Heart rate variability changes as an indicator of decompression-related physiological stress

Many aspects of the physiological stress related to the exposure to the hyperbaric environment have been studied, but no research has been made to evaluate the impacts of scuba diving on heart rate variability (HRV). We investigated the effects of a simulated dive to 557 KPa (45 meters of salt water) for a 30-minute bottom time on the frequency and time domains estimators of HRV. Electrocardiogram records were obtained with superficial electrodes for 30 minutes before the simulated dive and, subsequently, for one hour after the dive. Each of these time-series was then subdivided into non-overlapping windows of 256 consecutive R-R intervals. A control group was submitted to the same protocol, breathing the same gases used in the simulated dive, while not being exposed to the hyperbaric environment. In the control group we observed a significant increase in SDNN (the square root of the variance of the R-R intervals), RMSSD (the square root of the mean squared differences of successive R-R intervals), and in two bands (high and low) of the power spectrum of frequencies. The subjects in the simulated dive presented only an increase in the low-frequency estimator without any further relevant changes in other estimators of HRV. This study suggests that the low-frequency increase without concomitant high-frequency increase might be an indicator of the physiological stress caused by decompression and that such a dissimilarity in responses might be correlated to the dive-related impairment of the endothelial function.