Apple has released the first official update for watchOS in the form of a beta to developers registered in Apple's beta program. This release comes days after the public release of watchOS 5 and unlocks the ECG functionality for the Apple Watch Series 4.
During the initial announcement on September 12, Apple demonstrated how their new wearable would carry out analysis of the electrical activity of it's wearer's heart. However, this functionality was left out of watchOS 5 when it was released for internal reasons. Fortunately, it looks like this functionality will be making a return to Apple Watch Series 4 owners with the watchOS 5.1 update.
Developers can download the latest watchOS beta by heading to developer.apple.com or as an OTA update directly on their registered devices by heading to the Settings section on that device.