Analysing Decompression Sickness Risk

BY DENNIS GUICHARD

Few things are more exciting than the long boat ride to the far reefs north of Sodwana Bay. The sight of the seemingly endless primary dune coastline from out on the backline delights the cockles of one’s heart. There’s just something about the high-pitched drone of the boat engines and the fresh seawater spray aroma that rejuvenates the soul as nothing else can.

Named after marine geologist Dr Peter Ramsey, Ramsey’s Reef is located about 12 kilometres from Jesser Point at Sodwana Bay. It’s a reef that is rarely dived but ranks as one of the most beautiful and a personal favourite at Sodwana. Home to thousands of species of juvenile fish, mind-blowing swim-throughs, endless coral, and many larger pelagics, it’s a dive not to be missed.

Being the analytical data nerd that I am, one of my recent dive profiles gave me an ideal opportunity to showcase the level of tissue saturation and decompression stress risk profiling that is easily possible to do.

By accessing the tissue data off my Shearwater Peregrine dive computer, I can run that through an Excel-based algorithm analysis tool I’ve developed to visually model tissue saturation and desaturation in accordance with either the Bühlmann ZH-L16C or ZH-L16A algorithm.

Below you can see what my tissue loads would look like diving on either air or EANx36 The substantial benefits of both EANx32 and EANx36 are easily demonstrated with reduced saturation loads and resulting in reduced DCS risk compared to air diving.

Different tissues absorb and eliminate inert gas according to their defined half-life times The two graphs below represent the sixteen hypothetical tissue types assumed in the Bühlmann ZH-L16 based decompression algorithm These sixteen tissue saturation curves are displayed below for the two comparable gas mixtures simulated, ie, air versus EANx36.

In terms of decompression sickness risk, we are interested in knowing the peak surfacing gradient factor (GF) in the leading tissue Tissues 5 & 6 are running ‘hot’ on the air dive, and tissue four is doing the same on the reduced-nitrogen-load EANx36 dive.

The most saturated tissue is where bubbles might form and define the nature of any potential DCS injury But DCS risk is also heavily influenced by the peak desaturation gradient and the extent of time our tissues carry that excess load We refer to this as the ‘Integral Tissue Super-Saturation’ value (ISS) measured in bar minutes.

By running various ‘what-if’ scenarios through my spreadsheet, we can quickly review the impacts that different lengths of safety stop times and breathing mixes have on tissue saturation and desaturation loads (refer to Table 1) The benefit of extended safety stops is easily demonstrated.

We can also plot out the Integral Super-saturation across each of the sixteen Bühlmann tissues (refer to Table 2) to see which leading tissue presents the most significant risk for bubble formation. The heat maps visualise which tissues are running ‘hot’ and carrying the most significant desaturation loads over time.

The valuable difference between diving EANx36 over air on this dive is also seen in the total off-gassing times - only 112 minutes on EANx36 rather than 241 minutes on air from in-gassing all that extra nitrogen through the air dive An analysis spreadsheet tool of this nature is invaluable because it empowers us to evaluate the impacts of our diving behaviourisms and breathing gas choices I always do 5-minute safety stops as a part of my strategy to minimise DCS risk and post-dive fatigue.

Safety is paramount, and information is King..

Dennis Guichard is a multi-agency qualified Scuba Instructor Trainer & a DAN ‘Master Dive Pro’ member. He is a qualified Diver Medic and Saturation Life Support Technician, freelancing as a hyperbaric technologist at the Netcare St Augustine’s Hospital Hyperbaric Medicine and Wound Care Unit, Durban, South Africa.

Developed and reviewed with gratitude to Dr Lyubisa Matity at the Gozo General Hospital Hyperbaric & Tissue Viability Unit.