It can be done if you try hard enough.
Say you're at 40 m with air diluent, thats a PO2 of 1.05. Now you turn off the O2 and breathe it down.
When the PO2 gets to 0.7 you're depleting the oxygen, and oh my gosh you're increasing the fraction of nitrogen. To how much?
Ptotal = 5 ata = Po2 + Pn2
Po2 = 0.7
Pn2 = 5 - 0.7 = 4.3
END = 10 * (4.3 / 0.79 - 1) = 44 m
So that feels like 44 m
at 0.4 it feels like 48 m
at 0.21 it feels like 51 m
Thats if you did let it go down like that. btw thats just thinking N2 is the only narcotic.