MIXED GASES IN DIVING

B.R. Wienke
Applied Theoretical Physics Division
Los Alamos National Laboratory
Los Alamos, N.M. 87545

"INTRODUCTION"
Much interest in the use of mixed breathing gases, across a spectrum of diving, has errupted in the past few years or so, mostly in mixtures of nitrogen and oxygen that differ from pure air, and especially those with higher oxygen content than air, termed enriched, which can be employed efficiently in shallow diving. Non-enriched mixtures of nitrogen/oxygen (nitrox), helium/oxygen (heliox), and helium/nitrogen/oxygen (trimix), of course, have long been employed commercially in deep and saturation diving. Recently, mixtures of hydrogen/oxygen (hydrox) have also been tested. A closer look at these inert gases in a range of diving applications is illuminating, particularly gas properties, advantages and disadvantages, and interplay.

A keynote in mixed gas diving is the oxygen partial pressure. Inspired partial pressures of oxygen must remain below 1.6 atm (52.8 fsw) to prevent toxicity, and above .16 atm (5.3 fsw) to prevent hypoxia. This window, so to speak, is confining, some 1.44 atm (47.5 fsw). Balancing diver mobility within this window at increasing depth is a delicate procedure at times.

"INERT GAS PROPERTIES"
Nitrogen is limited as an inert gas for diving. Increased pressures of nitrogen beyond 200 fsw lead to excessive euphoria, and reduced mental and physical functional ability, while beyond 600 fsw loss of consciousness results. Individual tolerances vary widely, often depending on activity. Symptoms can be marked at the beginning of a deep dive, gradually decreasing with time. Flow resistance and the onset of turbulence in the airways of the body increase with higher breathing gas pressure, considerably reducing ventilation with nitrogen-rich breathing mixtures during deep diving. Oxygen is also limited at depth for the usual toxicity reasons. Dives beyond 300 fsw requiring bottom times of hours need employ lighter, more weakly reacting, and less narcotic gases than nitrogen, and all coupled to reduced oxygen partial pressures.

A number of inert gas replacements have been tested, such as hydrogen, neon, argon, and helium, with only helium and hydrogen performing satisfactorily on all counts. Because it is the lightest, hydrogen has elimination speed advantages over helium, but, because of the high explosive risk in mixing hydrogen, helium has emerged as the best all-around inert gas for deep and saturation diving. Helium can be breathed for months without tissue damage. Argon is highly soluble and heavier than nitrogen, and thus a very poor choice. Neon is not much lighter than nitrogen, but is only slightly more soluble than helium. Of the five, helium is the least and argon the most narcotic inert gas under pressure.

Saturation and desaturation speeds of inert gases are inversely proportional to the square root of their atomic masses. Hydrogen will saturate and desaturate approximately 3.7 times faster than nitrogen, and helium will saturate and desaturate some 2.7 times faster than nitrogen. Differences between neon, argon, and nitrogen are not significant for diving. Comparative properties of hydrogen, helium, neon, nitrogen, argon, and oxygen are listed in Table 1. Solubilities, S, are quoted in milliliters of dissolved gas per milliliter of solvent, weights, A, in atomic mass units, and relative narcotic potencies,  nu , are dimensionless (referenced to nitrogen in observed effect). The least potent gases have the highest index.

Table 1. Inert Gas And Oxygen Molecular Weights, Solubilities, and Narcotic Potency.

@ H sub 2  @ He @ Ne @ N sub 2  @ Ar @ O sub 2

________@________@________@________@________@________@________

A ~~ (amu) @ 2.02 @ 4.00 @ 20.18 @ 28.02 @ 39.44 @ 32.00

________@________@________@________@________@________@________

S ~~ (ml/ml)@ @ @ @ @ @

blood @ .0149 @ .0087 @ .0093 @ .0122 @ .0260 @ .0241

oil @ .0502 @ .0150 @ .0199 @ .0670 @ .1480 @ .1220

_______@________@________@________@________@________@________

nu  @ 1.83 @ 4.26 @ 3.58 @ 1.00 @ 0.43 @

The size of bubbles formed with various inert gases depends upon the amount of gas dissolved, and hence the solubilities. Higher gas solubilities promote bigger bubbles. Thus, helium is preferable to hydrogen as a light gas, while nitrogen is perferable to argon as a heavy gas. Neon solubility roughly equals nitrogen solubility. Narcotic potency correlates with lipid (fatty tissue) solubility, with the least narcotic gases the least soluble.

