Equivalent narcotic depth

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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]

Contents

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]

Oxygen narcosis

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. [3]

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. [3]

Since there is some evidence that oxygen plays a part in the narcotic effects of a gas mixture, [5] 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. [1]

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.

Objective testing has failed to demonstrate oxygen narcosis, and research continues. There has been difficulty in identifying a reliable method of objectively measuring gas narcosis, but quantitative electroencephalography (EEG) has produced interesting results. [3] [6] Quantification of the more subtle effects of inert gas narcosis is difficult. Psychometric tests can be variable and affected by learning effects, and participant motivation. In principle, objective neurophysiological measurements like quantitative electroencephalogram (qEEG) analysis and the critical flicker fusion frequency (CFFF) could be used to get objective measurements. [3] [7]

Some studies have shown a decrease in CFFF during air-breathing dives at 4 bar (30 msw), but have not detected a change with partial pressure of pure oxygen within the breathable range. The results with CFFF for nitrogen do not scale well with partial pressure at greater depths. [3] [7]

Hyperbaric inert gas narcosis is associated with depressed brain activity when measured with an EEG. A functional connectivity metric based on the so-called mutual information analysis has been developed, and summarized using the global efficiency network measure. This method has successfully differentiated between breathing air at the surface and air at 50 m, and even showed an effect at 18 m on air, but did not show a difference associated with pressure for heliox exposures. The lack of change with heliox suggests that the effect of hyperbaric nitrogen is measured, and not a direct pressure effect. [3]

The EEG functional connectivity metric did not change while breathing hyperbaric oxygen within the safe range for testing, which indicates that oxygen does not produce the same changes in brain electrical activity associated with high partial pressures of nitrogen, which suggests that oxygen is not narcotic in the same way as nitrogen. [3]

Carbon dioxide narcosis

Although carbon dioxide (CO2) is known to be more narcotic than nitrogen – a rise in end-tidal alveolar partial pressure of CO2 of 10 millimetres of mercury (13 mbar) caused an impairment of both mental and psychomotor functions of approximately 10% – [5] [2] the effects of carbon dioxide retention are not considered in these calculations, as the concentration of CO2 in the supplied breathing gas is normally low, and the alveolar concentration is mostly affected by diver exertion and ventilation issues, and indirectly by work of breathing due to equipment and gas density effects. [8] [9]

The driving mechanism of CO2 narcosis in divers is acute hypercapnia. The potential causes can be split into four groups: insufficient ventilation, excessive dead space, increased metabolic carbon dioxide production, [10] and high carbon dioxide content of the breathing gas, usually only a problem with rebreathers. Insufficient ventilation can be a consequence of high work of breathing. [8]

Other components of the breathing gas mixture

It is generally accepted as of 2023, that helium has no known narcotic effect at any depth at which gas can be breathed, and can be disregarded as a contributor to inert gas narcosis. Other gases which may be considered include hydrogen and neon.

Standards

The standards recommended by the recreational certification agencies are basically arbitrary, as the actual effects of breathing gas narcosis are poorly understood, and the effects quite variable between individual divers. Some standards are more conservative than others, and in almost all cases it is the responsibility of the individual diver to make the choice and accept the consequence of their decision, except during training programs where standards can be enforced if the agency chooses to do so. One agency, GUE, prescribes the gas mixtures their members are allowed to use, but even that requirement and membership of the organisation is ultimately the choice of the diver. [11] Professional divers may be legally obliged to comply with the codes of practice under which they work, and contractually obliged to follow the requirements of the operations manual of their employer, in terms of occupational health and safety legislation.

