Americans have had these options since December, but their global expansion puts the technology squarely within the purview of public-health systems, which typically think carefully about how to screen for health conditions. The watch is also spurring debate about how doctors should handle the AFib that it and other consumer devices, such as AliveCor, detect.

AFib is the most common cardiac arrhythmia and occurs when the heart’s upper chambers do not beat in a co-ordinated fashion. Blood can pool in parts of the chambers and form clots. Patients with AFib are five times more likely to have a stroke. They can be treated successfully with blood-thinners, but these carry risks, primarily excessive bleeding. AFib is thought to occur in 2% of the population. However, as the risk of suffering from it increases greatly with age, it will be rare in Apple Watch owners, who are younger, richer and healthier.

Jonathan Mant, a professor of primary-care research at the University of Cambridge, runs a study of the over-65s that is hoping to discover if AFib screening can prevent stroke and other problems, such as heart attacks or even dementia. Dr Mant says AFib, picked up clinically, is important and worth treating, but that may not be true of the cases found by the watch. “We really have no idea what that would mean,” he says.

Some public-health scientists have warned that AFib screening leads to many false positives and negatives, and these problems could be made worse by consumer technologies. Carl Heneghan, a professor of evidence-based medicine at the University of Oxford, says these new technologies bypass the usual governance systems that ensure new screening programmes do not create harm. He also worries that false positives will generate a huge amount of pointless work.

The recent “Apple Heart Study”, covering 420,000 patients, looked at the predictive value of the device’s monitoring for irregular pulses. It found that the watch only agrees with a gold-standard method 84% of the time. The feature is intended to prompt wearers to use the ECG app, which is designed to deliver a diagnosis. A study conducted by a research organisation contracted by Apple found the app’s algorithm was able to correctly identify 98.3% of true positives and 99.6% of true negatives.

Yet neither trial included randomised controls, which would offer the kind of information doctors want. This is now planned among the over-65s. There is also an urgent need to understand how common intermittent AFib is, and its consequences. This is something the watch might help with, by providing reams of data that are otherwise difficult to come by.

Matt Kearney, the National Clinical Director for Cardiovascular Disease Prevention for Britain’s National Health Service, admits there are challenges with the watch and the rise of consumer-health technologies more generally. But he says the device will uncover cases of AFib that need treatment, and in younger people who have no other symptoms it will be an “opportunity for people to be advised about their risks”.

Health professionals will need to embrace new technologies, if only because they are inevitable, Dr Kearney adds. And health systems will need to adapt to the torrent of data. As for the broader impact of the Apple Watch, Dr Mant concludes it is “paradigm-shifting. I just don’t know if it is going to be in a good way or a bad way.”

This article appeared in the Science and technology section of the print edition under the headline "Don’t be still, my monitored heart"（Apr 4th 2019）