Respiratory Muscle Training Improves Exercise Endurance at Altitude
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People have been traveling to high altitude for centuries where they are faced with adverse environmental conditions. As one ascends to elevation, the barometric pressure is reduced and the air gets thinner. These changes affect the partial pressure of gases in the ambient air. The decreased partial pressures of oxygen affects physiological processes, such as increased ventilation (hyperventilation), vascular tone and the decreased capacity for cellular metabolism. These changes have serious implications on exercise capacity and overall safety of individuals exposed to these conditions as well as their health. Prior studies have shown voluntary isocapnic hyperpnea training (VIHT) to eliminate the hyperventilatory response associated with exercise at sea level. Since high altitude provokes hyperventilation at rest, and the addition of higher intensity exercise exacerbates this response, the respiratory muscles have a higher propensity to fatigue, further limiting exercise capacity. High ventilation rates, especially at altitude, cause significant decreases in the arterial partial pressure of carbon dioxide (P a CO 2 ) leading to the constriction of blood vessels. The constriction has potential detrimental effects on cerebral oxygenation and therefore central fatigue. These factors, taken together, produce severe limitations on exercise capacity at altitude. This study was designed to measure the effects of VIHT three days per week for four weeks on exercise performance at 10,000ft simulated altitude. Ten healthy non-smoking moderately active men were recruited, five of which completed the study. The subjects performed Pre- and Post-VIHT exercise endurance trials cycling at 60rpm against 75% of their predetermined maximal workload (determined at sea level) on an electrically break cycle ergometer in a hypobaric (decompression) chamber. Prior to the start of exercise and during exercise physiological responses were measured and recorded during exercise at altitude. Heart rate, arterial oxygen saturation (S a O 2 ), cerebral blood flow velocity (CBFv), diffused cerebral tissue oxygen saturation, end tidal CO 2 and mixed expiratory gases were measured as well as ventilatory characteristics such as minute ventilation, respiratory rate and tidal volumes were recorded. All subjects training minute ventilation rates improved over the twelve training sessions, 37% on average. At rest, subjects S a O 2 decreased 6.6% on average from sea level to simulated altitude while heart rates subsequently increased on average from 73 to 86 bpm. During exercise at altitude there was marked hyperventilation in both Pre- and Post-VIHT endurance trials, and more so in the post-VIHT trials, lowering end tidal CO 2 throughout the trial. Corresponding to the hyperventilation, CBF v also decreased while cerebral oxygen saturation remained constant. Exercise endurance times improved 64% on average from Pre- to Post VIHT trials. These results suggest that VIHT reduces respiratory muscle fatigue, allowing subjects to breathe at higher ventilation rates for extended periods of time, which improves exercise endurance at altitude. Key Words: Altitude, respiratory muscle training, exercise, cerebral blood flow velocity.