Heavy Gases

In school we were learning about heavy gases and the dangers they pose to people who work with or around them. For a project I have to basically list a lot of jobs when you would encounter heavy gases, since I love cars I decided to concentrate on Nitrous Oxide or N20.

My question is if someone had a lung capacity of say 6 litres and they breathed in 3 litres of N20 would they be able to then breathe in a further 3 litres of air and thus continue breathing or would the N20 prevent any more air entering their lungs so that they suffocate?