Some training agencies, such as CMAS, GUE, and PADI and include oxygen as equivalent to nitrogen in their equivalent narcotic depth (END) calculations. PSAI considers oxygen narcotic but less so than nitrogen. Others like BSAC, IANTD, NAUI and TDI do not consider oxygen narcotic. [3] [11]

Calculations

In diving calculations it is assumed unless otherwise stipulated that the atmospheric pressure is 1 bar or 1 atm. and that the diving medium is water. The ambient pressure at depth is the sum of the hydrostatic pressure due to depth and the atmospheric pressure on the surface. Some early (1978) experimental results suggest that, at raised partial pressures, nitrogen, oxygen and carcon dioxide have narcotic properties, and that the mechanism of CO2 narcosis differs fundamentally from that of N2 and O2 narcosis, [5] and more recent work suggests a significant difference between N2 an O2 mechanisms. [6] Other components of breathing gases for diving may include hydrogen, neon, and argon, all of which are known or thought to be narcotic to some extent. The formula can be extended to include these gases if desired. The argon normally found in air at about 1% by volume is assumed to be present in the nitrogen component in the same ratio to nitrogen as in air, which simplifies calculation.

Since in the absence of conclusive evidence, oxygen may or may not be considered narcotic, there are two ways to calculate END depending on which opinion is followed.

Oxygen considered narcotic

Since for these calculations oxygen is usually assumed to be equally narcotic to nitrogen, the ratio considered is of the sum of nitrogen and oxygen in the breathing gas and in air, where air is approximated as entirely consisting of narcotic gas. In this system all nitrox mixtures are assumed to be narcotically indistinguishable from air. The other common calculation assumes that oxygen is not narcotic and is multiplied by a relative narcotic value of 0 on both sides of the equation.

Metres

The partial pressure in bar, of a component gas in a mixture at a particular depth in metres is given by:

fraction of gas × (depth/10 + 1)

So the equivalent narcotic depth can be calculated as follows:

partial pressure of narcotic gases in air at END = partial pressure of narcotic gases in trimix at a given depth.

or

(fraction of O2 x (relative narcotic strength) + fraction of N2 x 1) in air × (END/10 + 1) = (fraction of O2 x (relative narcotic strength) + fraction of N2 x 1) in trimix × (depth/10 +1)

which gives for oxygen deemed equal in narcotic strength to nitrogen:

1.0 × (END/10 + 1) = (fraction of O2 + fraction of N2) in trimix × (depth/10 +1)

resulting in:

END = (depth + 10) × (fraction of O2 + fraction of N2) in trimix − 10

Since (fraction of O2 + fraction of N2) in a trimix = (1 − fraction of helium), the following formula is equivalent:

END = (depth + 10) × (1 − fraction of helium) − 10

Working the earlier example, for a gas mix containing 40% helium being used at 60 metres, the END is:

END = (60 + 10) × (1 − 0.4) − 10
END = 70 × 0.6 − 10
END = 42 − 10
END = 32 metres

So at 60 metres on this mix, the diver would feel approximately the same narcotic effect as a dive on air to 32 metres.

Feet

The partial pressure of a gas in a mixture at a particular depth in feet is given by:

fraction of gas × (depth/33 + 1)

So the equivalent narcotic depth can be calculated as follows:

partial pressure of narcotic gases in air at END = partial pressure of narcotic gases in trimix at a given depth.

or

(fraction of O2 + fraction of N2) in air × (END/33 + 1) = (fraction of O2 + fraction of N2) in trimix × (depth/33 +1)

which gives:

1.0 × (END/33 + 1) = (fraction of O2 + fraction of N2) in trimix × (depth/33 +1)

resulting in:

END = (depth + 33) × (fraction of O2 + fraction of N2) in trimix − 33

Since (fraction of O2 + fraction of N2) in a trimix = (1 − fraction of helium), the following formula is equivalent:

END = (depth + 33) × (1 − fraction of helium) − 33 [2]

As an example, for a gas mix containing 40% helium being used at 200 feet, the END is:

END = (200 + 33) × (1 − 0.4) − 33
END = 233 × 0.6 − 33
END = 140 − 33
END = 107 feet

So at 200 feet on this mix, the diver would feel the same narcotic effect as a dive on air to 107 feet.

Oxygen not considered equally narcotic to nitrogen

The ratio of nitrogen between the gas mixture and air is considered. Oxygen may be factored in at a narcotic ratio chosen by the user, or assumed to be negligible. In this system nitrox mixtures are not considered equivalent to air.

