Exhaled nitric oxide concentration and decompression-induced bubble formation: An index of decompression severity in humans?

• We evaluated the effect of diving-induced bubble formation and hyperoxia on exhaled NO measurement.
• We examined different levels of bubble formation between Air, Trimix and Oxygen dives.
• We highlighted an important decrease in FE NO after the deep Trimix dive.
• The decrease in FE NO after the Trimix dive was correlated with the level of bubble formation.
• We postulate that exhaled NO concentrations could be a useful index of decompression severity in healthy human divers.

IntroductionPrevious studies have highlighted a decreased exhaled nitric oxide concentration (FE NO) in divers after hyperbaric exposure in a dry chamber or following a wet dive. The underlying mechanisms of this decrease remain however unknown. The aim of this study was to quantify the separate effects of submersion, hyperbaric hyperoxia exposure and decompression-induced bubble formation on FE NO after a wet dive.MethodsHealthy experienced divers (n = 31) were assigned to either (i) a group making a scuba-air dive (Air dive), (ii) a group with a shallow oxygen dive protocol (Oxygen dive) or (iii) a group making a deep dive breathing a trimix gas mixture (deep-dive). Bubble signals were graded with the KISS score. Before and after each dive FE NO values were measured using a hand-held electrochemical analyzer.ResultsThere was no change in post-dive values of FE NO values (expressed in ppb = parts per billion) in the Air dive group (15.1 ± 3.6 ppb vs. 14.3 ± 4.7 ppb, n = 9, p = 0.32). There was a significant decrease in post-dive values of FE NO in the Oxygen dive group (15.6 ± 6 ppb vs. 11.7 ± 4.7 ppb, n = 9, p = 0.009). There was an even more pronounced decrease in the deep dive group (16.4 ± 6.6 ppb vs. 9.4 ± 3.5 ppb, n = 13, p < 0.001) and a significant correlation between KISS bubble score >0 (n = 13) and percentage decrease in post-dive FE NO values (r = −0.53, p = 0.03).DiscussionSubmersion and hyperbaric hyperoxia exposure cannot account entirely for these results suggesting the possibility that, in combination, one effect magnifies the other. A main finding of the present study is a significant relationship between reduction in exhaled NO concentration and dive-induced bubble formation. We postulate that exhaled NO concentration could be a useful index of decompression severity in healthy human divers.