The FDA has previously warned that the ECG app is for "informational use only" and "should not be used by people under 22 years old." The regulator also noted that user's should not take "clinical action" solely based on the device's data readings and should always consult with a qualified healthcare professional.
Some medical experts have also said that Apple Watch wearers may draw false positives from the app, resulting in unneeded trips to the doctors or even ER. Apple has today reiterated the FDA's warnings to The Verge, adding that the features are definitely not diagnostic tools. The company also recently extended its Apple Watch return period, from two weeks to 45 days, but only for heart feature issues.
For now, the ECG function is only available in the US, and should be rolling out via the WatchOS 5.1.2 update.