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.