There are two female nurses going at it in this scene. The nurse with glasses seems to be the boss. She slaps the younger nurse for making some kind of mistake.

The younger nurse apologizes. When the boss nurse turns away, an anesthetic gas mask is placed over her nose and mouth!

The mask is connected to two tanks labeled "O2" and "N2O", which is Nitrous Oxide (Laughing Gas). The gas knocks the older nurse out.

When she wakes up, she's strapped to an examination bed with her legs spread out.