Serum sphingomyelin levels define oxyhemoglobin desaturation-related metabolic threshold in symptomatic obstructive sleep apnea.

Hypoxia is a contributing factor for the morbidity and mortality in patients with obstructive sleep apnea (OSA). We aimed at identifying the percentage of sleep time with oxyhemoglobin desaturation below 90% (Tc90%) breakpoint from which the most significant changes occur in systemic metabolome of patients with OSA. In a prospective observational study on patients with polysomnography-confirmed symptomatic OSA, profiles of 186 metabolites including amino acids, biogenic amines, acylcarnitines (AC), lysophosphatidylcholines, phosphatidylcholines (PC) and sphingomyelins (SM) were analyzed with liquid chromatography-mass-spectrometry in peripheral blood, obtained at 3 time points that covered patients' night sleep. Comparisons of rank-transformed data with general linear model for repeated measures after dichotomizing the study group at different Tc90% levels were applied to define the best cut-off, hypoxic metabolic threshold (HMT), based on Cohen's f. Fifty-one subjects were recruited with their median Tc90% of 2.1. The mean Cohen's f over the metabolites was highest (0.165) at a Tc90% of 1.8 representing the HMT. Of the different classes of metabolites, the Cohen's f value at HMT was highest for SM (0.322). Compared to patients with Tc90% < HMT, concentrations of 2 PC, 1 AC and 7 SM were significantly higher in patients with Tc90% ≥HMT. The HMT for patients with OSA described in this report for the first time is located at a Tc90% level of 1.8 with SM levels contributing most to the size of this threshold.