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Long-term Intermittent Exposure to High Ambient CO₂ Causes Respiratory Disturbances During Sleep in Submariners^*

^* From the Israeli Naval Medical Department (Dr. Margel), Haifa, Israel; Division of Sleep Medicine (Dr. White), Brigham and Women’s Hospital, Harvard Medical School, Boston, MA; and the Sleep Laboratory (Dr. Pillar), Rambam Medical Center and Faculty of Medicine, Technion, Haifa, Israel.

Correspondence to: Giora Pillar, MD, PhD, Sleep Laboratory, Rambam Medical Center, Haifa, Israel 31096; e-mail gpillar{at}tx.technion.ac.il

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.

Key Words: apneic threshold • CO₂ • respiratory control mechanism • sleep • submarine medicine