Cardiac arrest during physical activity question.?

All right so I was gong to try out for the basketball team but now I'm starting to worry because there could be something wrong with me that I don't know about with my heart. I started getting panic attacks thinking about this. So I went to the emergency room for this and and I had and EKG, blood work, and a chest x-ray and all come back good. So my question is would these test find if there were any underlying problems with my heart. I am just petrified that there may be something wrong and I might end up like Len Bias, Hank Gathers, that Russian hockey player. I told my doc about this fear and he told me it is rare for this to happen. I still can't help worrying though. Also what is an EKG anyways.