We've been through it, when we first moved here our lab stayed with my sister for a bit, until we could get a fence up for her, and somewhere in the shuffle she didn't get her heartworm treatment.
We had to go through the treatment twice, the 1st time she developed pneumonia, so we backed off, gave her antibiotics, and let her heal up from that, we then started again, the plan from our vet was to kill them as slowly as possibly. The problem with treating heartworms, is large chunks of the dead worms can dislodge at once and that is bad. So we had to keep her as calm as possible, no jumping around or running or anything that might cause a largish chunk of the worms at once to break loose. Cowhooves, bones, some new toys, etc to keep her occupied.
I believe she was 6ish when we treated her and she lived until she was 12.
LaurelGrove is right though, it was rough and scary but she was a trooper and pulled through it fine.
