The relative safety of forward and reverse diving profiles

A recent workshop found that with no-decompression dives, "reversed dive profiles" (RDP) did not increase the risk of decompression sickness (DCS). Thus in multi-level dives, the deeper part of a dive may be performed later in the dive, and repetitive dives may progress from shallow to deep. This contradicts the conventionally recommended forward dive profile (FDP) when the deeper dive, or deeper part of the dive, is performed first. The RDP Workshop recommendations were made despite the absence of adequate data. We performed two groups of experiments to test this hypothesis. We exposed two matched groups of 11 guinea pigs each to forward and reverse multi-level diving profiles to determine any substantial difference between FDPs and RDPs. There was no evidence of DCS in any of the FDP animals, while six (55%) of the RDP animals exhibited symptoms of severe DCS and died. This difference was statistically significant (P = 0.01). We then compressed two groups each of 11 guinea pigs to repetitive dives to determine any substantial difference in the risk of DCS when two equivalent sets of three dives were conducted from the deepest to most shallow on the one hand (FDP), and from the shallowest to the deepest on the other (RDP). Over two such series of dives (the second extended in time and depth to increase DCS risk), there was a significantly higher incidence of severe DCS in those animals in the RDP group. Seven of 21 exposures (33%) in the RDP group resulted in severe DCS versus none in the FDP group (P = 0.01). Our findings suggest that multi-level and repetitive dives performed in the established FDP manner are less hazardous than those performed in the reverse profile mode, at least for the exposures we chose. We believe the recommendations of the workshop should be re-examined.