Background: During most of the cruise, submarines are detached from their environment. Therefore, O2 levels are relatively low (19 kPa, 144 mm Hg) and CO2 levels are high (1 kPa, 7.6 mm Hg). There are, however, periods during ventilation of the submarine in which CO2 levels drop and O2 levels increase. The objective of this study was to determine whether these unique gas changes might result in sleep-disordered breathing in submariners.
Methods and materials: The sleep of eight healthy soldiers was assessed three times: (1) control night, in submarine docking; (2) at the beginning of the cruise (reflecting acute exposure to gas changes); and (3) at the end of the cruise (chronic exposure to gas changes). Each night was divided to three parts because of different CO2 levels (secondary to ventilation of the submarine). Sleep and breathing were measured using the portable Watch PAT100 device (Itamar Medical, Ltd; Caesarea, Israel) to detect breathing abnormalities during sleep.
Results: Sleep and breathing data were categorized according to four CO2 conditions: acute moderate (inhaled CO2 levels of 2.3 to 5 mm Hg during first 1 to 2 nights of the cruise); acute high (inhaled CO2 levels of 5 to 9.2 mm Hg during the first 1 to 2 nights of the cruise); chronic moderate (inhaled CO2 levels of 2.3 to 5 mm Hg during nights 9 to 10 of the cruise); and chronic high (inhaled CO2 levels of 5 to 9.2 mm Hg during nights 9 to 10 of the cruise). Respiratory disturbance index (RDI) was significantly higher in the chronic moderate CO2 condition than the chronic high condition (18.9/h vs 8/h, p < 0.005). RDI did not correlate with CO2 levels during the first nights of the cruise (R = − 0.2, not significant), but significantly negatively correlated with it during the last nights of the cruise (R = − 0.56, p < 0.05).
Conclusions: We conclude that during an 11-day cruise, submariners adapt to high CO2 levels, as evidenced by the significant dependence of RDI on CO2 during the final but not initial days of the cruise. This adaptation resulted in a significant increase in RDI when CO2 levels declined during the later nights of the cruise.