It sounds to me like they didn’t account for the oxygen in his lungs. They put a fucking mask on him and filled it with pure nitrogen, but it was probably a closed system so his own breath had nowhere to go. Most of the oxygen that goes to your lungs is not absorbed, and you have breathe in your own breath for a while before the CO2 content makes it poisonous.

That’s the wrong fucking way to do it.

CO2 poisoning is fucking brutal. Your brain recognizes that you can’t expell carbon dioxide, and your body panics. You’ll thrash about and struggle to breathe fresh air, even though you still have lots of oxygen in your lungs and in your blood stream.

The whole point of Nitrogen is that your body doesn’t realize it’s not getting Oxygen, and you slowly lose consciousness without any panic or reflex. To do that, you have to pump nitrogen into a chamber and then filter out oxygen and carbon dioxide. The chamber can be the size of a mask, but a sealed box would be less likely to leak. As you breathe, the oxygen and the carbon dioxide are continuously scrubbed from the air.

Basically, this guy was tortured to death. If you’ve ever had the sensation of being under water too long, and you have that shocking dull pain in your chest and a headache, it’s like that but for 23 minutes until you die.