The ECG app records a single-lead electrocardiogram using the electrical heart sensor — the same kind of rhythm information a single-lead medical ECG captures, but taken from the wrist. Its classification is deliberately narrow: it checks your rhythm for signs of atrial fibrillation (AFib) and reports Sinus Rhythm, AFib, Low/High Heart Rate, or Inconclusive.
What it can't do: it doesn't screen general heart health, it can't detect heart attacks, and an Inconclusive result isn't a verdict — just noise, a too-slow or too-fast rate, or an arrhythmia other than AFib.
Requirements: Apple Watch Series 4 or later and all Ultra models (not the SE), with an iPhone XS or later and current OS versions. Availability is regional, and Apple states the app is not intended for use by people under 22 years old.
Important: any result is a classification, not a diagnosis. If you have symptoms — or get an AFib result without a prior diagnosis — talk to a doctor; ECG recordings (including a PDF with the waveform) can be exported from the Health app to bring along.
Sources: https://support.apple.com/120278
— Osnivo Guides