I am sorry your dog is having trouble.
Did your vet run any blood or urine tests other than a heartworm test?
Is you dog on regular heartworm preventative?
Was a blood smear performed?
Did your vet take any chest x-rays?
does your dog have a heart murmur?
Does your dog have alot of tartar build up on her teeth? Bad breath?
Let me know and I will try to help..........