A new study suggests the Apple Watch was able to identify abnormal heart rates that could be linked to a serious heart condition. More than 400,000 Apple Watch users volunteered for the study. It was a project of researchers at the Stanford University School of Medicine. Apple, the American technology company, provided money for the study. Results were recently presented at a major conference on heart health in New Orleans, Louisiana. The researchers looked for signs of atrial fibrillation, the most common form of irregular heartbeat. The condition, also called a-fib, results from the abnormal firing of electrical signals in the upper chambers of the heart. Some people with atrial fibrillation feel changes in their heartbeat, while others do not. Because of this, many people do not know they have the condition.