Skelly is having a reaction to his meds

About 2 hrs after giving him his meds, he started acting very lethargic and his breathing appeared labored. I let him inside and found his leash. I could tell he was excited, but he could barely react. We left the house and I opened the back door to the car. He tried to jump in, but needed help. As soon as I closed the door, he curled up.

When we arrived at the Vet's office I opened the door, but he didn't budge. I had to carry him from the car to the vet. They are working with him now...

Update: they have given him an IV catheter and are administering drugs to combat the reaction...

Update: His blood pressure is very low. Reaction is pretty strong. I think they are about to send me home and keep him here while they work on him...

Update: They took me back to the treatment area. Skelly was on an IV getting some liquids. They gave him 2 injections to combat the reaction. His only movement was breathing. He was looking so pathetic.  As I was speaking to the technician about leaving and checking back in a couple hours, he moved nose to the edge of the cage. I gave him a little bit of one finger affection through the bars. His breathing sped up a bit when I did so.

Hopefully he'll stabilize and be ready to come home soon! We're crossing our fingers. Nobody at the vet's office spoke in definites. They were careful not to use phrases that might give false hope. This is a serious situation. Good luck Skells!!

Update: I called the vet's office to check on his condition. The doc was with another patient at the time. All they could tell me was that he was not interested in food and that he was resting comfortably.

I've done a bit of research. Apparently a reaction this severe is rare.
I found some pretty informative pages on the subject:
http://www.placervillevet.com/canine%20heartworm.htm
http://www.avma.org/animal_health/brochures/heartworm/heartworm_brochure.asp

Starting heartworm prevention

The first dose of heartworm prevention medicine will kill a large number of microscopic heartworm larvae, which can cause a reaction. We minimize this by pretreatment with a cortisone drug followed by the first heartworm pill about an hour later. This can be done at home.

Do this on a weekday, when our office will be open and you can be with your pet all day. Give the prednisone tablet(s) at 7:00 in the morning and the first heartworm pill an hour later. Give food and water normally. If there is going to be a significant reaction, it will probably happen within two to four hours after giving the heartworm pill. Signs of a significant reaction might be panting, weakness, vomiting, or diarrhea. If your dog does any of these things or acts sick in other ways, please call us. Although extremely rare, severe reactions require IV fluid support and additional treatment with cortisone drugs."

From what I understand, the death of the microfilariae and larvae in his bloodstream is causing an allergic reaction. Causing organ tissue to swell and white blood cells to go nuts performing cleanup. This weakens his entire body leaving him in a shock-like state.

Heartworms are a very strange and deadly parasite. I read that they are currently incurable in cats, but are also rarely transmitted FROM cats. However, cats and dogs are at equal risk of contracting them from mosquitoes.

Did I mention that I hate mosquitoes? Worst haematophagous vectors ever!!!

Update: The vet says he's "doing better." They are going to keep him for observation. Hopefully he'll be much better tomorrow.