I would def. find out if your dog got Immiticide (for adult heartworms) or Ivermectin (for baby/microfilarea), just to keep it straight, etc. Either the adults and/or babies dying off can cause embolisms with the Immiticide or slow kill methods....
We had the 2 shot protocol one day apart of the Immiticide last November. This was after Beau had xrays and bloodwork done to determine (I think educated guess now) what stage he was at and whether he was healthy enough to withstand the Immiticide. His xrays showed an enlarged heart which our vet said he could have been born with but could also be from heartworm disease. I don't believe seeing baby/microfilarea in a blood smear is a standard way to determine any stage before treating adult heartworm but just shows there are or had been adults... I've read that the babies can stay in circulation in a dog's blood after treatment for up to a year.
That all being said... Beau had been on Interceptor since we adopted him and when we had our vet records sent to them they helped pay for his Immiticide treatment and they sent us a year's supply of Interceptor. Just this summer we changed to Heartguard Plus as I just didn't think the microfilarea were dying off enough with the Interceptor.... who knows if I'm right.
I think our second followup to the adult heartworm treatment with Immiticide, our vet was concerned the babies were still pretty abundant in his blood smear so we asked if we could do anything for that. I think I asked about it as I'd read about a shot of Ivermectin (to help clear the babies) and she said that was possible to do. We did that and then continued to 'manage' Beau's activities to keep him as 'quiet' as possible, etc.
Now that it's a year later and he seems to still have alot of babies in the smears I don't know what stage he would be considered as he still doesn't show signs. My understanding is that when the dog does show signs, coughing, etc... the dog is in later stages and weaker. I hope my rambling helps someone.
I think also, vets are finding out the heartworms are becoming resistant to the adult treatment as well. And it's not a clear science how to rid a dog of them always... also... can't recall if this has already been discussed.. but here's a quote from a site "Ivermectin does not kill adult heartworms. It does weaken them and will speed up their death. Heartworms typically live two years. To further speed up the process of heartwormDoxycycline. The Doxycycline kills off an organism within heartworms called wolbachia. Wolbachia is a symbiotic organism that lives within adult heartworms. By treating an infested dog with a 30 day regimen of Doxycycline, you kill off the wolbachia, weakening the adult heartworms. This also is reported to lessen the inflammatory immune response of worm die off when the dog is receiving HW treatment of any kind. By weakening the heartworms and then continuing to treat with Ivermectin, you can clear a dog of heartworms typically in 18 months or less."