True, your breathing is mainly the body's response to an excess of CO2, not so much a need of Oxygen. Sure Oxygen is needed but CO2 levels increasing is what can cause people to inhale water, or inducing panic. An animal that's suffocates on pure nitrogen will have a much more pleasant death than one that suffocates in a CO2 rich environment. Nitrogen suffocation is considered by some to be a much more humane way of euthanasia of rodents for food use or to be dissected.