Hydreliox: the future of deep technical diving?

April 2024

Dosis sola facit venenum (the dose makes the poison) is a toxicological adage popularized by Paracelsus in the 16th century (and arguably earlier, by Mithridates VI in the 1st century BC). It means that in high-enough concentration, virtually every substance becomes toxic to the human body — including those that are otherwise vital to us, such as oxygen and drinking water.

In underwater diving, the pressure exerted by the water column increases the density of the gases delivered to the diver by his or her regulator. In fact, any form of deep diving (below 30 m) is first and foremost a challenge of dealing with the physiological responses to breathing pressurized gases. I have explored the pressure challenge involved in deep diving in another post, and thought I would here explore how familiar gases that are otherwise innocuous at the surface become problematic in various ways under hyperbaric (i.e., greater than atmospheric pressure) conditions.

Nitrogen is narcotic

The first physiological response that a diver is likely to encounter while breathing gases at depth is nitrogen narcosis, etymologically "the act of making numb". Virtually all gases are narcotic at some theoretical pressure. However, nitrogen (N₂), which makes up 78% of the standard air that we breathe, is the most proximate cause of diving narcosis, and can cause significant disorientation and mental confusion in divers. I have discussed nitrogen narcosis in both hyperbaric and normobaric conditions in another post. Nitrogen is generally considered to become inescapably narcotic from a depth of 30 m, when its partial pressure (PP) crosses 78% N₂ * [30 m / 10 m of seawater per bar + 1 bar of atmospheric pressure] = 3.12 bar PPN₂. So, how do we avoid the onset of nitrogen narcosis in diving?

Oxygen is toxic

A logical substitute for N₂ that comes to mind is oxygen (O₂), which is, after all, the only gas that is metabolically indispensable to the diver. In fact, a common recreational diving mixture is hyperoxic enriched air nitrox (EAN), or nitrox for short (nitrogen + oxygen), which is a fancy way of referring to standard air in which some of the N₂ has been replaced with additional O₂ (e.g., from 78/21 respectively in standard air to 68/32 in EAN 32).

Hyperoxic nitrox is used for another reason than mitigating narcosis, though: it is meant to decrease the loading of N₂ into the biological tissues, and thus extend the no-decompression limit (NDL) and bottom time of the dive. In fact, adding O₂ to standard air does not mitigate the effects of narcosis (Hobbs, 2014), because O₂ is even more narcotic than N₂ in hyperbaric conditions (roughly 1.7 vs. 1.0 narcotic potency, respectively) (Bennett & Rostain, 2003, p. 305), a fact that most divers are likely unaware of.

Furthermore, past a partial pressure that is fundamental to safe dive planning, oxygen becomes toxic to the central nervous system, resulting in convulsions and possible loss of consciousness. Breathing pure O₂ at the surface (i.e., at a PPO₂ of 1 bar) at the surface is fine; but at a depth of just 10 m, a regulator delivers the gas at twice its surface pressure (i.e., a PPO₂ of 2 bars), which is already beyond the commonly-accepted safety threshold for oxygen toxicity (conventionally, 1.4 bar PPO₂). Even the 21% of O₂ in standard air crosses that toxicity threshold at a mere depth of [1.4 bar PPO₂ / 21% O₂ - 1 bar of atmospheric pressure] * 10 m of seawater per bar = 57 m.

It follows that hyperoxic enriched air nitrox forces the diver to dive at shallower depths than otherwise permitted by standard air. In the example of EAN 32, the oxygen toxicity limit is reached is reached as early as [1.4 bar PPO₂ / 32% O₂  - 1 bar of atmospheric pressure] * 10 m of seawater per bar = 33.75 m.

Conversely, it also means means that an ever-increasing share of the O₂ in the mixture must be replaced with a third, non-narcotic diluent gas to dive deeper without exceeding the oxygen toxicity threshold.

