First of all: Don’t panic.
Then stick your fingers in your ears because you’re probably about to develop an acute case of tinnitus. Because of all off that repressed panic or something.
But don’t panic about that, either. I SAID BUT DON’T PANIC ABOUT THAT, EITHER!
Panikattacken: Einfache Ratschläge gegen Atemnot, Schwitzen und Herzrasen
Observing Hermann 
You can also start humming Brahms´s “Frei aber Einsam” (“Free but Lonely”) quietly to yourself. I hear it helps;-)