I had sedation for my endoscopy. The doctor performing the procedure explained that they would have to pump air down into my stomach to allow them to pass the camera down and it would make me burp a lot. She also said I would probably be aware of them trying to put the camera down and my body would gag and try and stop it, but they would win (in a nice way).
I did flitter to conciousness when they were trying to get the camera down my throat, it was a very strange sensation (and would be scary if I hadn't of expected it) but I relaxed back into it so they could get it down.
They also drugged me up to my eyeballs for my colonoscopy. All for my IBS, or whatever it is, we're still investigating.