If you can find an O2 tank you oughta have a verry good time... but other than that the trick is to get some air when you need it (breathe deep for a while to get O2 in, take a hit, then deep breaths of fresh air), make sure your diet is good (take vitamins perhaps) and get a good run in every now and then to make sure your ticker is in tip-top shape.

Some of it can be lack of O2 then paranoia, just lean back and breathe deep, put on some Floyd and meditate, breath in and out deep and relax, if you get all worry pushed out of your head while doing breathing exercies your heart rate sould be lowered majorly... when your in a pool next, try holding your breath for long amounts of time, the human body can do many things if you condition it to.