BackgroundIt is postulated that lack of hypoxic ventilatory response is a predictor for AMS. End-tidal carbon dioxide (ETCO2) is an accurate, noninvasive surrogate measure of ventilation.ObjectivesWe sought to determine if changes in baseline ETCO2predicts the development of AMS.MethodsThis prospective cohort study took place in three separate high-altitude hiking treks. Subjects included a convenience sample of hikers. Predictor variable was change in ETCO2levels and outcome variable was AMS. Measurements of ETCO2levels were obtained at the base and repeated daily at various elevations and the summit of each hike. Concurrently, hikers were scored for AMS by a trained investigator. We utilized correlation coefficients and developed a linear regression model for analysis.Results21 subjects in 3 separate hikes participated: 10 ascended to 19,341 ft over 7 days, 6 ascended to 8900 ft in 1 day, and 4 ascended to 11,006 ft in 1 day. Mean age was 40 years, 67% were males, mean daily elevation gain was 2150 ft, and 5 hikers developed AMS. The correlation coefficients for ETCO2and development of AMS were −0.46 (95%CI -0.33 to −0.57), and −0.77 (95%CI -0.71 to −0.83) for ETCO2and altitude. ETCO2predicted the development of symptoms better than the elevation with AUCs of 0.90 (95%CI 0.81–0.99) versus 0.64 (95%CI 0.45–0.83). An ETCO2measurement of ≤22 mmHg was 100% sensitive and 60% specific for predicting AMS.ConclusionsETCO2was strongly correlated with altitude and moderately correlated with AMS and it was a better predictor than altitude.