Actually no, nitrogen asphyxiation doesn’t have the ‘panic’ that is associated with built up carbon dioxide levels. You just wake up dead.
I worked in a semi facility where a respirator line was purged with nitrogen. Through an error, the nitrogen was not removed. 1 guy died. IIRC, no panic, pain or anything. He just passed out and died.
Needless to say, the lock out/tag out procedure got audited.
The animals were on a moving conveyor which slowly moved them thru a CO2 chamber for about a three minute period until they lost consciousness and exited. At the exit point we spun a cinching chain around their back leg and an electric hoist inverted them to head facing down. They were unconscious at this point. We then slit their throats and collected the blood in a gutter where it was sent for processing. The animal never felt a thing.
Now cattle were different. Their conveyor involved a pneumatic fired spike with an electrical charge right bettween the eyes. That was more worker safe and much higher speed in the process.
The gas filled rooms involved an elaborate safety system for hazardous confined entry permits. It was my job to fix anything that broke or stopped working. Atmospheric testing and flushing needed to be monitored by a person outside the room while you wore a harness so you could be dragged out to safety if you stopped talking on the radio which had to be every 30 seconds.