The influence of oxygen on the narcotic potency of the breathing mixture
Last updated: Author: Piotr Orlewicz
How to properly determine the relative narcosis of breathing mixture? The different sources are not consistent in this field, especially with regard to consideration of the oxygen influence. So what is the correct answer to this question? I invite you to discuss.
A few facts:
- Taking as reference the impact of nitrogen (ratio equal to 1), the relative narcotic potency is respectively: 0.23 for helium, for oxygen 1.7. These values confirm the Meyer-Overton theory, according to which the potency can be determined on the basis of the gas lipophilicity.
- Oxygen is the only metabolized. According to some sources, should therefore not be taken it into account when calculating END, according to others, the ratio should be corrected to the value of 1. Higher oxygen indices are recommended by groups associated with the cave diving.
- Regardless of the theory which is more true, for nitrox diving it does not matter. Assume for the moment that the influence of oxygen is the same as nitrogen and we have calculated END using a value 0. Is it practical impact? In the case of nitrox it does not matter. Narcotic potency of such a mixture is not greater than for air, and the maximum depth is less, because MOD specifies a higher fraction of oxygen.
- In the case of Trimix, the answer is not so simple.
In the case of normoxic mixtures, the narcotic ratio of trimix will never be higher than for air, and MOD is the same, so in practice it does not matter.
In the case of a hyperoxic mixture, maximum diving depth decreases, also the narcotic potency decreases, so again the method of calculating the END does not cause any problems in practice.
The trouble starts in the case of hypoxic mixtures (not only trimix), because the maximum diving depth determined by the oxygen fraction increases and with it the risk of narcosis.
- There is some confusion with simplified formulas for calculating END. But that we'll talk more widely on another occasion.
- What with the metabolism ?
The metabolism of oxygen is possible to made by the hemoglobin in our blood. Composition, division of hemoglobin, its subtypes and states are very extensive and it is a complicated topic. For our considerations just some basic information about it:
- oxygen is transported by hemoglobin, precisely by heme contained in it,
- the maximum amount of oxygen that can be bind to hemoglobin at any given time is finite,
- oxygen saturation of hemoglobin increases with partial pressure and is not linear,
- oxygen saturation of hemoglobin reaches a level close to 100% even at about 0.10-0.13 bar of oxygen partial pressure,
- under normal conditions, only about 1.5-2% of oxygen is dissolved in the plasma, the remainder is transported by hemoglobin,
- under normal condition, 100mls of blood can bind about 20ml of oxygen,
- a significant part of carbon dioxide passes quickly from the hemoglobin into the plasma,
- at rest state the blood circulates in the body at approximately speed of 5l/min.
- The mechanism of the narcosis effect of gases is not fully recognized. For today there is also no clear explanations and there is no scientifically proven data on the real influence of oxygen on narcosis potency of mixtures used in scuba diving. I found, that currently one of research centers invites to participate in a research study investigating the narcotic effects of oxygen at depth.
Due to the volume of this article (already too long; -]), I touched on only a few basic aspects. It seems that at low pressure of respirable mixtures the oxygen narcotic potency can be omitted. But how much is oxygen dissolved in the plasma at the deeper or longer dives? Do tiredness affect the participation of oxygen in the END. Is the impact of oxygen on narcotic effect linear? How does it affect the real narcotic potency of breathing gas?
Feel free to discuss. Do you take oxygen into account when calculating the narcotic potency? Maybe you find newer research or recommendations in this regard.