Related Research Articles

Nitrox refers to any gas mixture composed of nitrogen and oxygen. This includes atmospheric air, which is approximately 78% nitrogen, 21% oxygen, and 1% other gases, primarily argon. In the usual application, underwater diving, nitrox is normally distinguished from air and handled differently. The most common use of nitrox mixtures containing oxygen in higher proportions than atmospheric air is in scuba diving, where the reduced partial pressure of nitrogen is advantageous in reducing nitrogen uptake in the body's tissues, thereby extending the practicable underwater dive time by reducing the decompression requirement, or reducing the risk of decompression sickness.

<span class="mw-page-title-main">Nitrogen narcosis</span> Reversible narcotic effects of respiratory nitrogen at elevated partial pressures

Narcosis while diving is a reversible alteration in consciousness that occurs while diving at depth. It is caused by the anesthetic effect of certain gases at high partial pressure. The Greek word νάρκωσις (narkōsis), "the act of making numb", is derived from νάρκη (narkē), "numbness, torpor", a term used by Homer and Hippocrates. Narcosis produces a state similar to drunkenness, or nitrous oxide inhalation. It can occur during shallow dives, but does not usually become noticeable at depths less than 30 metres (98 ft).

<span class="mw-page-title-main">Trimix (breathing gas)</span> Breathing gas consisting of oxygen, helium and nitrogen

Trimix is a breathing gas consisting of oxygen, helium and nitrogen and is used in deep commercial diving, during the deep phase of dives carried out using technical diving techniques, and in advanced recreational diving.

Heliox is a breathing gas mixture of helium (He) and oxygen (O2). It is used as a medical treatment for patients with difficulty breathing because this mixture generates less resistance than atmospheric air when passing through the airways of the lungs, and thus requires less effort by a patient to breathe in and out of the lungs. It is also used as a breathing gas diluent for deep ambient pressure diving as it is not narcotic at high pressure, and for its low work of breathing.

<span class="mw-page-title-main">Partial pressure</span> Pressure of a component gas in a mixture

In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas mixture is the sum of the partial pressures of the gases in the mixture.

<span class="mw-page-title-main">Technical diving</span> Extended scope recreational diving

Technical diving is scuba diving that exceeds the agency-specified limits of recreational diving for non-professional purposes. Technical diving may expose the diver to hazards beyond those normally associated with recreational diving, and to a greater risk of serious injury or death. Risk may be reduced via appropriate skills, knowledge, and experience. Risk can also be managed by using suitable equipment and procedures. The skills may be developed through specialized training and experience. The equipment involves breathing gases other than air or standard nitrox mixtures, and multiple gas sources.

<span class="mw-page-title-main">Breathing gas</span> Gas used for human respiration

A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common and only natural breathing gas, but other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed habitats. Oxygen is the essential component for any breathing gas. Breathing gases for hyperbaric use have been developed to improve on the performance of ordinary air by reducing the risk of decompression sickness, reducing the duration of decompression, reducing nitrogen narcosis or allowing safer deep diving.

<span class="mw-page-title-main">Gas blending for scuba diving</span> Mixing and filling cylinders with breathing gases for use when scuba diving

Gas blending for scuba diving is the filling of diving cylinders with non-air breathing gases such as nitrox, trimix and heliox. Use of these gases is generally intended to improve overall safety of the planned dive, by reducing the risk of decompression sickness and/or nitrogen narcosis, and may improve ease of breathing.

High-pressure nervous syndrome is a neurological and physiological diving disorder which can result when a diver descends below about 500 feet (150 m) using a breathing gas containing helium. The effects experienced, and the severity of those effects, depend on the rate of descent, the depth and the percentage of helium.

Hydreliox is an exotic breathing gas mixture of hydrogen, helium, and oxygen. For the Hydra VIII mission at 50 atmospheres of ambient pressure, the mixture used was 49% hydrogen, 50.2% helium, and 0.8% oxygen.

