Tissue-specific gradient factors to address undeserved decompression sickness.

BACKGROUND: "Undeserved hit" is a term used to define a case of decompression sickness (DCS) when the diver has adhered to the limits set by a dive computer. In a recent study, Divers Alert Network (DAN) Europe has analyzed over 39,000 dives with 295 undeserved hits. These 295 dives, and an additional 22 that were borderline or above accepted safe limits of decompression, were processed to determine the inert gas load at the end of the dive, expressed in terms of gradient factors (GF) in a ZH-L16C framework. The data show that DCS was observed with GF values as low as 70, but mostly involved the intermediate tissues (27-minute half-time and higher).

MATERIAL AND METHODS: We propose a variation of the Bühlmann algorithm to incorporate this evidence in a way that does not penalize a recreational diver when not needed.

RESULTS: Our simulations show that by applying tissue-specific gradient factors (TSGF), the algorithm can automatically become more conservative as the severity of the dive profile increases and as the effect of repetitive dives becomes more pronounced.

CONCLUSIONS: Tissue-specific gradient factors can be used to increase the conservatism of the ZH-L16C algorithm for extreme exposures and repetitive dives, without being too limiting for the recreational diver.