[scirpaceus]

In the case of Herbert Nitsch's accident, he went so deep that even the short bottom time was sufficient to cause DCS on the ascent with just the standard air he was carrying. N2 diffusion is a function of both time spent at depth and the partial pressure of N2 from depth itself, and in his case, time was short but depth was extraordinary. In hindsight, his attempt would have qualified for a deco stop, had he had the lung capacity to do one on the way up.

[ethbr1]

Indeed, if you're doing extreme free diving, it would make more sense to breathe a more depth-compatible (nitrogen-replaced) mix at the surface before diving, no?

Then your lungs (and blood) would be full of a safer gas mix at depth.

I'm not sure if any of the deeper gas mixes are safe to breathe at surface level though? I think so?

[nradov]

Yes, some "technical" freedivers do pre-breathe nitrox or pure oxygen at the surface before descending.

https://www.deeperblue.com/technical-freediving-are-breathho...