My wife gets this sometimes when she hears something on the news thats disturbing or watches a scary movie. She feels like she cant breath and starts to freak out. Her doctor awhile back, prescribe relaxens but I suggested her not to use them because she may become depended on them.
The last time she felt this way was few weeks ago when we were away during our family vacation. I learned about breathing exercises on how they can help one relax. It actually worked and within 5 minutes she felt calm and stupid because she knew her fears are not real but only presumed to be real.
Here's how it works. It should be practiced a few times before an attack occurs, so you kind of get use to it. Breath in for about 5-7 seconds, hold your breath for about 4 times that (20-28 seconds), then exhale 10-14 seconds. The rule of thumb is, breath in one part, hold breath 4 parts, exhale 2 parts. Do this exercise 10 times. Its guarantee to work.