Last year, Apple’s Tim Cook shared widely the story of an 18-year-old Florida girl who claimed that her watch saved her life by alerting her that her pulse was 190, prompting her to go the hospital for evaluation. She was found to have kidney damage of which she was unaware and referred for treatment. My problem with that heart-warming story was that it was hard to believe she was not aware of that rapid a pulse on her own, and that it may have had nothing to do with her kidneys. This week we have been bombarded with news from a study reported at the Annual Meeting of the American College of Cardiology about the watch’s ability to detect previously unknown atrial fibrillation and thereby “save countless lives.”
First, a brief word about atrial fibrillation (AF). AF is a very common heart rhythm disorder that gets much more common with age. While rare in people under 50, it may affect as many as 10% of those over 75. Most people with AF are very aware of the condition because the heart beats very fast as well as erratically. A few have no symptoms, particularly if they are on a medicine for some other reason that slows the rate. AF can be associated with heart failure and, most ominously, it is associated with a greatly increased rate of stroke. The accepted wisdom is that most patients with AF should be on blood thinning drugs to prevent stroke except for the minority who have absolutely no other stroke risk factors beyond the AF.
The study looked at the ability of the watch to use an app to detect AF. Over 400,000 people self-enrolled on an invitation they got when they down-loaded the Heart Study app. A pulse notification was received by 2161 (0.5%) participants. Not surprisingly, notification was highest in those over 65 (3%) and lowest among those under 40 (0.2%). ECG patches were sent to 658 participants and returned by 450. AF was identified in 34% of those who were notified and wore the patch.
So, a few conclusions can be made. First, the watch can pick up some cases of AF. Second, it has many “false positives:” people who did not have AF despite what the watch said. A huge problem is that the study was not designed to detect “false negatives:” we have absolutely no idea how many of the participants did have AF that was not detected by the watch because no one got a patch who did not receive an AF alert. Since Apple watch wearers tend to be younger and healthier than non-wearers, many of the alarms are likely to be false positives, thereby causing anxiety and unwarranted health expenditures to evaluate the alarms.
Another problem with the study is that we really have no idea whether treating younger people who do truly have AF detected by the watch will help or harm them. The evidence that using blood thinners prevents strokes was all obtained back in the 1990s and came from people who had symptomatic AF. These drugs have enormous potential for harm from bleeding, and it is very possible that treating people found via apps may do more harm than good.
So, take the hype with a grain of salt. Enjoy your watch but do not depend on it to save your life.
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