Cats are usually pretty good at ridding heartworms on their own... however, in the off-chance they do become infected, it is often fatal, and often asymptomatic until they fall over dead. Feline Heartworms are often mistaken for feline asthma, since both share very similar clinical signs. My only suggestion is to get tested if you haven't already, eventhough tests aren't 100% accurate, its better than nothing. As mentioned above, if you want to know xrays would probably be your best bet, but again depending on severity they MAY not show up.

Are you on a monthly preventative already? If you are then the chances of it being heartworms is very, very slim. If you are concerned, talk to your vet about HARD, heartworm associated respiratory disease.

Catlady is pretty accurate; as to date, there is no safe or effective treatment for feline heartworms. A heartworm positive cat has a VERY slim chance of surviving heartworm treatment. However, there ARE options for treatment out there. But that is for you to discuss with your vet.