See this Apple post for more details
"The [ECG] feature "is really a game changer as the first wearable device with this feature," Evercore ISI analyst Ross Muken wrote in a note to investors. "This update really establishes the company’s increasing efforts to push the watch as a serious medical device."Apple Gets FDA Approval for New Watch, Touts Health Gains -- Bloomberg
The Watch has more-powerful sensors so it can spot when someone has a hard fall. It delivers an alert and calls emergency services if the user doesn’t move for one minute after a fall, Williams said. The ECG capability, available later this year, helps the device sense atrial fibrillation, an irregular heart rate that can increase the risk of stroke, heart failure and other heart-related complications.
“The FDA worked closely with the company as they developed and tested these software products, which may help millions of users identify health concerns more quickly,” FDA Commissioner Scott Gottlieb said in a statement. Health-care products on ubiquitous devices, like smartwatches, may help users seek treatment earlier and will empower them with more information about their health, he added."