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. 2017 Nov-Dec;44(6):589-600.
doi: 10.22462/11.12.2017.10.

Effects of prolonged and repeated immersions on heart rate variability and complexity in military divers

Nathaniel T Berry  1 , Laurie Wideman  1 , Christopher K Rhea  1 , Jeffrey D Labban  1 , Ki H Chon  2 , Barbara E Shykoff  3 , F J Haran  4   5 , John P Florian  3
Affiliations

Effects of prolonged and repeated immersions on heart rate variability and complexity in military divers

Nathaniel T Berry et al. Undersea Hyperb Med. 2017 Nov-Dec.

Abstract

Background: The influence of prolonged and repeated water immersions on heart rate variability (HRV) and complexity was examined in 10 U.S. Navy divers who completed six-hour resting dives on five consecutive days. Pre-dive and during-dive measures were recorded daily.

Methods: Dependent variables of interest were average heart rate (HR), time-domain measures of HRV [root mean square of successive differences of the normal RR (NN) interval (RMSSD), standard deviation of the NN interval (SDNN)], frequency-domain measures of HRV [low-frequency power spectral density (psd) (LFpsd), low-frequency normalized (LFnu), high-frequency psd (HFpsd), high-frequency normalized (HFnu), low-frequency/ high-frequency ratio (LF/HF)], and non-linear dynamics of HRV [approximate entropy (ApEn)]. A repeated-measures ANOVA was performed to examine pre-dive measure differences among baseline measures. Hierarchical linear modeling (HLM) was performed to test the effects of prolonged and repeated water immersion on the dependent variables.

Results: Pre-dive HR (P=0.005) and RMSSD (P⟨0.001) varied significantly with dive day while changes in SDNN approached significance (P=0.055). HLM indicated that HR decreased during daily dives (P=0.001), but increased across dive days (P=0.011); RMSSD increased during daily dives (P=0.018) but decreased across dive days (P⟨0.001); SDNN increased during daily dives (P⟨0.001); LF measures increased across dive days (LFpsd P⟨0.001; LFnu P⟨0.001), while HF measures decreased across dive days (HFpsd P⟨0.001; HFnu P⟨0.001); LF/HF increased across dive days (P⟨0.001); ApEn decreased during daily dives (P⟨0.02) and across dive days (P⟨0.001).

Conclusions: These data suggest that the cumulative effect of repeated dives across five days results in decreased vagal tone and a less responsive cardiovascular system.

Keywords: cardiac dynamics; complexity; heart rate variability; water immersion.

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Conflict of interest statement

The authors of this paper declare no conflicts of interest exist with this submission.