Argox is the informal name for a scuba diving breathing gas consisting of argon and oxygen. Occasionally the term argonox has been used to mean the same mix. The blend may consist of varying fractions of argon and oxygen, depending on its intended use. The mixture is made with the same gas blending techniques used to make nitrox, except that for argox, the argon is added to the initial pure oxygen partial-fill, instead of air.

In physiology, isobaric counterdiffusion (ICD) is the diffusion of different gases into and out of tissues while under a constant ambient pressure, after a change of gas composition, and the physiological effects of this phenomenon. The term inert gas counterdiffusion is sometimes used as a synonym, but can also be applied to situations where the ambient pressure changes. It has relevance in mixed gas diving and anesthesiology.

Hydrox, a gas mixture of hydrogen and oxygen, is occasionally used as an experimental breathing gas in very deep diving. It allows divers to descend several hundred metres. Hydrox has been used experimentally in surface supplied, saturation, and scuba diving, both on open circuit and with closed circuit rebreathers.

Hydrogen narcosis is the psychotropic state induced by breathing hydrogen at high pressures. Hydrogen narcosis produces symptoms such as hallucinations, disorientation, and confusion, which are similar to hallucinogenic drugs. It can be experienced by deep-sea divers who dive to 300 m (1,000 ft) below sea level breathing hydrogen mixtures. However, hydrogen has far less narcotic effect than nitrogen and is very rarely used in diving. In tests of the effect of hydrogen narcosis, where divers dived to 500 m (1,600 ft) with a hydrogen–helium–oxygen (hydreliox) mixture containing 49% hydrogen, it was found that while the narcotic effect of hydrogen was detectable, the neurological symptoms of high-pressure nervous syndrome were only moderate.

<span class="mw-page-title-main">Scuba gas planning</span> Estimation of breathing gas mixtures and quantities required for a planned dive profile

Scuba gas planning is the aspect of dive planning and of gas management which deals with the calculation or estimation of the amounts and mixtures of gases to be used for a planned dive. It may assume that the dive profile, including decompression, is known, but the process may be iterative, involving changes to the dive profile as a consequence of the gas requirement calculation, or changes to the gas mixtures chosen. Use of calculated reserves based on planned dive profile and estimated gas consumption rates rather than an arbitrary pressure is sometimes referred to as rock bottom gas management. The purpose of gas planning is to ensure that for all reasonably foreseeable contingencies, the divers of a team have sufficient breathing gas to safely return to a place where more breathing gas is available. In almost all cases this will be the surface.

<span class="mw-page-title-main">Scuba gas management</span> Logistical aspects of scuba breathing gas

Scuba gas management is the aspect of scuba diving which includes the gas planning, blending, filling, analysing, marking, storage, and transportation of gas cylinders for a dive, the monitoring and switching of breathing gases during a dive, efficient and correct use of the gas, and the provision of emergency gas to another member of the dive team. The primary aim is to ensure that everyone has enough to breathe of a gas suitable for the current depth at all times, and is aware of the gas mixture in use and its effect on decompression obligations, nitrogen narcosis, and oxygen toxicity risk. Some of these functions may be delegated to others, such as the filling of cylinders, or transportation to the dive site, but others are the direct responsibility of the diver using the gas.

Gas blending is the process of mixing gases for a specific purpose where the composition of the resulting mixture is specified and controlled. A wide range of applications include scientific and industrial processes, food production and storage and breathing gases.

<span class="mw-page-title-main">Physiology of decompression</span> The physiological basis for decompression theory and practice

The physiology of decompression is the aspect of physiology which is affected by exposure to large changes in ambient pressure. It involves a complex interaction of gas solubility, partial pressures and concentration gradients, diffusion, bulk transport and bubble mechanics in living tissues. Gas is breathed at ambient pressure, and some of this gas dissolves into the blood and other fluids. Inert gas continues to be taken up until the gas dissolved in the tissues is in a state of equilibrium with the gas in the lungs, or the ambient pressure is reduced until the inert gases dissolved in the tissues are at a higher concentration than the equilibrium state, and start diffusing out again.

