I have chest wall dystonia and have horrible muscle spasms and sometimes my heart feels like its going to pop or an elephant is on my chest. It's mostly muscular and sometimes sympathetic/parasympathetic nerve stuff that can cause bronchiole dilation and constriction. If your pec muscles are taunt or the muscles that band around your ribs are taunt its muscles. Try an inhaler. I've gone into dr swearing I must have low oxy levels cause I can't breath and its fine. Baclofen helps some, but those symptoms are quite frightening whatever it is. Try breathing techniques (deep breathing and slow exhaling). If nothing else it helps keep the panic down. I use a sedative and baclofen and a huffer, heat (clay) on back and chest and lay down. Hope this helps. Get well soon. This too shall pass eventually as RSD can change in intensity on a dime. Best wishes.