As predicted, the Apple Watch's version 5.1.2 update went live today, giving anyone in the United States with an Apple Watch Series 4 the ability to take an ECG reading on the wrist. Alongside the active ECG, the Watch's passive irregular rhythm detection feature launched today as well.

"We are confident in the ability of these features to help users have more informed conversations with their physicians,” Dr. Sumbul Desai, Apple’s vice president of Health, said in a statement. “With the ECG app and irregular rhythm notification feature, customers can now better understand aspects of their heart health in a more meaningful way.”

Additionally, Apple has begun to share some of the data that allowed the much-lauded feature, first announced in September, to pass muster with the FDA.

"The ECG app’s ability to accurately classify an ECG recording into A-Fib and sinus rhythm was validated in a clinical trial of around 600 participants," the company wrote in a news post on its website. "Rhythm classification from a gold standard 12-lead ECG by a cardiologist was compared to the rhythm classification of a simultaneously collected ECG from the ECG app. The study found the ECG app on Apple Watch demonstrated 98.3 percent sensitivity in classifying A-Fib 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."

That clinical trial was separate from the more public Stanford Apple Heart Study, which provided the company with validation data on the passive irregular rhythm detection feature. 

"A subset of the data from the Apple Heart Study was submitted to the FDA to support clearance of the irregular rhythm notification feature," Apple wrote. "In that sub-study, of the participants that received an irregular rhythm notification on their Apple Watch while simultaneously wearing an ECG patch, 80 percent showed A-Fib on the ECG patch and 98 percent showed A-Fib or other clinically relevant arrhythmias."

The data is stored in Apple's Health app once collected. Interestingly, it's not clear that Apple is supporting the sharing of ECG data directly via Apple Health Records, even at hospitals whose EHR is integrated with the app. Instead, users will have to export a PDF of their ECG to share with their provider.