The Decompression Application Risk Assessment (DARA) project began to gain a better understanding of the methods and techniques used by technical divers. In 2007, we conducted a survey to evaluate the concerns of technical divers facilitate better communication of those concerns to the diving medical community prior to the Divers Alert Network sponsored Technical Diving Conference.(1, 2) Forty-three percent of the respondents listed decompression theory as an area where they would like to see research focused.(1)
Probabilistic decompression models are designed to calculate the risk (or probability) of decompression sickness (DCS) occurring on a given decompression profile. The probabilistic models can be used to optimize decompression under different constrains, one of which is to minimizing total stop time for a desired risk or probability of DCS. The model does this while minimizing the total decompression time. This process can also work in reverse allowing one to calculate the probability of DCS for any decompression schedule. It is this aspect of probabilistic decompression models we use to evaluate risk in various conditions.
The models we use are three compartment decompression models that are parameterized to decompression data that includes the time of occurrence of decompression sickness. USN93 (3) and BVM3 (4) are fit to the same dataset, while NMRI98 (5) was fit to an expanded data set including dives with high fractions of oxygen. USN93 and NMRI98 have risks as a function of compartment gas content, while BVM3′s risks are a function of bubble volume.
Phase I – Decompression Software Profile Risk (Nitrox):
Our first evaluation was a risk assessment for many of the various decompression calculation software packages available on the market.(6) A goal in performing this analysis is to demonstrate for novice technical divers the generally accepted trend that estimated risks of decompression sickness increase with bottom time for common decompression schedules.(7)
We produced decompression tables utilizing six different models for 32% nitrox dives and a single 100% oxygen decompression gas with a final stop depth of 6 meters (20 fsw). Default settings for each of the four software programs were used in the table calculations.
The lowest risk is not for the shallowest dives because this analysis starts a decade of time prior to the no-stop limit and continues to a bottom time limited by the divers gas supply.
There is a high degree of confidence that the actual risks of the profiles for all the models increase as their bottom times increase because of the agreement of the three probabilistic models from two different classes of models. At most, there is a difference of 50% in the estimated risks for these products which does not present a clear advantage of one product over another for nitrox diving.
Risk modification is necessary when comparing to dives performed by technical divers. The models used are calibrated to data with moderate work during the bottom time (1.5 – 2.0 L/min VO2), and divers wearing wetsuits in cold water. Work levels are done by technical divers are lower and include swimming and use of Diver Propulsion Vehicles and should therefore reduce the inert gas volume seen with moderate work.(8) Decompression tests show required decompression can be dramatically reduced based on diver thermal status.(9) While the absolute values of estimated risk from the probabilistic models may not apply directly to technical diving, the relative values are of use for comparing schedules.
This risk analysis was performed on square dive profiles with a direct descent to depth, maximum stay at the maximum depth followed by a direct ascent to the first stop depth. These parameters are used to plan adequate decompression for the maximum possible dive within these confines. It should be recognized that for most recreational technical dives, majority of the dive profile will not be spent at the maximum depth. This deviation from a square profile reduces the estimated risk when dove with the planned decompression schedule.
We can suggest that with the estimated risk being similar, other product features can be used to select which software to use (although this trial was nitrox only).
Phase II – Optimal Decompression Depth and Breathing Gas:
Our second phase of this project was to evaluate the risk of various decompression strategies utilized by US Navy and Technical divers. We evaluated the use of oxygen versus 50% nitrox(10) as well as the selection of a final decompression stop depth(11).
Divers commonly use oxygen or 50% Nitrox for accelerated decompression. 50% Nitrox as a single decompression gas has been proposed to be iso-risk with pure oxygen profiles where using 50% results in an overall shorter decompression time. We used validated probabilistic models to estimate the risk of DCS for planned decompression dives using oxygen or 50% Nitrox as a single decompression gas. Methods derived from earlier analysis were again used for these calculations however, tables were generated tables using only common two algorithms: Buhlmann with Gradient Factors and the Variable Permeability Model.
In this analysis, we have determined that decompression using oxygen decreases the probability of DCS compared to decompressing with 50% nitrox, irrespective of overall decompression time.(10)
Oxygen is commonly utilized for accelerated decompression with the last stop at 20fsw to take advantage of the increased partial pressure gradient of retained nitrogen. The relative risk of this practice, compared to traditional Navy profiles with a final 10 foot decompression stop, is unknown.
The probability of DCS on these profiles was not reduced by planning the last oxygen decompression stop at 20fsw instead of 10fsw.(11)
Phase III – Decompression Software Profile Risk (Trimix):
Our third evaluation is a risk assessment for many of the various decompression calculation software packages with trimix dive profiles. Profiles are currently being formatted for final risk assessment.
1. Hobbs, GW; Armstrong, BM; Armstrong, HC; Schreiber, JS; Kaylor, ZM; Vann, RD. What can the medical community do for technical divers? Undersea and Hyperbaric Medical Society Annual Scientific Meeting, Kapalua Maui, Hawaii, USA. Undersea and Hyperbar Med 2007; 34(4)
2. Vann RD, Mitchell SJ, Denoble PJ, Anthony TG, eds. Technical Diving Conference Proceedings. Durham, NC: Divers Alert Network; 2009; 394 pages. ISBN# 978-1-930536-53-1
3. Thalmann ED, Parker EC, Survanshi SS, Weathersby PK. Improved probabilistic decompression model risk predictions using linear-exponential kinetics. Undersea Hyperb Med. 1997 Winter;24(4):255-74.
4. Gerth WA, Vann RD. Probabilistic gas and bubble dynamics models of decompression sickness occurrence in air and nitrogen-oxygen diving. Undersea Hyperb Med. 1997 Winter;24(4):275-92.
5. Parker EC, Survanshi SS, Massell PB, Weathersby PK. Probabilistic models of the role of oxygen in human decompression sickness. J Appl Physiol. 1998 Mar;84(3):1096-102.
6. Hobbs GW, Gault KA. Decompression Risk Evaluation of Commercially Available Desktop Decompression Software Algorithms. Undersea and Hyperbaric Medical Society Annual Scientific Meeting, Las Vegas, Nevada, USA. Undersea and Hyperbar Med 2009; 36(4): 321
7. Thalmann ED, Kelleher PC, Survanshi SS, Parker EC, Weathersby PK. 1999. Statistically Based Decompression Tables XI: Manned Validation of the LE Probabilistic Model for Air and Nitrogen-Oxygen Diving. Navy Experimental Diving Unit Panama City Fla. Technical Report 01-99.
8. Dick AP, Vann RD, Mebane GY, Feezor MD. Decompression induced nitrogen elimination. Undersea Biomed Res. 1984 Dec;11(4):369-80.
9. Gerth WA, Ruterbusch VL, Long ET. 2007. The Influence of Thermal Exposure on Diver Susceptibility to Decompression Sickness. Navy Experimental Diving Unit Panama City Fla. Technical Report 06-07.
10. Walker JR, Hobbs GW, Gault KA, Howle LE, Freiberger JJ. Decompression risk analysis comparing oxygen and 50% nitrox decompression stops. Undersea and Hyperbaric Medical Society Annual Scientific Meeting, St Pete Beach, Florida, USA. Undersea and Hyperbar Med 2010; 37(4)
11. Walker JR, Hobbs GW, Gault KA, Howle LE, Freiberger JJ. The effect of final oxygen decompression stop depth on DCS risk: 20 fsw vs. 10 fsw. Undersea and Hyperbaric Medical Society Annual Scientific Meeting, St Pete Beach, Florida, USA. Undersea and Hyperbar Med 2010; 37(4)
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