General info I found:
On ProHeart 6:
Pfizer Animal Health Dogs Health Management
Immiticide to rid dogs of adult Heartworms is not best choice of treatment for some dogs.
Original slow kill method is being revamped.
Adult Heartworms can live around 5 years.
Combination therapy with Ivermectin (monthly) and Doxycycline (kills a bacteria in the Adult Heartworm, weakening them) appears to be the new treatment, but not yet approved for use in the US (?). I believe this protocol may clear Adult Heartworms from the dog's system within 36 months.(?)
Heartworms can live in the lungs. There is not likely any protocol using ProHeart 6 and anything else in curing an Adult Heartworm infection, as it is intended as a preventative and is marketed as such.
As your dog was given the ProHeart Vaccine, I think your vet may be leary of giving her anything else between now and at least 6 months. Did they give your dog Ivermectin injections also? (I think I remember you stated that)
Are you giving a monthly Heartworm preventative now? If yes...what kind? And how did they tell you to give the Doxycycline?
What tests were done to determine the extent of Heartworm infection in your dog?
How well does she tolerate exercise? Has she ever coughed up blood?
Did you find a new vet since she first got her vaccines and got sick?