Human physiology of underwater diving is the physiological influences of the underwater environment on the human diver, and adaptations to operating underwater, both during breath-hold dives and while breathing at ambient pressure from a suitable breathing gas supply. It, therefore, includes the range of physiological effects generally limited to human ambient pressure divers either freediving or using underwater breathing apparatus. Several factors influence the diver, including immersion, exposure to the water, the limitations of breath-hold endurance, variations in ambient pressure, the effects of breathing gases at raised ambient pressure, effects caused by the use of breathing apparatus, and sensory impairment. All of these may affect diver performance and safety.

References

  1. 1 2 3 "Mixed-Gas & Oxygen". NOAA Diving Manual, Diving for Science and Technology. 4th. National Oceanic and Atmospheric Administration. 2002. [16.3.1.2.4] ... since oxygen has some narcotic properties, it is appropriate to include the oxygen in the END calculation when using trimixes (Lambersten et al. 1977,1978). The non-helium portion (i.e., the sum of the oxygen and the nitrogen) is to be regarded as having the same narcotic potency as an equivalent partial pressure of nitrogen in air, regardless of the proportions of oxygen and nitrogen.
  2. 1 2 3 Fogerty, Reilly (6 September 2019). "Calculated Confusion: Can O2 Get You High?". InDepth. Retrieved 3 April 2023.
  3. 1 2 3 4 5 6 7 8 9 10 Vrijdag, Xavier (1 February 2023). "Is Oxygen Narcosis A Thing?". gue.com. Retrieved 30 March 2023.
  4. ↑ Menduno, Michael (14 May 2020). "Measuring Inert Gas Narcosis". alertdiver.eu. DAN Europe. Retrieved 4 April 2023.
  5. 1 2 3 Hesser, C.M.; Fagraeus, L; Adolfson, J. (December 1978). "Roles of nitrogen, oxygen, and carbon dioxide in compressed-air narcosis". Undersea Biomed Res. 5 (4): 391–400. PMID   734806.
  6. 1 2 Vrijdag, Xavier C. E.; van Waart, Hanna; Sames, Chris; Mitchell, Simon J.; Sleigh, Jamie W. (20 July 2022). "Does hyperbaric oxygen cause narcosis or hyperexcitability? A quantitative EEG analysis". Physiological Reports. 10 (14): e15386. doi:10.14814/phy2.15386. PMC   9300958 . PMID   35859332.
  7. 1 2 Vrijdag, X.C.; van Waart, H.; Sleigh, J.W.; Balestra, C.; Mitchell, S.J. (20 December 2020). "Investigating critical flicker fusion frequency for monitoring gas narcosis in divers". Diving Hyperb Med. 50 (4): 377–385. doi:10.28920/dhm50.4.377-385. PMC   7872789 . PMID   33325019.
  8. 1 2 Mitchell, S. J.; Cronjé, F. J.; Meintjes, W. A. J.; Britz, H. C. (February 2007). "Fatal respiratory failure during a "technical" rebreather dive at extreme pressure". Aviat Space Environ Med. 78 (2): 81–6. PMID   17310877 . Retrieved 2008-06-15.
  9. ↑ Anthony, Gavin; Mitchell, Simon J. (2016). Pollock, N.W.; Sellers, S.H.; Godfrey, JM (eds.). Respiratory Physiology of Rebreather Diving (PDF). Rebreathers and Scientific Diving. Proceedings of NPS/NOAA/DAN/AAUS June 16–19, 2015 Workshop. Wrigley Marine Science Center, Catalina Island, CA. pp. 66–79.
  10. ↑ Drechsler, M.; Morris, J. (January 2023). "Carbon Dioxide Narcosis". StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. PMID   31869084.
  11. 1 2 Menduno, Michael (20 May 2020). "Rapture of the Tech: Depth, Narcosis and Training Agencies". alertdiver.eu. DAN Europe. Retrieved 4 April 2023.
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