Monitoring cardiovascular health has become a core capability of modern wearables. Starting with Apple Watch Series 4 (excluding SE models), Apple integrated electrical heart sensors into the Digital Crown and back sapphire crystal. This hardware enables users to generate a single-lead electrocardiogram (ECG) similar to a Lead I ECG used in clinical settings. The ECG app records electrical impulses generated by heartbeats, checks for signs of Atrial Fibrillation (AFib), and creates medical-grade PDF waveform documents that can be shared directly with healthcare providers.
How the Apple Watch Electrical Heart Sensor Works
Unlike optical heart rate sensors that measure blood flow through green LED photoplethysmography, the ECG app measures actual electrical signals:
- Closed Electrical Circuit: When wearing your Apple Watch snugly on your wrist and resting a finger from your opposite hand against the Digital Crown, you create a closed electrical circuit across your chest and arms.
- Single-Lead Measurement: The sensor samples microvolt electrical impulses across a continuous 30-second window, plotting standard P-waves, QRS complexes, and T-waves.
- FDA-Cleared Algorithmic Classification: Upon recording completion, Apple’s proprietary algorithm categorises the rhythm into one of several clinically verified diagnostic buckets.
Setting Up the ECG App on iPhone
Before taking your first recording on Apple Watch, the feature must be initialized via the paired iPhone Health app:
- Open the Health app on your iPhone.
- Tap the Browse tab in the bottom-right corner and select Heart.
- Tap Electrocardiograms (ECG) and select Set Up ECG App.
- Enter your date of birth (the ECG app is regulatory-cleared for individuals aged 22 and older).
- Review the informational slides explaining what the ECG app can and cannot detect (it does not check for heart attacks, strokes, or blood clots).
- Tap Continue to finish setup. The ECG app icon (a white background with a red heart and waveform) will now appear on your Apple Watch.
Taking an Accurate ECG Recording on Apple Watch
For the cleanest electrical waveform with minimal muscular artifact noise, follow these posture guidelines:
- Ensure your Apple Watch is worn snugly on the wrist selected in your Watch settings (verify in Settings → General → Orientation).
- Sit down comfortably at a table or desk, resting your forearms flat against the table. Relax your hands and shoulders.
- Open the ECG app on your Apple Watch.
- Rest your index finger lightly on the Digital Crown. You do not need to press the crown inward during the test.
- Keep your finger steady on the crown. A 30-second countdown timer begins with a real-time animated electrocardiogram trace.
- Remain completely still and silent until the timer elapses and a chime sounds.
Understanding and Interpreting ECG Results
Once the 30-second test completes, your watch displays an immediate rhythm classification:
| Classification | Clinical Meaning | Recommended Action |
|---|---|---|
| Sinus Rhythm | Heart beats in a consistent, uniform pattern between 50 and 100 BPM. | Normal baseline result. Continue routine monitoring. |
| Atrial Fibrillation (AFib) | Heart beats in an irregular, chaotic pattern between 50 and 120 BPM. | Indicates an irregular heart rhythm. Contact a doctor or cardiologist. |
| Low or High Heart Rate | Heart rate was below 50 BPM (bradycardia) or above 120 BPM (tachycardia). | Waveforms cannot be checked for AFib at extreme heart rates. Retest when resting. |
| Inconclusive | Recording was distorted by movement, poor contact, or an unclassified arrhythmia. | Rest arms on a flat surface, clean sensors, ensure snug strap fit, and retest. |
| Poor Recording | The circuit was broken repeatedly during the 30-second window. | Moisten dry fingertips slightly and ensure finger remains still on the crown. |
Exporting ECG Waveform Reports as a PDF for Your Doctor
Every recorded ECG is saved in full clinical resolution within the iOS Health app:
- Open the Health app on your iPhone.
- Navigate to Browse → Heart → Electrocardiograms (ECG).
- Tap on the specific ECG recording you wish to review.
- Tap Export a PDF for Your Doctor.
- The Health app compiles a standardized 25mm/s grid chart detailing your average heart rate, single-lead voltage waveform, and rhythm classification. Tap the Share icon to print, email, or save the document directly to your medical records folder.