Vegan

for up to a minute

I'm not too well versed in animal physiology, but for humans it's closer to 3-4 minutes. And panicking doesn't influence the speed or terror much.

Drowning in water and suffocating with CO2 are the worst feelings of dread in existance. Animal or human, period.

And panicking won't speed it up, nor will doing medidation slow it down much since the body literally fights for its life and the adrenalin makes you very much awake and hyperalert during pretty much the entire process.

I think, it was by using nitrogen

You think right.

When panicking for lack of oxygen, the nervous system (mamallian and human alike) doesn't actually look for the lack of oxygen for some reason, but for the overabundance of certain "common displacers". Such as water and CO2. And that's pretty much the exhaustive list.

Nitrogen doesn't cause a panic response. Neither does CO (carbon monoxide).

You don't feel anything if you're suffocating in nitrogen or are poisoned by CO for a few minutes. Then you might feel a bit drowsy for some 10 seconds right before you lose consciousness.

Then you don't feel anything again. A few more minutes, and you're gone without even knowing.