Heart rate variability findings in neurological decompression sickness: an exploratory case series

Background Neurological decompression sickness (NDCS) is a potentially severe complication of diving, but its effects on autonomic cardiovascular regulation remain poorly understood. Heart rate variability (HRV) has emerged as a noninvasive method for assessing autonomic nervous system dynamics in both physiological and pathological conditions. Objective To describe multidomain HRV patterns in divers presenting with NDCS prior to hyperbaric oxygen therapy (HBOT). Methods This exploratory case series included eight divers diagnosed with NDCS at a tertiary hyperbaric medicine center. Continuous electrocardiographic monitoring was performed before HBOT, and HRV analysis was conducted using a standardized offline processing pipeline. Time-domain (SDNN, RMSSD, pNN50), frequency-domain (HF power, LF/HF ratio), and nonlinear metrics (Shannon entropy and two-dimensional Poincaré analysis) were evaluated. HRV parameters were analyzed both as absolute values and as Z-score normalized values relative to published reference populations. Results Conventional HRV indices demonstrated marked interindividual variability across cases. Some patients exhibited elevated global variability and vagally associated indices, whereas others showed reduced or near-reference values. Entropy-based measures also varied across the cohort, with near-reference values in some cases and lower values in others, particularly Cases 7 and 8. These findings suggest heterogeneous alterations in cardiac rhythm dynamics rather than a uniform HRV phenotype. Conclusions NDCS may be associated with measurable but heterogeneous disturbances in autonomic cardiovascular regulation detectable through HRV analysis. Entropy-based measures may provide exploratory information on NN interval distributional irregularity, but these findings should be interpreted cautiously because of the small sample size, retrospective design, spontaneous respiration, and methodological sensitivity of entropy estimates. Larger prospective studies are needed to determine whether HRV can contribute to physiological characterization or monitoring of decompression illness.