Nitrogen asphyxia. Instead of a poisonous gas, the gas mask delivered pure nitrogen. One feels panic due to accumulating carbon dioxide. Since the mask delivers pure nitrogen gas to replace exhaled air, there is no buildup of carbon dioxide. Therefore the brain does not perceive being asphyiated.
About forty years ago I took a hit of 100% Nitrogen, things got very dark, very fast. That’s what makes nitrous oxide so dangerous, in its pure form it displaces oxygen. Fifty years ago in anesthesia it was possible for your patient to show up in recovery room perfectly pink and completely dead. Whether it’s pure nitrogen or a hypobaric chamber like they used to use for dogs, it’s very humane.