Autonomic and physiological stress responses in navy divers: the protective role of diving experience

Abstract

Introduction: Deep diving presents significant physiological stress, yet reliable indicators for monitoring autonomic and stress responses remain underdeveloped. This study examined how prior deep diving experience influences autonomic regulation and stress biomarkers during a simulated dive to a depth of 220 feet.

Methods: Twenty-eight Navy divers (15 experienced, 13 novice) underwent psychological assessments (perceived stress, anxiety, fear) and salivary biomarker analysis (cortisol, amylase) pre- and post-dive. Heart rate variability (HRV), including linear (rMSSD, HF) and non-linear indices (SD1, SD1/SD2, sample entropy), was measured at four dive stages (pre-dive, bottom, decompression, and post-dive).

Results: After adjusting for age and perceived stress, experienced divers exhibited greater post-dive reductions in cortisol (p = 0.015) and amylase (p = 0.022). Additionally, after controlling for age, perceived stress, and respiratory rate, they also demonstrated significantly higher parasympathetic activity (p ≤ 0.001) and greater HRV complexity (sample entropy, p = 0.023) during decompression. No significant differences were found in self-reported psychological stress.

Discussion: Diving experience facilitates enhanced autonomic control and stress adaptation. These findings support the use of real-time physiological monitoring and experience-based training protocols to mitigate risk during deep dives.

Keywords: amylase activity; autonomic nervous system; deep diving; heart rate variability; psychological distress; salivary cortisol.

FIGURE 1

Diving profile. Modified from U.S. Navy Diving Manual, Revision 7 (2016). Divers descended to a maximum depth of 220 feet with a bottom time of 5 min. Including ascent and decompression stops, the total dive duration was 90 min. T1 to T4 represents four designated time points for 5-min HRV recordings: T1 (pre-dive), T2 (bottom), T3 (decompression), and T4 (post-dive).

FIGURE 2

Group means with 95% confidence intervals for HRV indices across diving stages. Results were derived from generalized estimating equation (GEE) models adjusted by age, pre-dive perceived stress (PSS), and respiratory rate. Group-by-diving stages interactions were significant for several HRV parameters (see Table 4). (A) Mean heart rate, (B) SDNN, (C) rMSSD, (D) natural log-transformed LF power (LF In), (E) natural log-transformed HF power (HF In), (F) SD1, (G) SD1/SD2, and (H) SampEn. SDNN, standard deviation of normal-to-normal intervals; rMSSD, the root mean square of successive differences; LF, low frequency of heart rate variability; HF, high frequency of heart rate variability; ln, natural logarithm; SD1, Standard deviation 1 is the standard deviation of the distances of each data point from the line of identity (y = x) in the Poincaré plot; SD2, Standard Deviation 2 is standard deviation along the line of identity in the Poincaré plot; SD1/SD2 ratio, the ratio of short to long-term variability; SampEn, Sample entropy.