The autonomic system responds not to lack of oxygen, but to a rise in carbon dioxide levels. (More specifically I think it has something to do with pH, which de

The autonomic system responds not to lack of oxygen, but to a rise in carbon dioxide levels. (More specifically I think it has something to do with pH, which decreases as carbon dioxide increases.) So as long as you can expire carbon dioxide, you'll pass out without any sensation of asphyxiation or struggle. You still need to inhale to exhale, though, and the inhaled air needs to have a sufficiently low partial pressure of carbon dioxide.