The watch will also have an “irregular rhythm” notification feature, which alerts people to potential problems. There’s every reason to believe it will generate many false positives. Before granting clearance, the F.D.A. reviewed data collected by the Stanford Heart study for 266 people who got such a notification. Most of the notifications were wrong.

The study wasn’t peer reviewed, so we don’t know for sure, but this was also a population for whom atrial fibrillation might be more common than in those who might use the watches. People who buy the latest Apple watch will most likely be younger, healthier, wealthier and more plugged into the health care system — and less likely to remain undiagnosed.

This is one of the major problems with such a device. The people most in need of it, those who might benefit from tests and distance monitoring, are the least likely to get it. If we truly believed this was a medical test beneficial to the general population, insurance should pay for it. No one is suggesting that should happen.

In fact, many experts don’t think it makes sense to have universal cardiac monitoring of the general public. The United States Preventive Services Task Force has issued a “D” recommendation for screening asymptomatic adults at low risk. The group doesn’t think there’s enough evidence to recommend screening of adults at intermediate or high risk. It doesn’t even think there’s enough evidence to recommend screening adults 65 or older, who are at higher risk, for atrial fibrillation.

The task force bases these recommendations on good research. A large randomized controlled trial of echocardiographic screening for many heart problems did not demonstrate that such screening offered any benefits in reducing death or the risk of heart attacks or stroke in middle-aged people. And these are scans much more robust than will be available with the new Apple Watch.

None of this prevented the American Heart Association from heralding this new function, although it’s not clear where the group’s enthusiasm comes from. Dr. Ivor Benjamin, the association president, appeared at the official announcement of the watch and praised the advancement for tools that “help fight heart disease.” (The A.H.A. does not officially endorse the watch, or any other specific products.)

I happen to own an Apple Watch. I find the other functions useful and fun. I even enjoy aspects of the activity monitoring. But I’m under no illusion they will help me lose weight or exercise more or improve my heart health. I own one because I want it, not because I need it. That’s the same criterion you should use, too.