PURPOSE: This study evaluated the impact of passive and active intermittent hypoxic (IH) exposure pre-acclimatization strategies on temporal and spectral power measures of heart rate variability (HRV) in normobaric hypoxia (NH), and natural altitude.
METHODS: Thirty participants (17 male and 13 female, aged 20-62 years), matched by sex, age and maximal aerobic capacity (VO2peak), were randomly allocated to either a control, passive IH or active IH group. Experimental groups completed 10 x 2-h, passive (PIH) or active (AIH), normobaric IH exposures (FIO2 = 0.124, ~4,011 m) over the 14-day intervention period (weekends excluded). The control group received no IH exposure. During the intervention period, participants completed 20 minutes daily running training, at an individualised intensity equivalent to 80% heart rate reserve (HRR). Training workload was determined by regressing HR and running speed data from individual VO2peak tests in normal ambient conditions (control and PIH groups) or NH (AIH group, FIO2 = 0.124). AIH participants completed the exercise training sessions under supervision, during scheduled IH exposure sessions, while control and PIH groups completed training unsupervised in normal ambient conditions. Within 48 hours of completing pre-acclimatization, participants travelled by air from the UK to Nepal, a journey time of approximately 36 hours. Participants then trekked from 2800 m to 5300 m over 14 days. Temporal (RR, SDNN, RMSSD) and spectral power measures (LFnu, HFnu and LFHF ratio) of HRV were recorded, at rest with spontaneous breathing, in normal ambient conditions (FIO2 = 0.209), NH (FIO2 = 0.124, ~4011 m) and in hypobaric hypoxia (HH) at 4356 m and 5350 m, during ascent.
RESULTS: Two-way ANOVA (group x condition) with repeated measures revealed neither significant interactions (P>0.05), nor between-group (P>0.05) nor within-group (P>0.05) differences for temporal or power spectral HRV measures between baseline, pre-IH and post-IH. No significant interactions, between-group or within-group changes were noted between post-IH, 4300 m and 5300 m (P>0.05) natural altitude.
CONCLUSION: Pre-acclimatization using active and passive intermittent hypoxic exposure did not significantly alter heart rate variability responses during ascent to very high altitude.