Equivalent narcotic depth

Equivalent narcotic depth (END) (historically also equivalent nitrogen depth) is used in technical diving as a way of estimating the narcotic effect of a breathing gas mixture, such as nitrox, heliox or trimix. The method is used, for a given breathing gas mix and dive depth, to calculate the equivalent depth which would produce about the same narcotic effect when breathing air.^[1]

The equivalent narcotic depth of a breathing gas mix at a particular depth is calculated by finding the depth at which breathing air would have the same total partial pressure of narcotic components as the breathing gas in question.^[1]

Since air is composed of approximately 21% oxygen and 79% nitrogen, it makes a difference whether oxygen is considered narcotic, and how narcotic it is considered relative to nitrogen. If oxygen is considered to be equally narcotic to nitrogen, the narcotic gases make up 100% of the mix, or equivalently the fraction of the total gases which are narcotic is 1.0. Oxygen is assumed equivalent in narcotic effect to nitrogen for this purpose by some authorities and certification agencies.^[2] In contrast, other authorities and agencies consider oxygen to be non-narcotic, and group it with helium and other potential non-narcotic components,^[3] or less narcotic, and group it with gases like hydrogen, which has a narcotic effect estimated at 55% of nitrogen based on lipid solubility.^[4]

Research continues into the nature and mechanism of inert gas narcosis, and for objective methods of measurement for comparison of the severity at different depths and different gas compositions.^[3]