Looking around the Net, I have to think that your barkeep is trying to knock you out.
Other sites report that Guinness uses Nitrogen, but not Nitrous Oxide (Laughing Gas). Laughing Gas is an anesthetic which produces unconsciousness.
But...canned whipped cream in the U.S.A. Does use Nitrous Oxide. I've seen it listed on the label. I'm assuming that the fat content or sugar content protects the consumer from experiencing a blackout, but I don't know for sure.
Nitrous Oxide? - Realbeer.com Beer Community