I’ve only experienced one or two in my life thankfully (both brought on by my hypochondria. I became convinced something was seriously medically wrong with me and that brought on a panic attack). I’m sure different people experience them differently. For me the main symptom was shortness of breath, increased heart rate…the more I thought about what was happening, the worse it got: the more my breathing rate increased and the more my heart pounded. I soon felt like I wouldn’t be able to breathe if it kept up, which of course, just made it even worse. It’s a positive feedback loop. I also felt like I was going insane. The only thing that stopped it was splashing cold water on my face. Once I could tell that my heart rate was actually beginning to slow, that allowed me to relax and finally it abated. Not a fun experience. Hope this helped your question, though.