This happened to me driving home from a baseball playoff game a year and a half ago. I actually passed out behind the wheel but came to in time to keep the car on the road. Scary thing is, I had the cruise control on and had I not come to, me and my son would probably be dead. It was the scariest thing that has EVER happened to me. I had a heart fibrillation, then I momentarily blacked out. After coming to, I immediately went into full panic attack, which does feel JUST like a heart attack. My doctor said it was a caffeine overdose and since then, I have greatly limited my caffeine consumption.