From various discussions on various boards - can't cite actual sources as most of it was hearsay from other people's vets and my own.
They should be gassed with ISO, preferably in a chamber, until the rat has lost consciousness. My vet, and others, allow the rat to remain in the chamber for an additional 5-15min to ensure they're totally out. Then the rat is removed and the IC injection of the euth drug is given.
It is *not* acceptable to inject any drug into the stomach, as this is incredibly painful. It obviously is not acceptable to administer the heart stick while the right is still conscious as it will be incredibly painful and will stress the rat out even further.
I know some vets, for rats with severe resp issues, will administer an anesthetic IM in order to knock them out prior to the IC.
Aha! Here is the
American Veterinary Medical Association's Guidelines for Euthanasia. It's long, can get technical, and some of the practices aren't something you would do for a 'pet', but it's comprehensive and detailed.