Williams, Prior and Bryson report in the Wilderness and Environmental Medicine Journal direct evidence that one dive can cause dehydration, a problem long thought to increase the risk for decompression sickness.

OBJECTIVE: Direct evidence that dehydration results from scuba diving is scanty. Increased hematocrit (Ht) is a commonly used proxy measure for dehydration. This study sought evidence that an increase in Ht occurs over the course of a scuba dive in tropical conditions. As a secondary outcome, evidence was sought that the degree of Ht increase is correlated to pressure exposure. METHODS: Twenty male and 21 female scuba divers were recruited at a remote tropical dive site. Water temperature was 30 degrees C (+/- 1 degrees C). Each diver gave venous blood relating to 1 dive only. Mean maximum dive depth was 13.6 m (+/- 3.7 m [SD]) and mean duration 39.5 minutes (+/- 4.5 minutes [SD]) using air as the breathing gas. Blood was taken at a mean of 12.4 minutes (+/- 3.5 minutes [SD]) before diving and a mean of 16.2 minutes (+/- 3.7 minutes [SD]) after diving. After centrifugation of microcapillaries, Ht was estimated on a visual plate reader. RESULTS: A paired Wilcoxon test showed evidence (P < .001) for a change in Ht. The mean difference between predive and postdive measurements was 0.0073 (95% confidence interval: 0.0104-0.0042), equating to a mean relative Ht increase of 1.78%. Similar results were found for the sexes individually. A correlation between maximum depth of dive and Ht increase was statistically significant, although the correlation itself was weak (P = .049, Spearman's r = .326). CONCLUSIONS: There is evidence of a statistically significant increase in Ht over the course of a single warm-water scuba dive. This increase is small and is within the range of error associated with the techniques of Ht estimation employed in this study. Depth exposure was found to correlate with Ht increase. In view of the small magnitude of change in the Ht, there is no reason to amend protocols for fluid resuscitation of recreational scuba divers suspected to have experienced decompression injury in tropical locations.