Gradient Factors can be used to control for depth-time exposure and alleviate the risk of decompression sickness in recreational scuba diving

An interesting study conducted by DAN Europe and presented at the European Underwater and Baromedical Society (EUBS) 2015 meeting in Amsterdam studied risk factors of decompression sickness in recreational divers and proposed how to reduce the risk of getting bent.

The study used data recoded in DAN Europe Dive Safety Laboratory (DSL) Database and compared dives that resulted with DCS to dives that left diver’s symptom free. There were 327 DCS cases (206 males and 121 female divers) and 65,304 symptom free dives. Data included electronic dive profiles, diver data and outcome data. Researchers explored possible contribution of dive profiles, age, gender, height, weight of divers, workload during dive, dry vs. wet suit, water temperature, acute health problems and other problems during dive.

The most significant difference between dives resulting with DCS and symptom free dives was in the dive profile. DCS dives were deeper (33.8 vs. 29.1 msw) and of longer run time (50 minutes vs. 39 minutes). Women were overrepresented among divers with DCS (37%) in comparison to their representation in DSL database (17%). It is important to note that most of DSL dives were collected prospectively while most DCS dives were include retrospectively and thus do not provide for calculation of rates. The mean age appeared higher in DCS cases but the difference was too small and of no practical significance. Other tested factors like weight, height, workload, type of dive suit, water temperature, acute health problems and problems during dive did not appear different between the two groups.

The study went a step further and tried to find a practical measure of dive exposure severity that divers could use to control their risk of DCS. They estimated the gradient factor (GF) in each dive and did group comparisons. The gradient factor is the mathematically estimated ratio of the maximum tolerable supersaturation in tissues, an estimated saturation during decompression from a given dive while assuming “standard” conditions. When saturation of the tissue equals the theoretical supersaturation the GF equals 1. When saturation of the tissue exceeds maximum tolerable supersaturation, theoretically gas cannot stay in solution any longer and free gas occurs in tissues and circulation. Over time, we have learned empirically that pushing the limits (diving until the GF gets close to theoretical limit) results in increased risk of DCS and that GF should be less than 1. The empirical wisdom was confirmed with this study too, as is shown in Figure 1. The GF was greater in the DCS group (0.47 to 1.15 and mean value of 0.79) compared to the symptom free group of dives (0.21 to 1 with a mean value of 0.67). The possible practical implication is that by setting dive computer GF limits to lower levels divers may reduce their risk of DCS. Depending on personal risk tolerance, the GF setting may vary but should not be greater than 0.80.

It is important to note that in deeper diving the GF game becomes more complex and the optimal GFs vary with depth. It is beyond the scope of this blog to discuss it. For more detail read the DAN Decompression Sickness Health and Diving References Series booklet. Divers must learn in-depth theory behind the GF practice before they start to make adjustments. It is also important to warn that a GF is not measured but rather mathematically estimated based on depth, time and inert gas without taking into consideration other factors that can greatly change gas solution dynamics.

This study also shows that the effect of depth-time on outcome of decompression is overshadowing possible effects of other observed factors.  However, this was an observational field study where such factors are not really measured and so the reader should not take it as far as to conclude that workload during dives does not matter. Instead, whenever exerting oneself during a dive more than usual, a diver should cut the time at depth short. This may compensate for the increased blood flow and on-gassing caused by exercise. The diver could also extend decompression time.

The presentation of this study at EUBS was a preliminary study. More thorough analysis will be presented soon in a scientific journal.

DCI quartiles

 

Referrences

Pieri M, Cialoni D, Balestra C, Marroni A. Possible risk factor correlated with decompression sickness: Analysis of DAN Europe DSL Data Base. Presentation at 41st Annual Scientific Meeting of EUBS Amsterdam (August 19-22 2015)

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