Unfortunately, your post is a warning for the many, many people who neglect monthly HW prevention. It kills a lot of dogs. It's not a good death either. If you can't treat and fight this, the dog needs to be humanely let go, not allowed to drown in its own fluids.
Personally, I would NOT give up on this dog! Maybe Diesel will yet live, if you can get better vet care. If Diesel dies, maybe find a very gentle second dog as a companion for Ranger.
I deal with lots and lots of heartworm in dog rescue in Louisiana, and I will help them fight as long as they've got it in them. We've saved nearly all of them -- a few died during treatment, but many many dogs lived, including some with "advanced" disease. I don't view HW as an automatic death sentence -- I view it as expensive, challenging, and dangerous, but not hopeless. I've got too many foster alumni who made it through to the other side of this disease and are now living happily for me to ever give up hope.
Let's walk through next steps:
1. Does your dog have "caval syndrom"? Ask the vet that question! It's critical information. Caval syndrome is the final, deadly stage -- it means you're facing an emergency:
UC Davis Caval Syndrome Text
They have pee with blood in it that looks like red wine, among other things.
Dogs that are caval may sometimes be saved by emergency surgery by a cracker-jack surgeon who knows how to physically remove HWs from the heart and pulminary arteries. Most vets haven't done this surgery, and they don't even know how -- but it CAN be done.
If you are in the South and can get your dog to Cypress Lake Animal Hospital in Prairieville, LA, people come from all over the country to see Dr. Kristen -- she's done this surgery successfully, MANY times. There are pictures of it on the clinic FB page. There was recently a patient flown from Maine to see her -- she's that good. She's also not expensive--very reasonable compared to most other options. See the post on May 2 for pictures of the 27 worms successfully pulled out of a dog by this vet:
3. Advantage Multi (RX ONLY) is proving extremely successful at killing adult worms. It's a monthly prevention product that is unique -- it's the ONLY one that is killing adults this way. I've had several advanced HW cases in foster dogs (not yet caval) clear in 9-12 months or less on Advantage Multi, with a month of Doxy -- it's much easier on them than immiticide/fast kill, so if they're too weak for immiticide, we start this in our rescue IMMEDIATELY. We have whole thread on this by Gatorbytes, with links to vet journal articles --
4. I would assume that your vet already put your dog on Doxycycline and Prednisone, right? If not, why not??? The doxy is starts to sterilize the HWs and kill a very nasty symbiotic bacteria that the HWs carry, which causes some of the symptoms. The pred is critical to reducing inflammation and the secondary effects of advanced HW disease.
5. Your puppy should be started on HW prevention around 12 weeks if you're in a heavy HW area. Advantage Multi is labeled for 9 weeks. If your puppy is older than that, PLEASE put him on Advantage Multi ASAP, and keep him on that product for the first year -- not something cheaper -- to kill juvenile heartworms already in him. Worms can't be tested for until the worms are 6-7 months old, but by that point, they're already too old to kill with other prevention products. You'll have a HW+ dog again if you don't get the pup on prevention soon enough. After a year, if he's still HW negative, you can assess whether to buy a cheaper product like TriHeart Plus, or keep him on Ad Multi. In heavy HW areas, I'd stay on Ad Multi for life, as it's so effective.