Operational Characteristics of a Normabaric Hypoxic System

Abstract

Introduction: Several studies have demonstrated beneficial performance effects in endurance athletes following completion of approximately four weeks of “high-low” altitude training (Levine and Stray-Gundersen 1997; Rusko 1996). Athletes who utilize “high-low” altitude training typically live/sleep at a simulated or natural elevation > 2000 m, and train at a relatively low elevation (< 750 m). The Hypoxico Altitude TentTM (HAT) is a commercially available, normobaric hypoxic system that allows one individual to rest/sleep in an artificial altitude environment, and purportedly simulates a maximum elevation of 4200 m. The HAT has been marketed as a safe and effective way for athletes to “sleep high and train low”. However, there are minimal data on the safety and efficacy of these commercial simulated altitude systems, particularly among the population most likely to utilize them, i.e., elite athletes (Wilber 2001). Therefore, the purpose of this study was to evaluate the operational characteristics of the Hypoxico Altitude TentTM using international level athletes as subjects. Methods: Female (n = 3) and male (n = 7) U.S. National Team athletes slept in the HAT for five consecutive nights, ∼ 8 hrs per night, at a simulated altitude of 2500 m (FIO2 ∼15.3%). The athletes were distance runners, racewalkers, rowers, and paddlers who lived and trained at ∼150 m at the U.S. Olympic Training Center (Chula Vista, CA) during the non-sleep portion of the day. Included in this group were four athletes who competed in the 2000 Olympics. Temperature and relative humidity (RH) were measured every 30 min using a digital barotemp device (Fisher Scientific). Measurements of FIO2 and FICO2 were made every 30 min via zirconium oxide (S-3A; Applied Electrochemistry) and infrared (LB-2; Applied Electrochemistry) gas analyzers, respectively. A repeated measures ANOVA was used to evaluate mean differences over time. Results: Mean temperature within the HAT was 20 ± 2°C, which remained relatively stable provided the HAT was used in an air-conditioned room. Mean RH within the HAT was 86 ± 4%. For one subject, RH increased to as high as 95% by the end of an overnight exposure. [CO2] increased rapidly and stabilized within 60 min. Mean FICO2 over the duration of the 8-hr sleep period was 0.7 ± 0.1%, which was significantly higher (p < 0.05) compared with baseline values (0.03–0.04%). [O2] decreased by 10% to 15% in the initial 2 to 3 hrs of sleep. Mean FIO2 over the duration of the 8-hr sleep period was 14.4 ± 0.4% (∼3000 m), which was significantly lower (p < 0.05) versus the baseline FIO2 of 15.3 ± 0.2% (∼2500 m). Questionnaires collected each day within 30 min of awakening indicated minimal physical and/or psychological side effects. Discussion: The most important finding of the present study was the significant 16-fold increase in [CO2] inside the HAT that occurred in the initial 60 min of the 8-hr sleep period. Although mean FICO2 (0.7%) did not approach unhealthy levels (> 3.0%), we were concerned that it might compromise the overnight recovery of the athletes. This does not appear to have been true, as evidenced by the fact that the athletes reported minimal discomfort or side effects on post-sleep questionnaires. The other interesting finding was the significant decrement in FIO2 (10–15%) that occurred primarily in the initial 2 to 3 hrs of sleep. This progressive decrease in FIO2 was observed to a greater degree in taller/larger athletes, and therefore may be related to differences in thoracic cavity size and VE. We concluded that despite some concerns with [CO2] and [O2] stabilization, the HAT has the potential of providing a relatively comfortable normobaric hypoxic environment, thereby allowing athletes to “sleep high and train low”. Acknowledgements: Supported by the United States Olympic Committee