Watch Out for A-fib


You might have seen the wave of press surrounding the preliminary results from the Apple Heart Study, presented at the American College of Cardiology annual meeting earlier this year. The 15-month study, conducted by Stanford researchers and funded by Apple, included nearly 420,000 adults (half under age 40) and tested the ability of the Apple Watch to detect episodes of atrial fibrillation (a-fib), a heart rhythm abnormality that increases the risk of stroke and heart failure.

The study included adults who owned an Apple Watch (Series 1, 2, or 3) and an iPhone and who had not been previously diagnosed with a-fib. (The Apple Watch, Series 4, was too new to be included and the Series 5 had not yet been released.) The participants, who enrolled themselves online, downloaded a free app that used data from the watch’s heart rate sensor to detect possible rhythm irregularities. If it picked up an abnormality, users received a notification and request to schedule a video consultation with a study doctor, after which they would be sent an electrocardiogram (ECG) patch to monitor the electrical activity of their heart over the next week.

Overall, 0.5 percent of participants (2,161 people) got a notification, including 3.2 percent of those over age 65 and only 0.16 percent of those under 40. This isn’t surprising, since a-fib affects mainly older adults. Fewer than half of the notified participants followed up with the video consultation. Of the 450 who did and then wore the ECG patch, about one-third (153 people) were found by the ECG to have a-fib. According to the researchers, the watch was accurate 84 percent of the time, compared to the ECG patch, in picking up a-fib episodes, a high “positive predictive value” that was widely cited in press releases and media reports.

But the research had shortcomings. The self-selected participants were mostly young and not representative of the general population, nor of those most at risk of a-fib. (Less than 6 percent were over age 65.) What’s more, there is no way to know how many participants had a-fib that the watch missed. Then there is the issue of unnecessary anxiety for those who got notifications and wore the ECG patches but turned out not to have a-fib.

Even if the Apple Watch detected some arrhythmias that would have been missed otherwise, it’s not clear how many strokes or other problems might be prevented by that knowledge. Finally, the study was paid for by Apple—so it merits the same healthy skepticism as any industry-funded, not-yet-published study.

Bottom line: The Apple Watch can be a fun (albeit expensive) gadget. But it’s way too early to rely on it for monitoring heart health.