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 (SDI of Poincare plot analysis) in detecting changes in vagal tone and to evaluate possible changes in the fractality of heart rate dynamics (alpha(1) 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, Poincare plot analysis and a, 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. SDI and high frequency (HF) power increased significantly but alpha(1) decreased during hyperbaric hyperoxia (PO2 235 kPa). HF power and SDI also correlated significantly. However, HF power and SDI correlated inversely with alpha(1). During hyperbaric hyperoxia, SDI reflects vagal activity and can be used instead of HF power, if stationary conditions cannot be achieved. The decreasing alpha(1) indicates more random heart rate dynamics during hyperbaric hyperoxia.