Helium causes tremors

So, past the typical recreational diving depth of 40 m, we must partially substitute another gas for both N₂ and O₂ to avoid nitrogen narcosis and oxygen toxicity, respectively. Not a natural thing to do, considering that these two gases make up 99% of standard air!

In technical diving, the substitution gas of choice is helium (He). The resulting mixture of O₂ + N₂ + He takes the self-explanatory name of trimix, and the poetic name heliox when only O₂ and He are present. Helium is not only negligibly narcotic (0.045 relative potency vs. 1.0 for N₂), it also has a lower density than standard air, thus reducing the mechanical effort required to breathe.

For example, in a 50 m deep technical dive, a typical trimix mixture might be 21/40, which means that it contains 21% O₂, 40% He, and the balance (39%) of N₂. At that depth, the partial pressure of oxygen is 21% O₂ * (50 m / 10 m of seawater per bar + 1 bar of atmospheric pressure) = 1.26 bar PPO₂, which is still beneath the toxicity threshold of 1.4 bar. The partial pressure of nitrogen is 39% N₂ * [50 m / 10 m of seawater per bar + 1 bar of atmospheric pressure] = 2.34 bar PPN₂, which is narcotically equivalent to diving on standard air at [2.34 bar PPN₂ / 78% N₂ - 1 bar of atmospheric pressure] * 10 m of seawater per bar = 20 m of depth.

In principle, all N₂ in trimix can be replaced by He, resulting in a pure heliox mixture of no more than 21% O₂ and no less than 79% He. As long as the PPO₂ remains below 1.4, such mixture eliminates the risks of both oxygen toxicity and nitrogen narcosis. So, why not just dive with heliox then?

There are two reasons why heliox is not a silver bullet. The first is cost: helium is getting scarcer, and therefore more expensive, every year. The second is the High-Pressure Nervous (or Neurological) Syndrome (HPNS), which is a central nervous system disorder that appears at depths of 150 m and greater. HPNS is caused by the surrounding pressure exerted on the body's excitable membranes (the carriers of the electrical signals in living organisms) and manifests itself as tremors, cognitive impairment, and possibly loss of consciousness.

HPNS can be mitigated by slowing down the descent, which is an option in saturation diving (where commercial divers will stay for long periods of time anyway in a deep underwater habitat), but not in technical diving (where divers aim to "bounce dive" and minimize the bottom time, as staying longer at depth comes with an exponential decompression obligation).

Ironically, HPNS can also be mitigated by re-introducing a small amount of nitrogen to the mixture; the reason being that N₂ is narcotic at depth (as we saw) and thus has an anesthetic effect on the nervous system, delaying the onset of HPNS. For example, in a 2020 rebreather (closed circuit) deep dive planned to 250 m, the Wet Mules diving team considered a bottom trimix mixture of 4/91, which means that it contained 4% O₂ + 91% He + 5% N₂. At that extraordinary depth, the PPO₂ would have been 4% O₂ * (250 m / 10 m of seawater per bar + 1 bar of atmospheric pressure) = 1.04 bar, and the PPN₂ would have been 5% N₂ * (250 m / 10 m of seawater per bar + 1 bar of atmospheric pressure) = 1.3 bar, which are both well within the tolerance for oxygen toxicity and nitrogen narcosis, respectively.

In fact, the team could have added even more nitrogen and yet remain below the narcosis-inducing PPN₂ of about 3 bars. But, as the astute reader will have guessed, nothing is ever so simple in deep diving.

CO₂ causes black-outs

Nitrogen is about seven times denser than helium (1.251 vs. 0.178 kg/m³). So, re-introducing N₂ into the breathing mixture to offset the HPNS risk results in a higher gas density. Yet, at a depth of 250 m, any gas mixture is already 26 times denser than at the surface. When combined with the physical exertion of a technical dive, the physiological effort involved in breathing can become overwhelming, resulting in shallow ventilation, excessive carbon dioxide (CO₂) retention, and eventually, a hypercapnic black-out from a CO₂ burst. This phenomenon is believed to have contributed to the death of Dave Shaw in 2005 at a depth of 270 m.

