Instantaneous beat-to-beat variability reflects vagal tone during hyperbaric hyperoxia

Hyperbaric hyperoxia affects heart rate variability (HRV) by increasing parasympathetic activity. The purpose of this study was to evaluate the applicability of instantaneous beat-to-beat variability (SD1 of Poincaré plot analysis) in detecting changes in vagal tone and to evaluate possible changes in the fractality of heart rate dynamics (alpha1 of detrended fluctuation analysis) during hyperbaric hyperoxia. Continuous three-lead ECG recordings were taken in ten divers who were treated at 2.5 ATA with air (PO2 47 kPa) and oxygen (PO2 235 kPa) for 60 min. Power spectral analysis, Poincaré plot analysis and alpha1 were analyzed before compression, after 30 min and after 55 min at 2.5 ATA. Correlations between the variables were calculated after 55 min exposure. SD1 and high frequency (HF) power increased significantly but alpha1 decreased during hyperbaric hyperoxia (PO2 235 kPa). HF power and SD1 also correlated significantly. However, HF power and SD1 correlated inversely with alpha1. During hyperbaric hyperoxia, SD1 reflects vagal activity and can be used instead of HF power, if stationary conditions cannot be achieved. The decreasing alpha1 indicates more random heart rate dynamics during hyperbaric hyperoxia.