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.

Background: Heart rate variability (HRV) monitoring during real-world closed-circuit rebreather (CCR) diving remains methodologically challenging. This exploratory case report describes phase-related HRV variations obtained from a wearable device during a military CCR dive. Methods: A trained male military diver underwent continuous monitoring with a RootiRx^® single-lead wearable device placed in a custom waterproof enclosure. Three consecutive 20 min segments were analyzed: pre-dive, CCR dive, and immediate post-dive. The post-dive segment was not acquired under standardized resting conditions, and the participant continued to wear the device throughout this period. RR interval data were analyzed using Kubios HRV Scientific 4.3.0 with medium-threshold artifact correction, smoothness-prior detrending (λ = 500), and 4 Hz interpolation. Time-domain, frequency-domain, and nonlinear HRV indices were evaluated descriptively. Results: Mean heart rate was 88.1 beats/min pre-dive, 102.8 beats/min during the dive, and 86.4 beats/min post-dive. SDNN values were 35.3, 29.9, and 49.2 ms, while RMSSD values were 29.7, 24.6, and 40.3 ms, respectively. The PNS index was −1.44, −2.08, and −1.10, and the SNS index was 1.66, 2.98, and 1.26 across the three phases. LF/HF values were 1.77, 1.46, and 3.92, respectively. Automatically corrected beats represented approximately 0.34%, 0.54%, and 0.46% of beats, with effective analyzable data proportions exceeding 99.7% in all three segments. Conclusions: An analyzable RR interval series was obtained across the three operational phases in this individual recording. However, the single-subject uncontrolled design, non-standardized recording conditions, multiple confounding factors, and absence of independent underwater validation prevent causal attribution of the observed differences to immersion, CCR exposure, or autonomic adaptation. Further controlled studies using validated underwater ECG systems are required.