Does a change in end-tidal carbon dioxide level predict high altitude mountain sickness?

It 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.

We sought to determine if changes in baseline ETCO₂ predicts the development of AMS.

This prospective cohort study took place in three separate high-altitude hiking treks. Subjects included a convenience sample of hikers. Predictor variable was change in ETCO₂ levels and outcome variable was AMS. Measurements of ETCO₂ levels 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.

21 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 ETCO₂ and development of AMS were -0.46 (95%CI -0.33 to -0.57), and -0.77 (95%CI -0.71 to -0.83) for ETCO₂ and altitude. ETCO₂ predicted 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 ETCO₂ measurement of ≤22 mmHg was 100% sensitive and 60% specific for predicting AMS.

ETCO₂ was strongly correlated with altitude and moderately correlated with AMS and it was a better predictor than altitude.