"INERT GAS DIVING MIXTURES"
Different uptake and elimination speeds suggest optimal means for reducing decompression time using helium and nitrogen mixtures. Following deep dives beyond 300 fsw breathing helium, switching to nitrogen is without risk, while helium elimination is accelerated because the helium tissue-blood gradient is increased when breathing an air mixture. By gradually increasing the oxygen content after substituting nitrogen for helium, the nitrogen uptake can also be kept low. Workable combinations of gas switching depend upon the exposure and the tissue compartment controlling the ascent.

In deep saturation diving, normoxic breathing mixtures of gases are often advantageously employed to address oxygen concerns. A normoxic breathing mixture, helium or nitrogen, reduces the oxygen percentage so that the partial pressure of oxygen at the working depth is the same as at sea level, the obvious concerns, again, hypoxia and toxicity.

Critical tensions can be employed in helium saturation diving in much the same fashion as nitrogen diving. A critical tension, recall, is the maximum permissible value of inert gas tension (M-value) for a hypothetical tissue compartment with specified half-life. An approach to helium exchange in tissue compartments employs the usual nitrogen set with half-lives reduced by 2.7, that is, the helium half-lives are extracted from the nitrogen half-lives following division by 2.7, and the same critical tension is assumed for both gas compartments. Buhlmann extensively tested schedules based on just such an approach. Tissue tensions scale as the relative proportion of inert gas in any mixture. More so than in air diving, computational methods for mixed gas diving and decompression are often proprietary information in the commercial sector.

Helium (normal 80/20 mixture) nonstop time limits are shorter than nitrogen, as reported by Duffner, and follow a t sup 1/2 law similar to nitrogen, that is, depth times the square root of the nonstop time limit is approximately constant. Using standard techniques of extracting critical tensions from the nonstop time limits, fast compartment critical tensions can be assigned for applications, as detailed by Workman. Modern bubble models, such as the varying permeability model, have also been used strategically in helium diving.

Today, the three helium and nitrogen mixtures (nitrox, heliox, trimix) are commonly employed for deep and saturation diving, with a noted tendency towards usage of enriched oxygen mixtures in shallow (recreational) diving. The use of enriched oxygen mixtures by recreational divers is the subject of controversy, aptly a concern over diver safety. Breathing mixture purity, accurate assessment of component gas ratios, oxygen toxicity, and appropriate decompression procedures are valid concerns for the mixed gas diver. Care, in the use of breathing mixtures, is to be underscored. Too little, or too much, oxygen can be disastrous. The fourth hydrogen mixture (hydrox) is much less commonplace.

"NITROX"
Mixtures of oxygen and nitrogen with less oxygen than 21% (pure air) offer protection from oxygen toxicity in moderately deep and saturation diving. Moderately deep here means no more than a few hundred feet. Hypoxia is a concern with mixtures containing as much as 15% oxygen in this range. Saturation diving on oxygen-scarce nitrox mixtures is a carefully planned exposure. The narcotic effects of nitrogen in the 100 fsw to 200 fsw depth range mitigate against nitrox for deep diving.

Diving on enriched nitrox mixtures must also be carefully planned exposures, but for opposite reason, that is, oxygen toxicity. Mixtures of 30% more of oxygen significantly reduce partial pressures of nitrogen to the point of down loading tissue tensions compared to air diving. If standard air decompression procedures are employed, enriched nitrox affords a diving safety margin. However, because of elevated oxygen partial pressures, a maximum permissible depth (floor) needs be assigned to any enriched oxygen mixture. Taking 1.6 atm (52.8 fsw) as the oxygen partial pressure limit, the floor for any mixture is easily computed. Enriched nitrox with 32% oxygen is floored at a depth of 130 fsw for diving, also called the oxygen limit point. Higher enrichments raise that floor proportionately.

Decompression requirements on enriched nitrox are less stringent than air, simply because the nitrogen content is reduced below 79%. Many equivalent means to schedule enriched nitrox diving exist, based on the standard Haldane critical tension approach. Air critical tensions can be employed with exponential buildup and elimination equations tracking the (reduced) nitrogen tissue gas exchange, or equivalent air depths (always less than the actual depths on enriched nitrox) can be used with air tables. The latter procedure ultimately relates inspired nitrogen pressure on a nitrox mixture to that of air at shallower depth (equivalent air depth). For instance, a 74/26 nitrox mixture at a depth of 140 fsw has an equivalent air depth of 130 fsw for table entry. Closed breathing circuit divers have employed the equivalent air depth approach for many years.

"HELIOX"
The narcotic effects of nitrogen in the several hundred feet range prompted researchers to find a less reactive breathing gas for deeper diving. Tests, correlating narcotic effects and lipid solubility, affirm helium as the least narcotic of breathing gases, some 4 times less narcotic than nitrogen according to Bennett, and as summarized in Table 1. Deep saturation and extended habitat diving, conducted at depths of 1000 ft or more on helium/oxygen mixtures by the US Navy, ultimately ushered in the era of heliox diving. For very deep and saturation diving above 700 fsw or so, heliox remains a popular, though increasingly expensive, breathing mixture.

