-risk populations. Take alerts seriously. Take false alarms in stride. Do not let the device convince you that you have a heart problem you do not.
