Heart Rate Variability During and After a Closed-Circuit Rebreather Dive: A Case Study in Autonomic Regulation

Background/Objectives: Heart rate variability (HRV) analysis is a valuable noninvasive tool to evaluate autonomic nervous system activity, especially under environmental stress. Diving, which combines immersion, hyperbaric exposure, and altered respiratory gas dynamics, presents a unique physiological challenge. This single-subject case study aimed to assess autonomic modulation during and after a closed-circuit rebreather (CCR) dive using comprehensive HRV analysis. Methods: A healthy male diver underwent continuous RR interval monitoring before, during, and after a CCR dive using the MCM100 military device. HRV parameters were evaluated across time-domain, frequency-domain, and nonlinear domains. Data processing was performed using the Kubios software, and statistical comparison between phases were carried out using paired-sample tests in SPSS. Results: During the dive, HRV indices showed a significant decrease in RMSSD (p < 0.01), SDNN (p < 0.05), and SD1 (p < 0.01) compared to the baseline, indicating marked parasympathetic withdrawal. A concurrent increase in the LF/HF ratio (p < 0.05) suggested sympathetic predominance. In the post-dive phase, parasympathetic indices such as RMSSD and SD1 improved relative to the dive phase (p < 0.05) but remained below baseline. Nonlinear measures, including SD2/SD1 ratio and sample entropy, also varied significantly across phases. Conclusions: This case study demonstrates significant autonomic shift associated with a CCR dive, characterized by acute parasympathetic withdrawal and a partial recovery post-dive. HRV profiling may offer a sensitive, noninvasive approach for monitoring stress and recovery dynamics in professional divers.