Yo-Yo Diving and Risk of Decompression Sickness in Trainee Military Divers.

Abstract

Readers are invited to submit letters for publication in this department. Submit letters online at http://joem.edmgr.com. Choose “Submit New Manuscript.” A signed copyright assignment and financial disclosure form must be submitted with the letter. Form available at http://www.joem.org under Author and Reviewer information. To the Editor: Yo-yo diving is a common practice in commercial diving (fish farm workers, harvest divers) corresponding to a series of multiple short-duration descents and ascents to the same depth with short surface intervals. This method of diving is traditionally recognized as a dangerous procedure in the medical community, mainly due to the lack of validated decompression tables to guide this type of repetitive dives.1,2 However, a study of the Scottish fish farms divers by Wilcock et al3 suggests that there was no evidence that yo-yo diving with an average number of surface excursions less than eight and a maximum depth at 16 msw for a mean time of 41 minutes produce a higher risk of decompression sickness (DCS) than that generally accepted by the diving industry. Furthermore, theoretical studies using mathematical models of decompression to analyze the potential risk of DCS development with this diving pattern in comparison with a single dive of equivalent duration and depth gave conflicting results.4,5 Recently, some researchers conducted a field study over 6 years in a cohort of 77 aquaculture divers using Doppler ultrasound to investigate decompression stress (ie, vascular bubble formation) produced from working bounce-dive series that were performed with specific dive tables based on no-decompression time limits (DCIEM dive table).6 They found that 97% of yo-yo dives profiles with a median number of seven bounces per series were low stress. In addition, none of the divers developed DCS during the study period. From the above findings, we therefore examined the characteristics and incidence of DCS in the population of military divers trained to yo-yo diving (Fig. 1) during their course of “ship diver” in the French Navy diving school.FIGURE 1: Yo-yo dive profile routinely performed by trainee military divers in French Navy diving school.All the medical records and military statutory declaration forms concerning diving parameters, demographic, and clinical characteristics of student divers treated for DCS in the recompression chamber of the French Navy diving school (Saint-Mandrier, France) between January 2003 and September 2015 were retrospectively analyzed. Inclusion criteria were the subjects who presented neurological symptoms or inner ear disorders following scuba air dives including a minimum of three bounces. Suspected cases of cerebral arterial gas embolism and inner ear barotrauma were excluded from the database. The incidence rate (IR) of DCS was calculated using the number of injuries sustained by the trainee divers as the numerator and the total number of yo-yo dives performed during the training program over the study period as the denominator. This number was accurately estimated from the number of divers registered for each course (6 per year) combined with the exact number of yo-yos dives performed by each diver to be qualified (ie, 10 series). The 13-year study period captured 23,400 person-dives with nine DCS cases that occurred after yo-yo diving, yielding an IR of 3.8 per 10,000 dives (0.038%). Detailed analysis of DCS characteristics revealed that mean age of injured divers was 27 ± 3 years for a mean depth of 16 ± 2 msw and a mean bottom time of 40 ± 20 minutes followed by mandatory decompression stops of 1-minute at 6 msw and 5-minutes at 3 msw (ascent rate 15 m/minute). Inner ear DCS was considered as the main injury (7/9, 77%) with a median delay of symptoms onset of 30 minutes [IQR, 11–90] after surfacing. All patients underwent hyperbaric oxygen treatment (US Navy 5 or GERS B table) in less than 3 hours (73 ± 45 minutes) and none of them exhibited residual deficit. Follow-up assessment of right-to-left shunt with transcranial Doppler examination revealed that 40% of divers had this condition. It is noteworthy that no DCS case was declared in the population of diving instructors using nitrox 40% as breathing gas during the same period. To conclude, yo-yo diving practiced in the French Navy diving school is a risky procedure for trainee divers if we consider the 0.01% to 0.03% overall DCS incidence range in commercial diving.3,6 Future field studies are needed for the validation of safer diving profiles in this population.