Redistribution of Decompression Stop Time from Shallow to Deep Stops Increases Incidence of Decompression Sickness in Air Decompression Dives
Technical rept. 1 Nov 2005-30 Apr 2011
NAVY EXPERIMENTAL DIVING UNIT PANAMA CITY FL
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Classical decompression algorithms limit hypothetical tissue gas contents and prescribe decompression schedules with most of the total stop time TST allocated to shallow decompression stops. More recent bubble-model-based algorithms limit hypothetical bubble profusion and size and prescribe decompressions with TST skewed toward deeper stops. A large man-trial compared the efficiency of these approaches. Divers wearing swimsuits and tshirts breathing surface-supplied air via MK 20 UBA, and immersed in 86 F water were compressed at 57 fswmin to 170 fsw for a 30 minute bottom time during which they performed 130 watt cycle ergometer work. They were then decompressed at 30 fswmin with stops prescribed by one of two schedules. The shallow stops schedule, with a first stop at 40 fsw and 174 minutes TST, was prescribed by the, deterministic, gas content, VVAL18 Thalmann Algorithm. The deep stops schedule, with a first stop at 70 fsw, was the optimum distribution of 174 minutes TST according to the probabilistic BVM3 bubble model. Decompression sickness DCS incidence following these schedules was compared. The trial was terminated after the midpoint interim analysis, when the DCS incidence of the deep stops dive profile 11 DCS198 dives was significantly higher than that of the shallow stops dive profile 3192, p0.030, one-sided Fisher Exact. On review, one deep stops DCS was excluded, but the result remained significant p0.047. Most DCS was mild, late onset, Type I, but two cases involved rapidly progressing CNS manifestations. Results indicate that slower tissue gas washout or continued gas uptake offsets the benefits of reduced bubble growth at deep stops.
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