Does oxygen-enriched air better than normal air improve sympathovagal balance in recreational divers?An open-water study

Effects of the hyperbaric environment on the autonomic nervous system (ANS) in recreational divers are not firmly settled. Aim of this exploratory study was to (1) assess ANS changes during scuba diving via recordings of electrocardiograms (ECG) and to (2) study whether nitrox40 better improves sympathovagal balance over air. 13 experienced divers (40yrs) performed two open-water dives each breathing either air or nitrox40 (25m/39min). 3-channel ECGs were recorded using a custom-made underwater Holter-monitor. The underwater Holter system proved to be safe. Air consumption exceeded nitrox40 consumption by 12% (n = 13; p < 0.05). Both air and nitrox40 dives reduced HR (10 vs 13%; p < 0.05). The overall HRV (pNN50: 82 vs 126%; p < 0.05) and its vagal proportion (RMSSD: 33 vs 50%; p < 0.05) increased during the dive. Moreover, low (LF: 61 vs 47%) and high (HF: 71 vs 140%) frequency power were increased (all p < 0.05), decreasing the ratio of LF to HF (22 vs 34%). : Conventional open-water dives distinctly affect the ANS in experienced recreational divers, with sympathetic activation less pronounced than vagal activation thereby improving the sympathovagal balance. Nitrox40 delivered two positive results: nitrox40 consumption was lower than air consumption, and nitrox40 better improved the sympathovagal balance over air.