Overnight changes in exhaled carbon dioxide, oxygen, and nitrous oxide in obstructive sleep apnea: associations with disease severity and polysomnographic findings

BACKGROUND: Exhaled breath analysis is a non-invasive approach for assessing physiological alterations in obstructive sleep apnea (OSA). This study evaluated overnight changes in exhaled CO₂, O₂, and N₂O concentrations and their associations with OSA severity and polysomnographic parameters. Methods: In this prospective observational study, 62 adults undergoing diagnostic overnight polysomnography were enrolled and classified as having no OSA (n = 21), mild-to-moderate OSA (n = 21), or severe OSA (n = 20) according to the apnea-hypopnea index. Exhaled CO₂, O₂, and N₂O concentrations were measured immediately before and after overnight polysomnography under standardized conditions. Results: Morning exhaled N₂O concentrations were significantly higher in patients with severe OSA than in the other groups (p = 0.001). Overnight changes in exhaled CO₂ (ΔCO₂), N₂O (ΔN₂O), and O₂ (ΔO₂) differed significantly according to OSA severity (all p < 0.001). Compared with the no OSA group, patients with OSA exhibited greater overnight increases in exhaled CO₂ and N₂O concentrations together with greater decreases in exhaled O₂ concentrations. Overnight ΔN₂O and ΔO₂ showed strong correlations with AHI (ρ = -0.760 and 0.912, respectively; both p < 0.001) and ODI (ρ = -0.713 and 0.913, respectively; both p < 0.001). Conclusion: Patients with OSA exhibited more pronounced overnight alterations in exhaled gas composition, particularly greater changes in exhaled O₂ and N₂O concentrations. These findings suggest that dynamic overnight exhaled gas changes may reflect physiological alterations associated with OSA severity and warrant further validation in larger prospective studies. .