You're putting her through treatment, right? If you guys take her through the treatment and have her on about 5-6 weeks of bed rest, she will most likely pull through.

Check to see if your heartworm prevention is expired. If not, you need to talk extensively to your vet to find out how this happened.

Is Misty an outside dog?

I would hold off on the new addition because Misty will need a lot of quiet rest during her treatment. A new dog around (especially if its a puppy) could get her excited. Her heart rate cannot go up while she's recovering or she could die. Also, you should figure out how your dog got heartworm twice before you get a new one. The treatment is VERY hard on them and she'll need to be inside in a crate when no one's home. When someone is home, she should still be inside but doesn't have to be in her crate as long as she's resting. Some dogs won't settle down so have to be crated just about all the time during their bed rest.

You said she's had it before so I assume you know all this? I've fostered heartworm dogs through their treatment so if you have any questions, you can email me or post here.

Oh yeah, you said she must of just gotten it but that can't be possible if she's that sick. When they get to the point where they aren't doing well, they've had it a loooong time. It won't even show up on the heartworm test until 6 months after they contract it.