Metabolic, Cardiac, and Hemorheological Responses to Submaximal Exercise under Light and Moderate Hypobaric Hypoxia in Healthy Men

SIMPLE SUMMARY: The lower atmospheric partial pressure of oxygen under hypobaric hypoxia decreases oxygen saturation and arteriovenous oxygen difference. Exercise under hypoxia decreases arterial oxygen saturation, which reduces the ability to deliver oxygen to active muscles and consequently worsens aerobic capacity and exercise performance. Previous studies on metabolic and cardiac responses to submaximal exercise under hypoxia have been well documented, but information on hemorheological responses is relatively insufficient. In this regard, a review of hemorheological responses to exercise under hypoxia could provide further information on reduced aerobic capacity and exercise performance caused by acute hypoxia. We conducted a randomized crossover trial to compare the effects of acute exercise under light and moderate hypobaric hypoxia versus normoxia on metabolic parameters, cardiac function, and hemorheological properties in healthy men. The main findings of our study revealed that endurance submaximal exercise under light (596 mmHg, simulated 2000 m) and moderate (526 mmHg, simulated 3000 m) hypoxia induced greater metabolic and cardiac responses than exercise under normoxia. However, exercise under hypobaric hypoxia did not affect hemorheological properties, including erythrocyte deformability and aggregation. These results can be used as basic data for understanding hemorheological responses in light and moderate hypobaric hypoxia. ABSTRACT: We compared the effects of metabolic, cardiac, and hemorheological responses to submaximal exercise under light hypoxia (LH) and moderate hypoxia (MH) versus normoxia (N). Ten healthy men (aged 21.3 ± 1.0 years) completed 30 min submaximal exercise corresponding to 60% maximal oxygen uptake at normoxia on a cycle ergometer under normoxia (760 mmHg), light hypoxia (596 mmHg, simulated 2000 m altitude), and moderate hypoxia (526 mmHg, simulated 3000 m altitude) after a 30 min exposure in the respective environments on different days, in a random order. Metabolic parameters (oxygen saturation (S(P)O(2)), minute ventilation, oxygen uptake, carbon dioxide excretion, respiratory exchange ratio, and blood lactate), cardiac function (heart rate (HR), stroke volume, cardiac output, and ejection fraction), and hemorheological properties (erythrocyte deformability and aggregation) were measured at rest and 5, 10, 15, and 30 min after exercise. S(P)O(2) significantly reduced as hypoxia became more severe (MH > LH > N), and blood lactate was significantly higher in the MH than in the LH and N groups. HR significantly increased in the MH and LH groups compared to the N group. There was no significant difference in hemorheological properties, including erythrocyte deformability and aggregation. Thus, submaximal exercise under light/moderate hypoxia induced greater metabolic and cardiac responses but did not affect hemorheological properties.