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 carbon dioxide (CO2) 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 yr) on separate days to two ambient CO2 levels (0.1% vs. 1.0% indoor sea-level equivalents; 0.76 vs. 7.60 mmHg partial pressure) during 6 h 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 ([Formula: see text]), brain, and muscle (tissue saturation index derived from near-infrared spectroscopy), respiration, and cognitive function (sustained attention, working memory, and hand-eye coordination) hourly. In addition, we conducted capillary blood gas analyses at baseline, 15 min, and 6 h after hypoxia onset. During hypobaric hypoxia, ambient CO2 enrichment on average increased Pco2 from 36.3 to 38.3 mmHg, Po2 from 60.9 to 68.3 mmHg, and minute ventilation from 9.7 to 10.4 L/min, whereas reducing the time fraction of [Formula: see text] < 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 the muscle. High ambient CO2 had no effect on cognitive performance. Taken together, enrichment of cabin air with CO2 during hypobaric hypoxia may improve blood and brain oxygenation.NEW & NOTEWORTHY Elevating ambient CO2 can improve blood and brain oxygenation in healthy individuals during moderate hypoxia as experienced in an airliner cabin. The findings offer a new approach potentially benefiting the health and safety of airline passengers and support investigating CO2 enrichment in broader passenger populations, especially in those at risk of hypoxemia. Exploration of CO2-based interventions to enhance passenger oxygenation may also prompt regulatory reassessment of cabin CO2 limits and technological innovations in ventilation systems.