Pete,
Well, simply tested. Try a static one day without the sub-neutral breathing. Just relax and don't even think about your breathing, so that you end up breathing as you would if you were listening to music sitting at home (or whatever your interest). If you find your static experience to be the same, then I stand corrected about you still ventilating CO2. However, I don't think you will find that to be the case.
Let me know.
