This documentary probably had something to do with it: https://vimeo.com/83750163

In it, a man investigates various means of execution, and concludes that nitrogen asphyxiation is the most 'humane' because the victim just passes out and then perishes. The presenter isn't necessarily wrong, but the caveats include a) the victim must breathe pure nitrogen, any oxygen present will prolong the experience; and b) if the victim is aware they are being executed, it is unlikely to be peaceful regardless.

Edit to add: Also, the physical discomfort from holding one's breath comes from increased concentration of CO2 in the blood, not the absence of oxygen. So as long as one can expel CO2 as usual, the lack of oxygen does not cause physical distress.