50M Chamber Diver

By Dennis Guichard, UHMS Hyperbaric Technologist with edits by Dr Frans Cronje

There is so much to be learnt and so much fun to be had, from a 50m chamber deep dive experience that is performed in a safe, controlled manner by diving & hyperbaric specialists who have your ultimate well-being at heart, wherever in the world you might get to enjoy the experience of one.

Any chamber facility, of course, has a responsibility to plan a safe dive due to current knowledge around decompression sickness and arterial gas embolism risk. But whilst we can do everything in our power to plan a safe dive within reasonable limits, there are things outside our control that we can’t ever know about our guests.
 
To be fair, a mathematical decompression algorithm can’t simulate the complexity of the human body perfectly. John Scott Haldane believed originally that each of the tissue groups in the body absorbed and eliminated gas independently and exponentially and that bubbles didn’t form until a critical super-saturation limit was exceeded. We now know that this isn’t entirely correct; ALL dives deeper than 6 meters (in saturation) have the potential of generating asymptomatic microscopic bubbles.

So decompression theories have evolved to try and more closely predict how we think bubbles might form and how we might better model decompression physiology.

The original US Navy Dive Tables closely followed the Haldanian bubble hypothesis, although they had a known 30-40% bend rate. A newer algorithm released by the US Navy in 1984 known as the Exponential Linear model, developed by Dr Edward Thalmann, assumed an exponential in-gassing rate and more accurately included consideration of microbubble formation slows off-gassing such that the decompression rate would be linear rather than exponential.

A linear off-gassing hypothesis results in a slower ascent rate, longer decompression stops, and a slower surface off-gassing rate. These, i.e., more conservative deco profiles, were safer than previous USN algorithms. Following some still-high DCS risk rates, Dr’s Gerth and Doolette published a revised version of the VVal-18 Thalmann algorithms. Version 6 of the 2008 USN Diving Manual included new tables for these adapted approaches to minimising DCS risk. Tests by the Swedish navy demonstrated a 10% bend rate with those v6 USN Tables on a series of 40m/20-minute dives.

A newer Version 7 of the USN Tables, based on the VVal-79 algorithm, was released in 2016, which requires slightly longer deco stops. They seem to minimise DCS risk, although high VGE microbubble count is still measured post-dive. Bubble loads can take up to an hour post-dive and persist at peak for over 2-hours. It’s critical if we rely on the USN Tables for dive planning to ensure we’re always using the most current versions.

When I run my Excel spreadsheet tissue super-saturation algorithm analysis over some early chamber dives I did and plot the resultant tissue heat maps, I can see how the air deco profiles (performed in alignment to a now-outdated 1999 Version 4 of the USN dive tables) pushing a 20-minute bottom time at 50m, gave me a 70% pure-Bühlmann super-saturation gradient factor in my leading Tissue 3 or instead drove me to within 90% of my typical 85% gradient factor comfort zone limit. And what’s surprising to realise is that peak super-saturation doesn’t happen at the surface. Instead, it occurs during the ascent phase just as I approach my first deco stop.

Thanks to extensive back-and-forth discussion and dive protocol development with DAN’s leading most senior diving physician Dr Jack Meintjes, as well as with Dr Lyubisa Matity from the Gozo hyperbaric facility (to whom I am both profoundly indebted), we have been able to substantially reduce the risk of these deep dives by shortening the 50m bottom run time to just 15-minutes and also including for 100% oxygen decompression in the chamber to help with accelerated tissue off-gassing.
 
Running and analysing my tissue saturation spreadsheets for my various previous deep dives, I can clearly see why the newer tables require more extended/deeper deco stops - when the maximum tissue saturation is formed on the ascent approaching the deco stops, it makes absolute sense to stay slightly deeper to limit microscopic bubble size growth so that gas can be efficiently released. The off-gassing gradient formed by breathing 100% oxygen in the chamber via our hyperbaric hoods most efficiently helps with nitrogen off-gassing, so there is no benefit to ascending shallower when doing oxygen decompression like there is keeping a progressively shallower depth-driven off-gassing gradient high with air decompression.

espite all the safety factors designed into our dive profiles, some risk is still there with these deep dives. We don’t know (and you probably don’t) whether you might have an atrial septal defect or a PFO that triggers an atrial shunt resulting in an arterial gas embolism.

We don’t know the integrity of your blood vessel walls due to your age or due to COVID-19 damage in previous years. So it is always thus critical that a diving physician is present at any chamber dive to assess divers afterwards for any DCS-related symptoms, which can undoubtedly occur albeit rarely. 
One of the lingering concerns with 50m dives is that unless guests have deep water technical diving certification, it is unlikely that their private medical aids or DAN membership will cover them for any DCS or gas embolism that occurs when a neurological insult can have you in a hospital neuro ward for up to 10-days with specialist care and daily long Table 6 and Table 5 hyperbaric treatments, all at a phenomenal expense. So, one has to wonder why we even risk doing them in the first place and whether the same benefits and learning experiences can be enjoyed by constraining the dives to just that 40m advanced sports diver limit instead.

By analysing and adapting our dive procedures with current knowledge and skills, we can reduce the pure Bühlmann-based DCS gradient factor risk profile from 70% down to just 33% on dives. Although it is just a mathematical approximation of hypothetical bubble behaviour, it gives us a better comfort zone to buffer safety against physiological occurrences like bubble clamping and individual physiological concerns that we can’t reasonably foresee in the diving guests we get.
If you’ve ever done a 50m chamber dive, we’d love to hear about your experience and how much fun you had. If you haven’t, they’re worth doing for the knowledge you should gain from experience - go and do one!