Heart rate variability changes in sea level residents during a 12 hour exposure to Hypoxia
Introduction--High altitude illness, specifically acute mountain sickness, is a potentially life threating ailment for individuals that travel to high altitude. Acute mountain sickness is characterized by the following symptoms; headache, lethargy, dizziness and gastrointestinal issues. To date there have been several attempts to validate the use of heart rate variability in an effort to predict when an individual is more likely to become sick at altitude, but no studies have included a model with exercise that is known to increase acute mountain sickness.--Purpose-- The purpose of this study is to examine changes in autonomic output via heart rate variability to see if there is a relationship between autonomic output and severity of acute mountain sickness during a 12-hour exposure to normobaric hypoxia.--Methods--Eleven participants (7 male and 4 female) were recruited for this study, of which eight (6 male and 2 female) finished the study. Participants were exposed to two 12-hour altitude sessions (FIO2 12.4%) which was roughly 4,200 meters. Baseline heart rate variability, blood pressures, heart rate, oxygen saturations and Lake Louise scores were obtained with participants seated for five minutes prior to entering the altitude tent. Subsequent measurements were taken at hours 3, 4, 5, 6 and 12 with participants seated for at least five minutes and were taken prior to any exercise. A two-way repeated measures ANOVA was used to determine any significant differences and Tukeys post hoc was used to determine where those differences occurred between groups. A Pearsons correlation was used to determine any relationships between the changes in HRV and AMS scores from hours 6 to 12. An alpha level of 0.05 was used for significance.--Results--The main finding of the study was a main effect for treatment and time for HRV measures. Significant differences between groups were observed and a significant difference between hour 3 and 12 of the non-exercise trial with LnrMSSD values being 3.73±0.45 and 3.40±0.36 respectively for hours 3 and 12. An interaction effect of treatment by time was observed for heart rate. Heart rates were significantly higher in the exercise trial at hours 4, 5, 6 and 12 when compared to all timepoints except hour 12 during the non-exercise trial. A Pearson value of 0.41 and 0.14 were calculated for non-exercise and exercise differences between LnrMSSD and LLS between hours 6 and 12 which yielded p-values of 0.30 and 0.75 respectively.--Conclusions--Although significant differences were found regarding HRV, no significant relationships between HRV and AMS were observed. This leads to the conclusion that HRV is not a valid tool for predicting AMS during acute normobaric hypoxic exposures.