The ‘helium penalty’: between model hypotheses and the ethics of science

From a medical point of view, helium is of little relevance outside of diving. Accordingly, its kinetics have been insufficiently studied in comparison to other inert gases used in medical research and diagnostics, for example as tracers (1). To date, the helium algorithms used in diving medicine are based predominantly on the data set published by Bühlmann.

Foto: Karin Aggeler

Helium in the Bühlmann model

According to Bühlmann, the diffusion rate of helium is 2.65 times higher than that of nitrogen due to its lower molecular weight. In his model, the half-lives of all compartments for helium are therefore reduced by the same factor. This also results in supersaturation tolerances which, depending on the tissue type, are approximately 1.4 (fast tissues) to 2 (slow tissues) times higher than for nitrogen. On this basis, a specific data set was created with half-lives adjusted for helium and coefficients a and b for supersaturation tolerance.

For trimix diving, Bühlmann weighted the nitrogen- and helium-specific coefficients a and b according to the respective gas proportions in the breathing gas and derived the permissible supersaturation from this. However, this approach has never been experimentally validated. In addition, Bühlmann's deep experimental dives were primarily conducted with heliox and not with trimix. The total pressure of the inert gases is calculated according to Dalton's law by adding the partial pressures of the individual gases.

The origin of the ‘helium penalty’

The shorter half-lives mean that tissue saturates more quickly when breathing helium than when breathing nitrogen. Conversely, this means that nitrogen saturation occurs more slowly than with helium. It can therefore be deduced that dives with nitrogen allow for shorter decompression times and shallower decompression stops than dives with helium. In other words, divers are ‘penalised’ for using helium with longer decompression times (2). The higher supersaturation tolerance cannot compensate for the shorter half-life in terms of decompression requirements. This effect has found its way into diving slang under the term ‘helium penalty’.

Experimental findings and doubts

But is this assumption actually correct? Measurements in fast tissues show identical time constants for helium and nitrogen (1). From this, it can be deduced that nitrox, heliox and trimix could possibly be treated equally from a decompression theory perspective. This hypothesis is supported by a study that compared compressed air and heliox (21% O₂) during dives to 18 m with bottom times between 70 and 100 minutes. The incidence of decompression sickness was identical in both groups, suggesting that the decompression requirements were comparable (2).

Another study attracted particular attention and further questioned the helium penalty (3). In this study, Trimix 12/44 and Heliox 12/88 were compared during dives to 200 fsw (feet depth) with a bottom time of 40 minutes. After 50 Heliox and 46 Trimix dives, two cases of decompression sickness occurred in the Trimix group – in sharp contrast to the accepted theory, which would predict twice the DCS rate under Heliox.

These findings cast doubt on the existence of a general helium penalty and raise the question of whether helium and nitrogen should be treated equally in decompression calculations. However, caution is advised here, as this assumption has not yet been systematically verified.

Methodological and ethical limitations of the Bühlmann model

Why does this uncertainty remain despite Bühlmann's extensive work? After all, he tested his model with heliox and also used it to conduct deep dives. The reason for this is likely to be ethical in nature. The research team at the time had significantly fewer technical options at their disposal than today – for example, ultrasound diagnostics were not available. Accordingly, only clinical endpoints could be recorded, i.e. only the occurrence or absence of decompression sickness. Parameters such as bubble formation or tissue-specific elimination half-lives could not be measured.

Bühlmann's work is therefore consistently clinical research. This fact makes it all the more valuable and particularly fascinates me from today's perspective, as it was necessary to completely dispense with the collection of surrogate markers.

At the same time, however, this approach also meant that there were clear ethical boundaries that could not be crossed. This applies in particular to fast-growing and vital tissues such as the central nervous system. For example, it would have been ethically unacceptable to deliberately expose test subjects to an increased risk of serious neurological damage, including paraplegia, solely to enable a more accurate estimation of helium half-lives – for example, under the hypothesis that short helium half-lives might apply to cartilage tissue but not to the spinal cord. This ethical framework could be one of the reasons why the helium half-lives used in the model for fast tissues were set shorter than current measurements would suggest.

Classification of current practice

In summary, it can be said that although there are doubts about the common practice of assigning shorter helium half-lives to all compartments across the board, these doubts are not sufficient to align the existing decompression algorithms for helium with those for nitrogen and thus shorten decompression times. Mitchell sums this up aptly by suggesting that, up to now, we may have been ‘doing the right thing for the wrong reasons’ – i.e. it is right to decompress for a long time, but not because of the helium itself, but because of the dive profile per se (4).

Possible explanation: dependence on the dominant tissue compartment

One possible explanation for the discrepancies between the Bühlmann model and modern diving research could be as follows: With breathing gas mixtures with a high helium content, there is no disadvantage in typical technical ‘bounce dives’ because their decompression is determined by the fast tissues, for which the half-lives of helium and nitrogen do not differ. However, as the dive time increases, a point is reached where decompression is limited by slower tissues. Assuming that the half-lives determined by Bühlmann for these compartments are correct, this would mean that these tissues would need to be decompressed for longer due to their higher helium saturation – the helium penalty would thus come into play. Whether this assumption is correct is currently unclear. At least for the inner ear, the skin and possibly also for cartilage tissue, the time constants for nitrogen appear to be significantly longer than for helium (1).

Practical consequences

At present, it is strongly advised not to circumvent the helium parameters implemented in decompression algorithms – for example, by assuming a helium content of 0% in calculations for trimix dives. Until further studies provide more clarity, the helium penalty must be accepted as a given – a minor drawback given the numerous other advantages of helium as an inert gas for dives to greater depths.

1. Doolette DJ, Mitchell SJ. Hyperbaric conditions. Compr Physiol. 2011 Jan;1(1):163-201. doi: 10.1002/cphy.c091004. PMID: 23737169

2. Doolette DJ et al. Recreational technical diving Part II. Diving and Hyperbaric Medicine. 43 (2): 96-104. 2013

3. Doolette et al. DECOMPRESSION FROM He-N2-O2 (TRIMIX) BOUNCE DIVES IS NOT MORE EFFICIENT THAN FROM He-O2 (HELIOX) BOUNCE DIVES. TA 13-04 NEDU TR 15-04 May 2015

4. https://www.shearwater.com/monthly-blog-posts/eliminating-helium-penalty/