Background: Current strategies to predict high altitude mountain sickness (AMS) have limitations. It has been postulated that hypoxic ventilatory response, or lack thereof, is a predictor for AMS. End-tidal carbon dioxide (ETCO2) is an accurate, noninvasive surrogate measure of ventilation. We sought to determine if a change in baseline ETCO2 predicts the development of AMS. Our secondary objective was to determine if there is a relationship between ETCO2 and altitude.
Methods: This study is a prospective cohort study which took place in three separate high altitude hiking treks over a 3 month period. Subjects included a convenience sample of consenting hikers who were present on each trek. Predictor variable was net change in ETCO2 levels from baseline and outcome variable was AMS as determined by validated 2018 Lake Louise Acute Mountain Sickness Score (LLAMSS). Baseline measurements of ETCO2 levels were measured using FDA-approved Masimo EMMATMMainstream Portable Capnometer at the base and repeated daily at various elevations and the summit of each hike. Concurrently, hikers were scored on the LLAMSS by a trained investigator. We recorded rate of ascent, age, living altitude and prophylactic medications. We used correlation coefficients and developed a linear regression model for analysis.
Results: There were a total of 20 subjects in 3 separate hikes: 10 ascended to 19341 ft over 7 days, 6 ascended to 8900 ft in 1 day, and 4 ascended to 11006 ft in 1 day. Mean age was 42 years, 70% were males, 50% took acetazolamide, 25% took dexamethasone, all resided at sea level, and mean daily elevation gain was 2150 ft. All 20 hikers completed the summits with only 5 developing mild AMS. Mean daily net ETCO2 change was 5 mmHg (range 0-13). There was a moderate negative correlation between ETCO2 change and development of AMS R2=0.61; and strong negative correlation between ETCO2 change in altitude R2=0.79. Correlations were stronger for higher altitudes and longer treks. Mean ETCO2 change was 8.2 mmHg (95%CI 5-11.4) for those with AMS and 4.8 mmHg (95%CI 4.3-5.3) for those without AMS
Conclusions: This is the first study to examine the relationship between ETCO2 and AMS. ETCO2 was strongly correlated with altitude and moderately correlated with AMS. ETCO2 may be a useful portable modality to predict and diagnose AMS.