Record an electrocardiogram with the ECG app on Apple Watch
Apple Watch Series 4 and later have an electrical heart rate sensor that, along with the ECG app 
, allows you to take an electrocardiogram (or ECG). To use the ECG app, update your iPhone 6s or later to the latest version of iOS and Apple Watch to the latest version of watchOS. The ECG app is not available in all regions.
Open the Health app on your iPhone, then follow the onscreen steps to set up ECG.
If you don’t see a prompt to set up, tap Browse at the bottom right, tap Heart, then tap Electrocardiogram (ECG).
Open the ECG app on your Apple Watch.
Rest your arm on a table or in your lap.
With the hand opposite your watch, hold your finger on the Digital Crown, then wait while Apple Watch records the ECG.
You don’t need to press the Digital Crown during the session.
At the end of the recording, you receive a classification. You can then tap Add Symptoms and choose your symptoms. Tap Save to note any symptoms, then tap Done. To view your results on iPhone, open the Health app on iPhone, tap Browse at the bottom right, then tap Heart > Electrocardiograms (ECG).
For more information about the ECG app, see the Apple Support article Taking an ECG with the ECG app on Apple Watch Series 4 or later, and the document Instructions for Use: ECG App.