Captain America and Black Panther were about to defend Earth from the villain Thanos when Kevin Foley first noticed something was wrong. Foley, a 46-year-old information-technology worker from Kyle, Texas, was heading into the theater to see Avengers: Infinity War when he realized he was having trouble breathing normally. The sensation struck again during another movie the following night, but more severe this time. Once the credits on the second film rolled, Foley took action: he looked at his wristwatch. It was a bigger step than you might imagine, because Foley was wearing an Apple Watch equipped with medical sensors and experimental software to track basic functions of his heart. And the watch was worried. It had, according to the display, detected signs of an irregular heartbeat.
Before long, Foley was in an emergency room, where doctors hooked him up to an electrocardiogram (ECG), which showed that he was in atrial fibrillation, or AFib, an irregular heartbeat that can lead to blood clots, stroke and other potentially fatal complications. Foley spent the next few days in the hospital while doctors worked to return him to a normal sinus heart rhythm–eventually turning to a procedure called electrical cardioversion to shock his heart back to normalcy. Foley is doing fine now. But he believes that, if not for the warning on his watch, he might not have sought help in time. “I would have never known,” he says.