Hypoxia-induced Increase of Heartrate is Attenuated in Endurance Trained Men

Chronic hypoxia elicits a multitude of physiologic responses leading to an increase in total hemoglobin mass (THM), an enhanced hypoxic ventilatory response (HVR), and many more. The magnitude of the acclimatization observed in these parameters is hypoxia dose-dependent, with changes at moderate altitude being less than at high altitude. Accordingly, high altitude has been the largest focus in this area with much less attention on moderate altitude, particularly as it pertains to deacclimatization. This is relevant as many more people reside at moderate altitude permanently or for weeks/months at a time for training, deployment, or recreation. These individuals will inevitably visit sea level (SL) for a short period of time for leave, competition, or leisure and then return to moderate altitude. It is not known how much, if any, deacclimatization in THM or HVR occurs when fully acclimatized individuals travel to SL for 3 days. PURPOSE: Test the hypothesis that following 3 days at SL THM will be significantly altered and HVR will remain unchanged. METHODS: To Date, seven healthy participants (22.1 ± 1.1 years, 5 females) have completed the study. All visits (6 total) were conducted in Flagstaff, AZ (altitude 2,100m). Three visits prior to descent to, and 66.3 ± 1.5 hours in, Phoenix, AZ (altitude 330m) and three upon return from SL. Visit 1 included consent and screening. In 4 visits, THM was quantified using the optimized carbon monoxide rebreathing technique and poikilocapnic HVR was determined using a step-down decrease in inspired oxygen fraction from 0.21 to 0.09. Pre-SL measurements were averaged and used to compare to post-SL measurements. A one-way ANOVA was computed on THM and HVR between preand post-SL measurements using GraphPad Prism. Significance was set at p = 0.05. RESULTS: At present, we have found no significant effect on THM (p = 0.07). HVR was unchanged from preto post-SL (p = 0.49). CONCLUSION: Following three days at SL, THM and HVR were unchanged. Our data suggest that 3 days at SL may not be sufficient to significantly alter HVR or THM. However, THM did decrease by 5% post-SL (p = 0.07) so a greater sample size may alter our findings.