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Equivalent narcotic depth
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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.
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.
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. In contrast, other authorities and agencies consider oxygen to be non-narcotic, and group it with helium and other potential non-narcotic components, or less narcotic, and group it with gases like hydrogen, which has a narcotic effect estimated at 55% of nitrogen based on lipid solubility.
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.
Although oxygen has greater lipid solubility than nitrogen and therefore should be more narcotic according to the Meyer-Overton correlation, it is likely that some of the oxygen is metabolised, thus reducing its effect to a level similar to that of nitrogen or less.
There are also known exceptions to the Meyer-Overton correlation. Some gases that should be very narcotic based on their high solubility in oil, are much less narcotic than predicted. Anesthetic research has shown that for a gas to be narcotic, its molecule must bind to receptors on the neurons, and some molecules have a shape that is not conducive to such binding. It is unknown if and how oxygen binds to neuronal receptors, so the measurable fact that oxygen is more oil-soluble than nitrogen, does not necessarily mean it is more narcotic than nitrogen.
Since there is some evidence that oxygen plays a part in the narcotic effects of a gas mixture, some organisations prefer assuming that it is narcotic to the previous method of considering only the nitrogen component as narcotic, since this assumption is more conservative, and the NOAA diving manual recommends treating oxygen and nitrogen as equally narcotic as a way to simplify calculations, given that no measured value is available.
The situation is further complicated by the effects of inert gas narcosis being significantly variable between divers using the same gas mixture, and between occasions for the same diver on the same gas and dive profile.
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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.
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.
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. In contrast, other authorities and agencies consider oxygen to be non-narcotic, and group it with helium and other potential non-narcotic components, or less narcotic, and group it with gases like hydrogen, which has a narcotic effect estimated at 55% of nitrogen based on lipid solubility.
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.
Although oxygen has greater lipid solubility than nitrogen and therefore should be more narcotic according to the Meyer-Overton correlation, it is likely that some of the oxygen is metabolised, thus reducing its effect to a level similar to that of nitrogen or less.
There are also known exceptions to the Meyer-Overton correlation. Some gases that should be very narcotic based on their high solubility in oil, are much less narcotic than predicted. Anesthetic research has shown that for a gas to be narcotic, its molecule must bind to receptors on the neurons, and some molecules have a shape that is not conducive to such binding. It is unknown if and how oxygen binds to neuronal receptors, so the measurable fact that oxygen is more oil-soluble than nitrogen, does not necessarily mean it is more narcotic than nitrogen.
Since there is some evidence that oxygen plays a part in the narcotic effects of a gas mixture, some organisations prefer assuming that it is narcotic to the previous method of considering only the nitrogen component as narcotic, since this assumption is more conservative, and the NOAA diving manual recommends treating oxygen and nitrogen as equally narcotic as a way to simplify calculations, given that no measured value is available.
The situation is further complicated by the effects of inert gas narcosis being significantly variable between divers using the same gas mixture, and between occasions for the same diver on the same gas and dive profile.