Apple Watch Series 4 models include an electrical heart rate sensor that's designed to take an electrocardiogram (ECG) using an ECG app, the built-in sensor, and the electrodes included in the Digital Crown, but the ECG functionality won't be coming until later this year.
According to Apple's Apple Watch Series 4 website, the ECG app that will enable the electrocardiogram feature will be available later in 2018. That means at launch, the ECG feature won't be functional.
As several MacRumors readers have pointed out, it appears ECG readings are going to be limited to Apple Watch models sold in the United States. Apple Watch Series 4 pages for countries like the UK and Canada contain no mention of the feature, and Apple's press release for Series 4 models says ECG functionality is U.S. only.
To take an ECG reading from the Apple Watch, users will need to place a finger on the Digital Crown while wearing the watch. The reading is completed in just 30 seconds, allowing users to determine whether their hearts are beating in a regular pattern or if there are signs of atrial fibrillation, which can be an indicator of serious health problems.
Atrial fibrillation is a health condition that often goes undiagnosed, so the ECG feature in the Apple Watch will be highly useful for detecting early signs of disease. ECG readings also normally require a full ECG machine and a visit to the doctor, which is far less convenient than a reading taken with the Apple Watch.
Apple says that the ECG feature in the Apple Watch has been granted a De Novo classification by the FDA as outlined on the FDA's website [PDF], and the need for regulatory approval is likely why the ECG feature is not available in other countries.
De Novo FDA approval is a preliminary approval process granted to new devices that are not likely to cause harm. According to the FDA, the Apple Watch's ECG feature should be used only for informational purposes, should only be used by those over 22 years of age, and is not recommended for people with known arrhythmias.
The ECG app is a software-only mobile medical application intended for use with the Apple Watch to create, record, store, transfer, and display a single channel electrocardiogram (ECG) similar to a Lead I ECG. The ECG app determines the presence of atrial fibrillation (AFib) or sinus rhythm on a classifiable waveform. The ECG app is not recommended for users with other known arrhythmias.
The ECG app is intended for over-the-counter (OTC) use. The ECG data displayed by the ECG app is intended for informational use only. The user is not intended to interpret or take clinical action based on the device output without consultation of a qualified healthcare professional. The ECG waveform is meant to supplement rhythm classification for the purposes of discriminating AFib from normal sinus rhythm and not intended to replace traditional methods of diagnosis or treatment.
The ECG app is not intended for use by people under 22 years old.
Apple has also obtained FDA approval for the feature in the Apple Watch Series 4 models that sends notifications when an irregular heart rhythm is detected through the ECG feature or through standard Apple Watch heart rate monitoring.
All ECG data gathered by Apple Watch Series 4 models is stored in the Health app in a PDF format that can easily be shared with physicians.