About an hour before a sudden cardiac arrest, you might feel nauseous, vomit, or have chest pain or shortness of breath. Immediate warning signs of sudden cardiac arrest include heart palpitations or a racing heartbeat, dizziness or lightheadedness, or just feeling more winded or fatigued than usual, Dr. Doshi says. But two-thirds of the time, sudden cardiac arrest has no symptoms. You simply lose consciousness—and your pulse. That’s why it’s extra important that you know what to do if you see someone suddenly go down. Here’s your course of action: First, call 911 immediately.