A huge study suggests the Apple Watch can detect a worrisome irregular heartbeat at least sometimes - but experts say more work is needed to tell if using wearable technology to screen for heart problems really helps.
More than 419,000 Apple Watch users signed up for the unusual study, making it the largest ever to explore screening seemingly healthy people for atrial fibrillation, a condition that if untreated eventually can trigger strokes.
Stanford University researchers reported on Saturday that the watch didn't panic flocks of people, warning just half a per cent of participants - about 2,100 - that they might have a problem.
But even among those flagged, "it's not perfect", cautioned Dr Richard Kovacs of the American College of Cardiology, who wasn't involved with the study.
People who received an alert were supposed to consult a study doctor via telemedicine and then wear an EKG patch measuring cardiac activity for the next week to determine the watch's accuracy. Some skipped the virtual check-up to consult their own doctors; overall, about 57 per cent sought medical attention.
Among those who got EKG monitoring through the study, a third had atrial fibrillation, according to preliminary results being presented at an American College of Cardiology conference in New Orleans.
A-fib tends to come and go, and a week of monitoring might have missed some cases, said Stanford lead researcher Dr Mintu Turakhia. But if the watch detected another irregular heartbeat while someone was wearing the EKG patch, 84 per cent of the time it really was a-fib, he said.
"This study we believe provides very encouraging evidence that a device, the Apple Watch, can be used to detect a-fib and to point out to people when additional monitoring or testing may be needed," said Dr Lloyd Minor, Stanford's dean of medicine.
Other cardiac experts said the study, which was funded by Apple, suggests screening with wearable technology might be technically feasible eventually, but needs lots more research.
"I would not advise this to the overall general population," said Dr Valentin Fuster, director of Mount Sinai Heart in New York and a former American Heart Association president, who wasn't involved with the study. Instead, he'd like to see it tested in seniors with risk factors like high blood pressure.
A mobile app uses the optical sensor on certain versions of the watch to analyse pulse rate data. If it detects enough variation from beat to beat over a 48-hour period, the user receives a warning of an irregular heart rhythm.
The latest version of the Apple Watch also allows wearers to push a button to take an EKG and share the reading with doctors. Saturday's study didn't include watches with that capability.
Australian Associated Press