Safety Ranges for Heart Rate Variability Parameters in Hyperbaric Environments

The Autonomic Nervous System (ANS) tries to maintain homeostasis in hyperbaric environments, but its activity may present large variability between subjects. The aim of this study is to establish safety ranges for ANS-related indices derived from the electrocardiographic signal (ECG) during diving and use them to identify subjects with abnormal ANS response and avoid potential diving accidents. A database with ECG recordings from 28 subjects introduced into a hyperbaric chamber was used. During immersion, five stages were studied at 1, 3 and 5 atm during descent and ascent. Indices of heart rate variability, extracted from ECG, reflecting the sympathetic and parasympathetic ANS response, were calculated and regularised with respect to their values at the initial stage at 1 atm. In particular, four time-related parameters extracted from the RR series and four frequency parameters based on the powers of the low and high frequency bands were used. High inter-subject variability in the ANS response was observed in all stages. The eight parameters were analysed for each stage and, as a result, some subjects presented highly uncommon responses with higher chances of suffering a diving accident, reflected in many parameters out of the interquartile range. This allows establishing safety ranges for ANS-related parameters that can help in the identification of subjects with potential health risk.