Navy Experimental Diving Unit Technical Report
Cognitive effects of inspired doses of CO2 during submerged working dives have previously not been explored. Three experiments using male volunteer Navy divers in the NEDU test pool under 12 feet of fresh water explored: (1) dose-related and on/off effects of 1.5% (Phase 1a, N=20) and 3% (Phase 1b, N=16) inspired CO2; (2) questions of whether switching to gas free of CO2 results in further changes in performance or restoration to baseline (Phase 2, N=34); and (3) differences in the effects of CO2 in air vice in O2 (Phase 3, N=16). End tidal CO2 was collected from all divers and correlated with cognitive performance. The Automated Neuropsychological Assessment Metrics, version 4 (ANAM4), was used before and four times during each dive — with three intermittent periods of mild or moderate exercise during each dive — to measure nine cognitive domains. No dose-related effect of CO2 was found. Basic cognitive domains of simple reaction time, visual scanning, visuo-spatial processing, and learning were unaffected, while fatigue and the higher cognitive functions of short-term memory (STM), long-term memory (LTM), working memory (WM), math processing, and sustained attention produced perplexing results. Most consistent of all differences was the decrease in LTM while divers were on CO2, a decrease that persisted in Phase 1 even after divers were removed from CO2 and returned to O2. Math processing, WM, and sustained attention increased among divers both during and after breathing CO2. STM decreased on CO2 in Phase 1 but not in Phase 2. No cognitive changes were detected on air, when end tidal CO2 remained closer to normal than on O2. While some participants reported mild to moderate symptoms (e.g., headache, shortness of breath, irritability, and lack of concentration), end tidal CO2 levels were mostly <7% Surface Equivalent Value (SEV). Because subjects were not hypercapnic, we cannot address the question of the study. Further investigation of the effects of inspired CO2 on higher cognitive domains — along with consideration of breathing resistance — is recommended.
Selkirk, A., Briggs, J. F., & Shykoff, B. (2010). Cognitive effects of hypercapnia on immersed working divers (NEDU TR 10-15). Panama City, FL: Navy Experimental Diving Unit.