maybe it's all over

Elise, the problem from my point of view is that the breathing reflex won't restart by itself after removal from a nitrogen atmosphere. There is still (generally) insufficient carbon dioxide present to restart the reflex, and so someone can die from a single deep breath of pure nitrogen. Actual death will not occur for a few minutes, but the person may collapse, unconcious, without feeling anything. Bystanders who come to their assistance may also succumb.

It'd make far more sense to have a low oxygen sensor than a high carbon dioxide sensor, as oxygen content is the more critical variable.