Judicious elevation of ambient carbon dioxide during hypobaric hypoxia to improve oxygenation in airline passengers - a randomized feasibility study

Despite pressurization of airliner cabins, some passengers experience in-flight hypobaric hypoxia with blood oxygen saturation dropping below 90%, potentially causing discomfort and increasing the risk of medical events. Enrichment of the cabin air with CO ₂ may augment passengers’ blood and tissue oxygenation by stimulating respiratory drive, thereby increasing health and safety during air travel. In a randomized double-blind crossover study we exposed 17 healthy adults (8 women; age range: 18-40 years) on separate days to two ambient CO ₂ levels (0.1 vs. 1.0% indoor sea level equivalents; 0.76 vs. 7.60 mmHg partial pressure) during 6 hours of hypobaric hypoxia (~565 mmHg total barometric pressure; corresponding to 2,438 m altitude) in an altitude chamber simulating long-haul flight conditions. We measured oxygen saturation of the blood (SpO ₂ ), brain and muscle (tissue saturation index derived from near-infrared spectroscopy), respiration, and cognitive function (sustained attention, working memory, hand-eye coordination) hourly. Additionally, we conducted capillary blood gas analyses at baseline, 15 minutes and 6 hours after hypoxia onset. During hypobaric hypoxia ambient CO ₂ enrichment on average increased pCO ₂ from 36.3 to 38.3 mmHg, pO ₂ from 60.9 to 68.3 mmHg, and minute ventilation from 9.7 to 10.4 l/min, while reducing the time fraction of SpO ₂ < 90% from 18.8 to 2.5%. Tissue oxygenation increased in the brain from 62.8 to 63.9% whereas no change was found in muscle. High ambient CO ₂ had no effect on cognitive performance. Taken together, enrichment of cabin air with CO ₂ during hypobaric hypoxia may improve blood and brain oxygenation.