In fact, at a depth of 250 m, the 4/91 trimix used by the Wet Mules team would have had a density of [4% O₂ * 1.428 kg/m³ + 91% He * 0.178 kg/m³ + 5% N₂ * 1.251 kg/m³] * (250 m / 10 m of seawater per bar + 1 bar of atmospheric pressure) = 7.3 kg/m³, which is already well above the recommended 6.0 kg/m³ density above which "there is an upward inflection in the risk of dangerous CO₂ retention" (Pollock et al., 2006, p. 73). Adding more nitrogen would only have exacerbated this risk. So, around that depth, a dilemma exists between mitigating the risks of HPNS and CO₂ retention, for which the respective solutions (increasing the nitrogen or helium concentrations) are mutually exclusive.

Hydrogen might just be the panacea

Eventually, the solution came from hydrogen (H₂), the only gas element on the periodic table that is even lighter than helium at 0.083 kg/m³. In combination with oxygen and helium, the hydrogen mixture takes the name of hydreliox, which is always fun to try to drop into a dinner party conversation. The use of H₂ in deep diving was pioneered by the French maritime engineering firm COMEX from as early as 1968 in a series of saturation dive experiments named Hydra. In 1996, Hydra X set an absolute record with COMEX diver Théo Mavrostomos reaching a simulated depth of 701 m while breathing hydreliox in an onshore hyperbaric chamber; he remains the deepest diver to this day. Of course, experimenting with hydreliox in a dry chamber is far safer than in a wet dive, as any medical emergency does not come with the added risk of drowning.

Still, the Hydra experiments usefully showed that H₂ combines two desirable properties that solve the earlier dilemma. First, it is slightly narcotic, which helps alleviate the HPNS risk the same way N₂ does. In fact, it is too narcotic to be used with O₂ alone (the mixture is then called hydrox); and thus helium is still needed as the main diluent gas. Second, it is less dense and easier to breathe than the other trimix options, and thus delays the risk of hypercapnia. Lastly, it is metabolically inert (other than for its slight narcotic property), and therefore safe to breathe. Its only downsides are that it conducts heat very well, resulting in a decrease of the diver's core temperature; and, that it is highly explosive, making it hazardous to handle (the Hindenburg disaster comes to mind).

In their 2020 rebreather dive experiment, the aforementioned Wet Mules team thus settled on a hydreliox mixture of 3/67, which is 3% O₂ + 67% He + 30% H₂. The choice of keeping O₂ to a mere 3% was dictated by the fact that H₂ only becomes flammable and explosive when mixed with 4% O₂ or greater. The resulting mixture density at the targeted depth of 300 m would have been [3% O₂ * 1.428 kg/m³ + 67% He * 0.178 kg/m³ + 30% H₂ * 0.084 kg/m³] * (300 m / 10 m of seawater per bar + 1 bar of atmospheric pressure) = 5.8 kg/m³, which is safely below the recommended 6.0 kg/m³ threshold for CO₂ retention risk.

Anecdotally, Richard "Harry" Harris, the Australian anesthesiologist notorious for co-rescuing the twelve Thai boys and their football coach from the Tham Luang Nang Non cave in 2018, was a member of the Wet Mules team (I talk about that extraordinary rescue in another post). He tested the mixture in his backyard's swimming pool, and thus determined empirically that a rebreather could be operated without the hydrogen combusting or detonating.

The Wet Mules team eventually dived successfully to 230 m using rebreathers and the 3/67 hydreliox mixture, thus proving that it was possible to safely push the limits previously imposed by HPNS and hypercapnia in deep diving. In doing so, they also paved the way for future exploration of ultra-deep diving using hydreliox.

References