Exercise-induced cardiovascular responses during combined normobaric vs. hypobaric and normoxic vs. hypoxic acute exposures in military air pilot trainee.

Abstract

Introduction: This study aimed to evaluate the putative effects of hypobaria on ventilatory, cardiovascular and muscle oxygenation during exercise in normoxia and hypoxia.

Methods: Eighteen healthy air pilot trainees (26±3 years, 177±10 cm, 70±11 kg) performed a 6-min moderate-intensity cycling exercise (1 W/kg) in four randomized conditions: normobaric normoxia (NN), hypobaric normoxia (HN), normobaric hypoxia (NH) and hypobaric hypoxia (HH) in a hypobaric chamber. Inspired oxygen pressure was matched between normoxic (NN vs. HN, 141.2±0.8 vs. 141.5±1.5 mmHg) and hypoxic (NH vs. HH, 75.7±0.4 vs. 74.3±1.0 mmHg) conditions. Gas exchanges, pulse oxygen saturation (SpO₂), heart rate (HR), middle cerebral artery blood flow velocity (MCAv), cerebral and muscular oxygenation (NIRS), and cerebral O₂ delivery (cDO₂) were recorded.

Results: SpO₂, brain and muscle oxygenation were significantly lower and ventilation higher in HH than in NN and HN, and NH, during both rest and exercise (exercise SpO₂ 99.0±1.5, 80.8±4.2, 97.6±1.9, 69.2±5.7 % and ventilation 12.5±2.3, 13.3±3.1, 12.4±2.6, 14.6±2.4, l/min in NN, NH, HN and HH respectively). cDO₂ was decreased to the same extent in HH and NH, compared with NN and HN (exercise 865.5±147.6, 731.8±152.2, 857.8±157.8, 755.8±163.3 cm.mlO₂/s.dl_bl). Specific effects of hypobaria in normoxia was lesser than in hypoxia, since only blood O₂ and CO₂ partial pressures were lower in HN than NN.

Discussion: Respiratory, cardiovascular responses and brain/muscle oxygenation were more altered in HH than in NH, which confirms the additive effects of hypobaria on exercise in severe hypoxia. However, the effects of hypobaria are likely of negligible clinical relevance in normoxia.