Time can be of the essence when it comes to heart trouble, so imagine a world where you can rely on your wristwatch to notify you if something seems awry. We're certainly not there yet, but a massive new study carried out by Stanford scientists (and funded by Apple) suggests this kind of functionality isn't unattainable either, with the Apple Watch able to detect irregular heartbeats in a meaningful amount of users.

Back in 2016, scientists at Stanford Medical School probed the practicalities of popular fitness trackers including the Apple Watch, Fitbit Surge and Samsung Gear S2, and their study returned mixed results. The data around energy expenditure seemed to be wildly inaccurate with huge error rates, but the researchers were surprised by the accuracy of heart rate monitoring, which they deemed to be largely reliable.

But still, offering reliable retrospective data at the end of the day is one thing, serving as an early warning tool for potential heart failure is another. The Apple Heart Study, which was fully funded by Apple and conducted by the Stanford School of Medicine, sought to investigate these possibilities by enlisting 400,000 subjects from the US.

More specifically, the scientists wanted to find out whether the Apple Watch could reliably detect a type of heartbeat irregularity called atrial fibrillation, which has no obvious symptoms and can therefore go undetected for long periods of time. This irregularity brings increased risks of stroke and other complications, including heart attack and failure.

Participants agreed to wear a Series One, Two or Three Apple Watch and download a purpose-built Apple Heart Study app onto their phones. When the heart-rate pulse sensor within the Apple Watch detected an irregular pulse, a notification was pushed to the user's phone asking them to schedule a video consultation with a doctor taking part in the study. Of the more than 400,000 participants, only 0.5 percent received notifications, boding well for the cumbersome problem of false positives.

Those that did contact a doctor were then sent ambulatory ECG patches which were used to monitor their hearts' electrical rhythm for up to a week. Of the users that wore the patches, 84 percent of the irregular heartbeat notifications were able to be verified as atrial fibrillation events by the ECG patch. Of that group of patients, 34 percent were found at the end of the week to have atrial fibrillation, though the scientists note it is an intermittent condition and would not be surprised if it did go undetected.

"The results of the Apple Heart Study highlight the potential role that innovative digital technology can play in creating more predictive and preventive health care," says Lloyd Minor, MD, dean of the Stanford School of Medicine. "Atrial fibrillation is just the beginning, as this study opens the door to further research into wearable technologies and how they might be used to prevent disease before it strikes – a key goal of precision health."

There's little room for guessing games when it comes to heart trouble, so this research isn't about to translate into ringing endorsements of Apple Watches and similar devices as fail-proof diagnostic devices, but further research may work toward that possibility.

Notably, the study didn't include the Apple Watch Series Four, which comes with onboard ECG monitoring, as it launched after the study commenced. Apple itself says its devices are not a guaranteed way of detecting heart irregularities like atrial fibrillation, only that they can flag potential problems for reporting to a doctor.

The Stanford researchers presented their findings over the weekend American College of Cardiology's 68th Annual Scientific Session and Expo.

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