Effect of Urinary Sheath Use on Hydration Status of Fighter Pilots Under Severe Thermal Stress: An Observational Study.

Introduction: Fighter jet pilots may adopt a voluntary hypohydration strategy hours before take-off to avoid urinating in flight. This may favor sortie-induced dehydration and potentially increase flying errors. Since 2015, French pilots have used a urinary sheath (US) that appears to have resolved this problem. However, its effect has never been assessed.

Methods: Urine specific gravity (USG) before and after sorties, inflight intake (water and food) and loss (sweat and urine), and changes in bodyweight were measured during 44 long (~370 minutes) sorties (22 in 2015 without a US, No-US group, and 22 in 2017 with a US, US group).

Results: The USG before sorties was lower in the US than No-US group (1.019 ± 0.008 vs 1.028 ± 0.006, p = 0.008) and the proportion of hypohydrated pilots (>1.020) was lower in the US than No-US group (29 vs 90%, p = 0.007). Total loss and intake were higher in the US group (1,793 ± 640 and 927 ± 585 g, respectively) than in the No-US group (1,337 ± 382 and 359 ± 191 g, respectively; p < 0.006). Changes in bodyweight, both absolute (-977 ± 367 g for the No-US and -866 ± 593 g for the US group) and relative (1.16 ± 0.51% for the No-US and 1.13 ± 0.77% for the US group) during sorties were not altered by wearing a US.

Conclusion: The use of a US successfully mitigates preflight dehydration, as reflected by decreases in USG, showing that pilots, at least partially, abandon preflight voluntary hypohydration strategies. It also favored water and food intake during flight without enhancing inflight dehydration, shown by the parallel increases in loss (urine and sweat) when wearing a US.