Apple Watch ECG, irregular-rhythm notifications and AF History answer three different questions: what a brief recording shows, whether occasional checks find a possible irregularity, and how often sampled rhythm shows signs of atrial fibrillation over a week. Their outputs cannot be read as interchangeable results. ECG, rhythm notifications, AF History
Atrial fibrillation, often shortened to AF, is a heart-rhythm problem. This comparison covers these three AF-related functions, rather than every heart feature on the watch. High- and low-heart-rate alerts, for example, are separate notifications. NHS explanation, Apple’s notification descriptions
Three functions, compared on the same criteria
The table compares current UK manufacturer descriptions by signal, observation period and output. Each feature has a different intended use; this is not an effectiveness ranking.
| Feature | What is checked | When | What is reported |
|---|---|---|---|
| ECG app | Electrical activity recorded through the watch’s electrodes | A user-started, 30-second recording | A saved trace and classification for that recording |
| Irregular-rhythm notifications | Pulse-rhythm observations | Occasional background checks, usually while still | A notification after readings suggest an irregular rhythm that may be AF |
| AF History | Periodic rhythm checks for signs of AF | Aggregated retrospectively across the previous week | An estimated percentage of time showing signs of AF |
Sources: ECG app, irregular-rhythm notifications, AF History.
An ECG is a snapshot
Apple describes the watch recording as similar to a single-lead ECG. Apple says the ECG app is not intended for people under 22. A successful recording can be classified, but inconclusive or poor-quality results can also occur. A sinus-rhythm classification applies to that recording; it does not establish that the rhythm stays the same throughout the day. The trace and classification can be exported together. ECG documentation
That differs from the broader clinical assessment. The NHS describes an ECG as a main test for AF and notes that longer monitoring or other tests may be needed. A saved watch trace can contribute information without replacing that assessment. NHS testing context
Background checks are not a continuous recording
Irregular-rhythm notifications are not designed for people already diagnosed with AF. Apple says the frequency of checks varies with activity, and some episodes can be missed. Silence therefore supplies neither a continuous record nor an all-clear result. These notifications are not intended for people under 22. Rhythm-notification requirements and limits
AF History changes the question—and the settings
AF History requires an existing AF diagnosis and is also not intended for people under 22. Apple specifies at least 12 hours of wear a day on five days a week for consistent estimates. Its percentage is retrospective: it does not notify someone that they are experiencing AF at that moment. Enabling AF History automatically turns off irregular-rhythm notifications. AF History requirements
The display also never reports zero: its lowest label is “2% or less”. That label is not a guarantee that no episode occurred. Estimate display and limitations
Compatibility belongs beside the comparison
Apple lists the United Kingdom for these functions in its country directory, but country availability alone does not establish that a particular watch has every feature. Hardware, software and current setup requirements remain part of the product description. Feature availability
The useful distinction is the question attached to the output: a recording, an occasional alert or a weekly estimate. HRV describes a different measurement, while hypertension notifications address a separate blood-pressure question. Neither should be folded into an AF feature ranking.