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To investigate the activity of the autonomic nervous system (ANS) during sleep in children with obstructive sleep apnea (OSA), in order to detect a possible cardiac ANS imbalance analyzing heart rate variability (HRV).43 subjects between 4 and 12 years of age (7.26 ± 2.8 years), undergoing a diagnostic assessment for OSA were evaluated. A time domain index (R-apnea index) was developed to evaluate HRV strictly related to obstructive events during sleep. Poincaré plot of RR intervals during the whole night was calculated.R-apnea index was negatively correlated with apnea hypopnea index (AHI) (r=-0.360, p=0.028). AHI and the duration of the disease were the only variables that were significantly correlated with R-apnea index. Three groups were subsequently created according to polysomnographic findings considering AHI. R-apnea index resulted significantly lower in patient with severe OSA compared to primary snoring/mild OSA subjects (p0.05). Looking at Poincaré plot, SD1 showed a diminishing trend with severity of OSA, however not reaching statistical significance.Our findings suggest an autonomic impairment in OSA children evidenced by the altered HRV both in the very short term (R-apnea index) and in short term (SD1).R-apnea index is an easy and cheap method to undelay early ANS imbalance.