A closer look
Your fancy step-counter is running a covert cardiology clinic on your wrist. Atrial fibrillation causes the upper heart chambers to quiver instead of fully contracting, allowing stagnant blood to pool and form massive, stroke-inducing clots. The green LED lights on the back of smartwatches bounce off your blood vessels hundreds of times per second to detect these microscopic rhythmic anomalies, alerting users to a life-threatening, totally asymptomatic condition before their brain loses oxygen.
