My dog had to use an inhaler with a paediatric spacer. I'm not sure it was very effective really because it didn't fit his muzzle and relied on him panting into it. That was a few years ago and my vet specialist wasn't aware of any inhalers specifically for dogs at the time.
My dog seems to have grown out of his lung problems. He was on so much medication he was rattling and I could see him going downhill quite rapidly, and he seemed almost depressed, so we made the decision to take him of all his medication and concentrate on quality of life rather than quantity. He's never looked back and we've managed to control his asthma attacks really well. In very hot or cold weather I only walk him on the lead and when he does run off lead (almost every day) I always allow time for him to stop panting before he goes back on the lead. This seems to be the crucial thing for us and has cut almost all of his attacks/fainting. He is seven now and his asthma rarely troubles him

Obviously it might be completely different for you and your dog but I thought you might be interested anyway.