LAUSR

OBJECTIVE Obstructive sleep apnea (OSA) is associated with sympathetic overactivity. Intermittent hypoxemia and repetitive arousals are the proposed mechanisms; however, the main contributing factor for sympathetic overactivity remains to be determined. We therefore investigated the independent effect of apnea or arousal on sympathetic activity in a large cohort of OSA patients using heart rate variability analysis. METHODS Frequency domain heart rate variability parameters were measured and compared between 782 patients with OSA (198 mild, 259 moderate, 325 severe) and 119 non-OSA controls. Univariate analysis was performed to identify associations between clinical and polysomnographic variables and heart rate variability measurements. Multivariate analysis was further performed to determine the potential contributing factors for sympathetic overactivity. RESULTS Patients with OSA exhibited overall alterations in heart rate variability measures indicating sympathetic overactivity compared to controls, and this tendency was more pronounced in those with severe OSA. Univariate analysis showed that both apnea-hypopnea and arousal indices correlated with increased sympathetic activity and decreased parasympathetic activity. In multivariate analysis, arousal index was found to be more closely associated with sympathetic overactivity than apnea-hypopnea index. CONCLUSION The present study showed that arousal may be a more potent contributing factor than apnea for sympathetic overactivity in OSA patients. Our results suggest that arousal index, rather than apnea-hypopnea index, could be considered as a surrogate marker for sympathetic overactivity in OSA. Further longitudinal studies should elucidate causal relationship between arousals and sympathetic overactivity in patients with OSA.