When we lived in Chicago we went to a cat only clinic. They did a type of anesthesia where they used a large fish tank and filled it with the gas. They said it was less affecting for the cat and they could more quickly come out of it.

Haven't seen that here though.

I have also seen the hood over the head with a hose (looks for all the world like the cat passed out at a party with a lamp shade on...) That is what my sister-in-law used in her clinic.

And the old standby intravenous...