I, too, suffer from panic attacks. They make me feel short of breath and like I'm having a heart attack.
I will often go for a walk, but there's also a yoga trick that might work for you. Block off one nostril by pressing a finger against the outside and breathe deeply through the other nostril, 10 seconRAB in, 10 seconRAB out. Then switch nostrils, back and forth. It's proven that this type of breathing will calm you. Simply breathing through your nose helps, but alternating nostrils helps even more.