Heart rate variability at rest during repeated 6‐hour water immersions

Abstract

Short‐duration water immersion (WI) alters heart rate variability (HRV), but the responses to long‐duration and repeated (WI) in comfortably warm water are unknown. Ten healthy young men were exposed to a 6‐hr resting WI breathing air (1.35 ATA) on 5 consecutive days. Heart rate (HR), SDNN and RMSSD (statistical measures of NN intervals), ApEn (approximate entropy), LFPSD and HFPSD (low and high frequency components of power spectral density), LFnPSD and HFnPSD (normalized units from PSD), and cardiac sympathetic and parasympathetic nervous system (SNS and PNS) dynamics using principal dynamic mode (PDM) analysis were measured. Data were compared to pre‐dive baseline HRV data and post‐dive recovery data obtained from the subjects. Compared to baseline, LFPSD increased on dive days 4 (p < 0.0001) and 5 (p=0.0005); HFPSD increased on dive days 2 (p=0.0066) and 3(p=0.0004); SNS increased on dive day 2 (p=0.0086); SDNN increased during dives 2–5 (p < 0.0001); RMSSD decreased during dive days 2 and 3(p < 0.0001). Significant and consistent decreases in ApEn measures compared with baseline were observed across dive days 1–5 (p < 0.0001). The results suggest that 1) the complexity of HR responses (ApEn) is reduced during single and repeated WI, and 2) time‐and frequency‐domain measures of HRV are altered with repeated WI.Support: ONR and NAVSEA DSBDP