Hour-to-hour variability of oxygen saturation in sleep apnea

Study objectives: Methods used to express the severity of oxygen desaturation during polysomnography include the average oxygen saturation (AO₂), lowest oxygen saturation (LO₂), and the percent of the total time with oxygen saturation level lower than 90% (T<90%). We wanted to determine which one of these methods is least variable during different hours of monitoring. Design: Prospective, observational study. Setting: Sleep center at a medical university. Patients: One hundred fifty patients with apnea- hypopnea index from 5 to 130. Measurements: AO₂, LO₂, and T<90% were calculated during each of the 8 h of polysomnography. Data for each hour were compared and the Cronbach alpha coefficients were calculated. Results: There was a high degree of correlation among the three methods as well as between each method and the severity of sleep apnea. The mean ± SD values for each method were as follows: AO₂, 92.7±5.6; LO₂, 68.5 ± 19.3; and T<90%, 15.7 ± 24.2. The alpha coefficients for these methods were AO₂, 0.98; LO₂, 0.88; and T<90%, 0.98. In all methods, the data of the first hour were significantly different from the data of the subsequent hours. Conclusion: Both AO₂ and T<90% methods show less hour to hour variability compared with LOs, and there is more variability in the first hour. Since the AO₂ values >90% may not convey the severity of O₂ desaturation, T<90% may be the best method of expressing oxygen saturation changes during polysomnography.