The purpose of this study was to investigate the effects of inhaling different percentages of oxygen (02) after maximal exercise on heart rate variability (HRV). Eight active college males (age, 19.9 +/- 1.5 years; height, 177.8 +/- 6.3 cm; weight, 76.2 +/- 12.7 kg) volunteered to participate in this study. Each subject inhaled normobaric 21% O-2 (NO), 12% O-2 (LO), and 60% O-2 (HO) for 40 minutes in random balanced order after exhaustive exercise on a treadmill. The beat-to-beat HRV was measured at the 10(th) (post-10, from 10 to 20 minutes after exercise) and 30(th) minutes (post-30, from 30 to 40 minutes after exercise) after maximal exercise for subsequent analysis. Time and frequency domain analyses of HRV were performed to determine the effect of inhaling different percentages of 02 on autonomic function. The results indicated that heart rate at post-30 minutes was significantly lower in the HO group than in the LO group (p < 0.05). The time domain indices of the RR interval at post-30 minutes was significantly lower in the LO than in the HO group (p < 0.05). There were no significant differences, either at post-10 or post-30 minutes, with regard to the time domain indices among the three treatments. The natural log of high-frequency (HF) powers and the coefficient of component variance of HF powers at hyperoxia were significantly higher than at hypoxia, but there were no significant differences between hyperoxia and normoxia. In conclusion, these results demonstrated that the inhalation of the gas mixture with 60% O-2 after exhaustive exercise, compared with hypoxia, increased the vagal modulation of the autonomic nervous system on the heart and improved physiologic recovery after intense exercise.