Helium uptake and elimination can also be tracked with the standard Haldane exponential expressions employed for nitrogen, but with a notable exception. Corresponding helium half-lives are some 2.7 times faster than nitrogen for the same hypothetical tissue compartment. Thus, at saturation, a 180  minute  helium compartment behaves like a 480  minute  nitrogen compartment. All the computational machinery in place for nitrogen diving can be ported over to helium nicely, with the 2.7 scaling of half-lives expedient in fitting most helium data.

When diving on heliox, particularly for deep and long exposures, it is advantageous to switch to nitrox on ascent to optimize decompression time, as discussed earlier. The higher the helium saturation in the slow tissue compartments, the later the change to a nitrogen breathing environment. Progressive increases of nitrogen partial pressure enhance helium washout, but also minimize nitrogen absorption in those same compartments. Similarly, progressive increases in oxygen partial pressures aid washout of all inert gases, while also addressing concerns of hypoxia.

An amusing problem in helium breathing environments is the high-pitched voice change, often requiring electronic voice encoding to facilitate diver communication. Helium is also very penetrating, often damaging vacuum tubes, gauges, and electronic components not usually affected by nitrogen. Though helium remains a choice for deep diving, some nitrogen facilitates decompression, ameliorates the voice problem, and helps to keep the diver warm.

"TRIMIX"
Diving much below 1400 fsw on heliox is not only impractical, but also marginally hazardous. High pressure nervous syndrome (HPNS) is a major problem on descent in very deep diving, and is quite complex. The addition of nitrogen to helium breathing mixtures (trimix), is beneficial in ameliorating HPNS. Trimix is a useful breathing mixture at depths ranging from 500 fsw to 2000 fsw, with nitrogen percentages usually below 10% in operational diving, because of narcotic effect.

Decompression concerns on trimix can be addressed, again, with traditional techniques. Uptake and elimination of both helium and nitrogen can be limited by critical tensions. Using a basic set of nitrogen half-lives and critical tensions, and a corresponding set of helium half-lives approximately 3 times faster for the same nitrogen compartment, total inert gas uptake and elimination can be assumed to be the sum of fractional nitrogen and helium in the trimix breathing medium, using the usual exponential expressions for each inert gas component. Such approaches to trimix decompression were tested by Workman and Buhlmann years ago, and many others after them.

"HYDROX"
Since hydrogen is the lightest of gases, it is reasonably expected to offer the lowest breathing resistance in a smooth flow system, promoting rapid transfer of oxygen and carbon dioxide within the lungs at depth. Considering solubility and diffusivity, nitrogen uptake and elimination rates in blood and tissue should be more rapid than nitrogen, and even helium. In actuality, the performance of hydrogen falls between nitrogen and helium as an inert breathing gas for diving.

Despite any potential advantages of hydrogen/oxygen breathing mixtures, users have been discouraged from experimenting with hydrox because of the explosive and flammable nature of most mixtures. Work in the early 1950s by the Bureau of Mines, however, established that oxygen percentages below the 3%-4% level provide a safety margin against explosive and flammability risks. A 97/3 mixture of hydrogen and oxygen could be utilized at depths as shallow as 200 fsw, where oxygen partial pressure equals sea-level partial pressure. Experiments with mice also indicate that the narcotic potency of hydrogen is less than nitrogen, but greater than helium. Unlike helium, hydrogen is also relatively plentiful, and inexpensive.

"POST SCRIPT"
Deep and saturation diving on mixed gases portend problems with temperature control, respiration, monitoring, compression, decompression, inert gas reactivity, and communication, to reiterate a few important operational concerns. Some of the solutions to these problems have just been discussed. But the long-term disability affecting deep sea divers, aspetic necrosis, is an insidious chronic complication. Affecting the long bones as secondary arthritis or collapse of a joint surface, lesions, detected as altered bone density upon radiography, are the suspected cause. Recent (early 1980) statistics compiled by the Royal Navy, US Navy, Medical Research Council, and other commercial operations indicated that some 8% of all divers exposed to pressures beyond 300 fsw exhibited bone damage, some 357 out of 4463. No lesions were seen in divers whose exposures were limited to 100 fsw. Some feel that high partial pressure of oxygen, for prolonged periods, is the ultimate culprit, maybe leading to fat cell enlargement in more closed regions of the bone core, a condition which reduces blood flow rate and increases local vulnerability to bubble growth. But the situation is not entirely clear.