surprisingly dehydration--even though you don't think you are dehydrated--can cause this. I have what's called Idiopathic Sinus Tachycardia. My heart rate goes sky high for no reason. I've had EP's, blood tests, holters, stress test, echo, you name and I've had it. The idiopathic means they don't know why. I get very lightheaded and it's like there's a rushing sound in my ears. sitting down does help, so does drinking a lot of cold water or running my wrists under cold water. The only reason I can figure that helps is the shock of the cold jars me. I've also laid my wrists on the cold window when I'm driving. I've been tried on beta blockers, which slow the heart down, but because it's not constant, it slows everything down too much and my bp goes too low. Take comfort in the fact that they did not see any fibrillation or anything else to concern them. It's a pain in rear that I've been living with for almost 30 years. Oh, for me, caffeene in any form makes it worse.