The study ended in August 2018. In all, only about 2,160 people, or 0.5% of study participants, received notifications of irregular heart rhythms. The rate was higher in people 65 and older—3.2%, verus 0.16% in people ages 22 to 39.
Of those notified by the watch, only about 450 people received and returned the wearable ECG patches to the researchers. Some didn’t get patches because they didn’t call the study doctor, or they revealed they had previously been diagnosed with atrial fibrillation, which disqualified them from the study, said Marco Perez, an associate professor of cardiovascular medicine at Stanford and one of the study leaders, in an interview.
Researchers found that the ECG patches confirmed atrial fibrillation in only 34% of the 450 people who returned patches. The remaining two-thirds had no confirmed atrial fibrillation during the time they wore the patches—raising questions about the watch’s accuracy.
Renato Lopes, professor of at Duke University School of Medicine, said the watch has potential to detect some atrial-fibrillation cases “you would not get otherwise.” But in a panel discussion after results were presented, he said the 34% confirmation rate was “not very high.”
Dr. Perez of Stanford said episodes of atrial fibrillation can be intermittent, which could help explain why a big proportion of ECG patch wearers had no confirmed atrial fibrillation during the week they wore it.
Doctors at the ACC meeting expressed concern that widespread use of the watch would lead people to undergo unnecessary tests and treatment, either because of false alarms or because they have atrial fibrillation that carries a lower risk of complications. Patients with lower-risk cases can be monitored instead of immediately undergoing treatment with blood thinners, doctors say.
“This also has the possibility to lead a lot of patients potentially to being treated unnecessarily or prematurely, or flooding doctors’ offices and cardiologists’ offices with a lot of young people,” Jeanne Poole, professor at the University of Washington in Seattle, said during the panel discussion after the results were presented
Measured another way, Dr. Perez said the likelihood that the watch detected atrial fibrillation as confirmed by the ECG patch was 84%, but this analysis was conducted in a smaller group, among 86 participants who received positive watch notifications of irregular heartbeats.
The Stanford researchers acknowledged the study’s limitations—it lacked a control arm and didn’t track patient outcomes such as stroke. And statistically, it didn’t meet researchers’ goals for confidence in some of the key measures. But they said it is a start in helping to assess the usefulness of wearable technologies.
“We’ve made some important measurements that are then going to be able to be used by clinicians to help guide what they want to do with the patient in front of them who has been notified,” said Dr. Perez.
Sumbul Desai, a Stanford professor and vice president of health at Apple, said the watch’s heartbeat tracker isn’t intended to be a diagnostic or screening tool. “I view the Apple Heart Study as just a first step,” she said in a panel discussion. She said Apple planned further research to better understand the medical value of the watch, including a new study in partnership with
Johnson & Johnson announced in January.
