Relationship Between Serum Substance P Levels and Daytime Sleepiness in Obstructive Sleep Apnea Syndrome

ObjectiveWe hypothesized that intermittent hypoxia might influence serum substance P levels, and that this effect might in turn contribute in excessive daytime sleepiness (EDS) in patients with obstructive sleep apnea syndrome (OSAS).Patients and methodsFifty-five patients with newly diagnosed OSAS and 15 age-matched nonapneic control subjects were enrolled in this study. Full polysomnography was performed in all patients. Single blood samples were drawn between 8:00 am and 9:00 am after the sleep study. Substance P levels were analyzed with a competitive enzyme immunoassay (substance P EIA kit; Cayman Chemical; Ann Arbor, MI).ResultsThere were no significant differences in age, gender, body mass index, smoking habit, and snoring between the two groups. Serum substance P levels in the OSAS group were significantly lower than that in the control group (p < 0.0001). Serum substance P levels were positively correlated with rapid eye movement sleep (r = 0.330, p = 0.049) and slow-wave sleep (r = 0.324, p = 0.049) phases. Serum substance P levels were negatively correlated with Epworth sleepiness scale score (r = − 0.253, p = 0.048), number of total apneas during the night (r = − 0.247, p = 0.036), number of respiratory events during the night (r = − 0.266, p = 0.024), apnea-hypopnea index (r = − 0.287, p = 0.015), respiratory arousal index (r = − 0.267, p = 0.026), time spent in apnea and hypopnea (r = − 0.307, p = 0.01), average oxygen desaturation (r = − 0.265, p = 0.026), and oxygen desaturation index (r = − 0.254, p = 0.031).ConclusionWe concluded that EDS seen in some of the OSAS patients might be associated with various pathophysiologic mechanisms including substance P levels.