The Effect Of Hypobaria On Ventilation And Vo2max During Maximal Running

Abstract

During hypobaric hypoxia (H), maximal exercise pulmonary ventilation (VEmax) is related to the arterial oxyhemoglobin saturation level (SaO2) and the maximal oxygen uptake (VO2max). Resistance to air flow is one of the factors limiting VEmax. It is therefore likely that the airflow resistance of the airway is reduced during maximal exercise under H conditions, since air density declines with descending atmospheric pressure. It is unknown, however, whether reducing airflow resistance by reducing the atmospheric pressure affects VEmax and thus VO2max, SaO2 and exercise performance. PURPOSE: To examine the effect of lowering the airflow resistance (hypobaria), itself [i.e., hypobaric normoxia (HN)], on VEmax, VO2max, SaO2 and exercise performance. METHODS: Ten young male subjects performed incremental treadmill running tests to exhaustion in a hypobaric chamber under conditions of normoxia [N; barometric pressure (BP)=760 mmHg (sea level)], H (BP=490 mmHg; 3,500 m above sea level) and HN (BP=490 mmHg; breathing 32.6% O2 / 67.4% N2) in random order. VEmax and VO2max were determined using the Douglas bag method. Oxyhemoglobin saturation (SaO2) was measured by oximetry. Exercise performance was evaluated based on the duration of the incremental running. RESULTS: For N, H and HN, VEmax was 146.4±7.6, 147.7±9.1 and 155.8±8.6 L/min, respectively; VO2max was 64.9±2.1, 46.5±2.8 and 65.5±3.2 ml/kg/min, respectively; and SaO2 was 90±3, 68±3 and 89±4 %, respectively. VO2max and SaO2 were significantly lower in H than in N. VEmax did not differ between N and H. VEmax was significantly higher in HN than N, though VO2max and SaO2 did not differ between NH and N. Running duration was significanly longer in HN than in N (885±76 s vs. 901±87 s; N vs. HN (t-test)). These results suggest that the reduced air density under the 3,500 m hypobaric condition increased VEmax and exercise performance, but a 6.6% increase in VEmax did not affect VO2max and SaO2 under the HN condition. CONCLUSION: Hypobaria, itself (hypobaric normoxia), induces an increase in VEmax and exercise performance, but VO2max and SaO2 do not change. Supported by grants from COE projects and from the Ministry of Education, Science, and Culture, Japan.