The feature was one of the most talked about when the Apple Watch Series 4 was first unveiled, allowing you to take an electrocardiogram right from their wrist, capturing heart rhythm when you experience symptoms like a rapid or skipped heart beat.
This could help to provide critical data to physicians, and help detect illnesses such as arrhythmias, coronary heart disease, or early signs of a heart attack.
As well as the ECG app, the irregular rhythm notification feature on Apple Watch can now also occasionally check heart rhythms in the background and send a notification if an irregular heart rhythm that appears to be atrial fibrillation (AFib) is identified.
The system works with a set of electrodes built into the back crystal and Digital Crown on Apple Watch Series 4.
To take an ECG recording you'll need to launch the new ECG app on Apple Watch Series 4 and hold your finger on the Digital Crown. As you touch the Digital Crown, a circuit is completed and electrical signals across your heart are measured.
After 30 seconds, the heart rhythm is classified as either AFib, sinus rhythm or inconclusive.
All recordings are stored securely in the Health app on iPhone, allowing you to share a PDF of the results with your doctor.
The system was tested in a clinical trial of around 600 participants, comparing the Apple Watch readings to a gold standard 12-lead ECG.
The test found the Apple Watch demonstrated 98.3 percent sensitivity in classifying AFib and 99.6 percent specificity in classifying sinus rhythm in classifiable recordings.
In the study, 87.8 percent of recordings could be classified by the ECG app.
To enable these new heart features, you'll be taken through an onscreen setup that includes details about who can use these features, what the features can and cannot do, what results users may get, how to interpret those results and clear instructions for what to do if users are feeling symptoms that require immediate medical attention.
In the US, Apple worked with the Food and Drug Administration (FDA) for a number of years to receive De Novo classification for the ECG app and the irregular heart rhythm notification.
These are now CE marked and cleared in the European Economic Area, making the features available in 19 European countries, including France, Germany, Italy, Spain and the United Kingdom.
The Apple Watch update, watchOS 5.2, is available to download today.