It's a drug called "acepromazine." It's not the anesthesia itself, but a type of PRE-anesthetic agent that is supposed to help make the actual anesthesia easier on the dog. But in Boxers and certain other breeds, it can cause a fatal, or near-fatal drop in blood pressure. There are other drugs that can be substituted. Do a search on here for "Ace" or "Acepromazine" and you will find more detailed information.
Genie