Heartworm and Asthma

Yesterday I took my cat to the vet for some checkups and information. I discussed with them a occassional problem that my cat experiences: sometimes he will stop and like cough or wheeze a little for maybe a minute or less. Then he returns back to normal. He is about a  little over a year now, and he was a stray kitten when we got him last winter. I've seen this problem with him shortly after we got him, and it only happens every so often. It never gets worse or more frequent.

Now the vet told me to keep an eye on him as she said she doubted that it was heartworm because it doesn't happen to terribly often. She also said it could be an allergy. I am taking her word on it, however I have been looking up cat heartworms and asthma, and I am frustrated by the lack of information. Sure they tell you the signs but they don't go into the details like the frequency of the symptoms and such.

What I would like to know is: what are the signs of heatworm and how often do they have to happen to be concerned? Also, I would like to know more about feline ashtma. I live in the southeastern part of the United States, and he currently is being treated for fleas and tapeworm(he is on Revolution) He is mostly healthy and energetic. He is of mixed breeds, he is not a purebred cat.  Any information would be a big help!