Narcosis

[DRE]

One thing to keep in mind is the difference between Equivalent Narcotic Depth and Equivalent Air Depth. Although some people use them interchangeably, there is in fact a difference, depending upon how you consider the O2.
O2 is a metabolic gas, which means the body uses it to sustain life, whereas He and N2 are inert gasses. However, it is believed that O2 does play a role in narcosis; test subjects were asked to perform certain tasks at a certain depth with varying O2 contents, and higher O2 contents did not improve their ability, whereas He did make a difference. That's why to calculate the END both the N2 and O2 are considered in the calculation. Now, to figure out what kind of decompression benefit one would get using a nitrox mix, the EAD concept comes into play, where you would just consider the N2 and figure out what air depth would be equivalent to the depth you're using nitrox at.
In short, when using nitrox you're END is always the same as diving air, but your EAD will be shallower.