Fitbit's ECG app has been cleared for use in the US, plus select countries in Europe and Asia, but approval came through just a little too late for it to be pre-installed on the device at launch.
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The app should be installed automatically when you sync your Fitbit Sense, but you can also install it manually through the App Gallery in the My Apps section of the Fitbit mobile app.
How to use the ECG app
Once that's done, sit down, open the ECG app, and tell it whether you're wearing your Sense on your left or right wrist. Place your thumb and index finger on opposite corners of the fitness tracker's metal case for 30 seconds, and wait for the scan to finish.
Once it's done, you'll see one of three possible results: atrial fibrillation, which is an irregular heart rhythm that should be checked out by a doctor; normal sinus rhythm, which means everything looks normal; or inconclusive, which means the app was unable to get a good reading. This might happen if your heart rate is below 50bpm or above 120bpm.
If the app has detected atrial fibrillation, you can use the Fitbit app to create a report that you can share with your doctor. Just tap the Discover tab, scroll down to Assessments and Reports, tap Heart Rhythm and select View Results.