Wearables giant Fitbit Inc. has kicked off its first large-scale, virtual study to evaluate how well its wrist-worn technology can spot episodes of irregular heart rhythm that might indicate atrial fibrillation (AF). The study, part of a broader plan to equip Fitbit devices with new tools to speed the detection of a range of conditions, is open to people in the U.S. ages 22 and older who own a Fitbit that tracks heart rate.