Heart rate variability: the impact of hypoxia and gender

Introduction: Non-invasive measurements of heart rate variability (HRV) are used to assess cardiovascular autonomic regulation. HRV can be measured using time and frequency domain indexes; changes in these indexes are considered to reflect changes in the parasympathetic and sympathetic modulation (1). A reduction in HRV, with an increase in low frequency (LF) component and LF/HF (high frequency) ratio are common findings when at high altitude, highlighting a shift towards sympathetic modulation and a decline in the vagal activity (2), Little is known regarding the impact of gender on HRV and existing literature is inconclusive suggesting that females exhibit either an increase (3) or a decrease (4) in HRV measurements when compared to males in normoxia. Therefore, the present aim was to investigate the effect of acute hypoxia and gender on the cardiovascular autonomic function. 
Methods: Sixteen healthy physically active subjects (6 males, 10 females) aged 26 ± 7 years, 29 ± 6 years (respectively) underwent a 5-minute supine HRV measurement in normoxia (21% O2) and following 4 hour exposure to normobaric hypoxia (12% O2). The following parameters of HRV were calculated: Time domain indexes: standard deviation of RR intervals (SDNN), root-mean-square of successive normal RR interval differences (RMSSD); Frequency domain indexes: Total power (TP, 0.0 - 0.40Hz), LF/HF ratio, HF normalised power (HFnu) calculated as the HFnu =(absolute HF power/TP)x100 and LF normalised power (LFnu) calculated as LFnu = (absolute LF power/TP)x100. Following confirmation of distribution normality using Shapiro-Wilks W tests, data were analysed using a 2-way repeated measures analysis of variance. Significance was established at a level of P<0.05 and data were expressed as mean ± SD. 
Results: Hypoxia induced a reduction in all the time domain indexes (P<0.05) as well as in HFnu, TP, but an increase in LFnu and a significant increase in LF/HF (P<0.05) in all groups. There was no gender difference in any measured HRV index (P>0.05). 
Conclusion: The findings are consistent with the concept that hypoxia induces modifications of the cardiac autonomous nervous system (5). However, the effect of gender remains undetermined in the current study both in normoxia and hypoxia, hence warranting further assessment.