Contrary to popular belief, CO2 is not the main breathing stimulus. It is true that under normal conditions (i.e. atmospheric pressure, atmospheric air), CO2 dominates the breathing reflex, but under unusual conditions (i.e. hyperbaric and/or pure oxygen), O2 dominates. If the O2 level is huge, breathing reflex is suppressed regardless of CO2 levels. When PaCO2 > 90mmHg, CO2 narcosis sets in, after which you have about 90 seconds before you black-out from CO2 narcolepsy/toxicity.
In the presence of a small amount of CO2, low O2 is the main breathing reflex.
So, the breathing reflex = F( CO2, O2 ), which forms a nice 2D surface.
So, during a pure O2 breath-hold, unless you hyperventilate, the breath-hold will end when you black-out from CO2 narcosis.
I tried a pure O2 breath-hold once in Florida. After 4 minutes I just stopped because I got bored.
Eric Fattah
BC, Canada
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