Differences in pulse oximetry saturation from sea level to high altitude among subjects with and without respiratory disease. A population-based, cross-sectional study

Introduction Oxygen saturation (SpO 2 ) is used to detect hypoxemia and guide oxygen therapy, but its interpretation varies with altitude, age, and lung disease. Although >500 million people live at ≥1,500 m, population-based data comparing SpO 2 across altitudes in adults with and without chronic respiratory disease are scarce. We aimed to compare SpO 2 between these groups from sea level to 3,030 m, identify factors associated with SpO 2 , and describe SpO 2 by altitude and age in adults without respiratory disease. We hypothesized that SpO 2 would be lower in participants with respiratory disease, with differences increasing with altitude. Methods PREPOCOL II is a population-based, cross-sectional study with a probabilistic, two-stage, clustered sampling design to estimate the prevalence and risk factors for respiratory disease among adults ≥40 years in Colombian cities at different altitudes (18-3030 m). All participants completed a standardized questionnaire, spirometry, and SpO 2 measurement. Participants with respiratory disease (RDP), including chronic obstructive pulmonary disease, asthma, chronic bronchitis, obstructive sleep apnea, and interstitial lung disease, were compared with controls using Mann-Whitney and Kruskal-Wallis tests, multivariable linear regression, and comparisons of altitude coefficients between groups. Results A total of 1146 RDP and 4613 controls were included. RDP were older, and had lower BMI and lung function. SpO 2 was lower in RDP than in controls at all altitudes and decreased in both groups as altitude and age increased (p<0.001). In multivariable models, altitude showed the strongest association with SpO 2 (standardized β −0.69 in controls and −0.71 in RDP), followed by age and BMI (and FEV 1 in RDP). The decrease in SpO 2 per unit of altitude was larger in RDP (B −1.15 vs. −0.80; difference p<0.001), and the between-group difference became evident at ~1,500 m (median 97.0% vs. 95.5%). Conclusion Among lifelong residents living at altitudes from sea level to 3,030 m, SpO 2 was lower in RDP than in controls. SpO 2 decreased with altitude and age in both groups, with altitude being the variable most strongly associated with SpO 2 . SpO 2 values should be interpreted in relation to specific altitude and age. In RDP, the lower SpO 2 was most evident at altitudes of ~1,500 m and above.