Effect of intermittent hypoxic exposure delivered via cyclic variation in altitude conditioning chamber on heart rate variability in aerobic athletes

Thesis (M.S.)--University of Hawaii at Manoa, 2006.Includes bibliographical references (leaves 101-105).x, 105 leaves, bound ill. 29 cmIn recent years, a focus of cardiovascular research has been heart rate variability (HRV). Heart rate variability has been defined as the beat-to-beat fluctuations in heart rate over time. In clinical studies, a low HRV has been related to increased risk of sudden cardiac death (Guzzetti et at., 2005). Analysis of HRV has been considered an effective, non-invasive method of assessing the circulatory system control~ as well as enabling a distinction between the sympathetic and parasympathetic components of the autonomic nervous system (Zuzewicz et at., 1999). Heart rate variability relies on a balance between the sympathetic and parasympathetic function, a\1owing for adjustments to heart rate when placed in changing environments. Because of the known changes that take place with acclimatization to altitude, athletes and scientists have had an interest in using attitude exposure to influence performance. Unfortunately, living at altitude is unrea1istic for most of the world's population, thus the use of attitude chambers have been tested for acclimati7mon to attitude. Altitude chambers a\1ow for short-term exposure to altitude on a regular basis, often referred to as Intermittent Hypoxic Exposure (IHE), and have been shown to elicit acclimatization responses (Ricart et at., 2000). According to a review by Levine (1997), nm is further classified into two subcategories, depending on the exposure time. One category, the "live-low train-high" method, receives exposure during exercise which enhances the training stimulus. The other category, the "live-high, train-low" method, receives exposure during rest which elicits the acclimatization effect. It has been reported that living and training at altitude has not been found very effective in improving performance at sea level (Wilber, 2001). Recently, however, Fulco et al. (2000) showed that living at high altitudes and training at lower altitudes (sea level) may result in the most beneficial combination to bring about the physiological changes to improve subsequent sea level performance in well-trained, competitive runners. It has been demonstrated and documented that changes in control of the autonomic nervous system occur due to altitude exposure; specifically, there is a sympathetic response, and subsequent a parasympathetic withdrawal, which is seen using spectral analysis of HRV (Zuzewicz et al., 1999; Roche et al., 2002). Hansen and Sander (2003), using direct measures of sympathetic activity, reported increased sympathetic activity at altitude versus sea level, and also reported increase sympathetic activity remained elevated 3-4 months after acclimation to altitude. Farinelli, et al. (1994) further investigated the changes due to altitude exposure and identified a desensitization of the heart to the sympathetic system because of the high sympathetic tone. When tested at altitude, acclimatized subjects, being unable to obtain an adequate sympathetic response, must call on a greater reduction in parasympathetic activity to compensate. Rodriguez, et al. (2000), reported that erythropoiesis was effectively stimulated after a relatively short exposure at rest, in a hypobaric chamber (90 minutes per day for 3 weeks at 5,500M); however, there is no indication of the amount of exposure needed to initiate changes in HRV. If IHE has a beneficial effect on HRV, then IHE may be a viable therapy for populations such as cardiac patients. To our knowledge, the effect of short-term intermittent cyclic hypoxic exposure on HRV has not been investigated. The purpose of this study is two-fold: 1) to investigate the acute effects of altitude acclimatization using the Cyclic Variation in Altitude Conditioning (CVACTM) chamber (described in detail below) on autonomic control of the heart during supine rest and a head-up tilt at sea level, and 2) to investigate the chronic effects of altitude acclimatization on HRV at sea level. It was hypothesized that: 1) there would be no significant differences in HRV variables at sea level immediately before or after cyclic nm (acute changes), and 2) there would be no significant differences in HRV variables at sea level due to nm throughout the 10 weeks of testing (chronic changes)