In the last several years, large observational studies24–25,55 of healthy children have increased our knowledge of normal respiratory parameters during sleep. This information adds to our knowledge of respiratory or arousal abnormalities noted in children with subtle SDB, such as those occurring without discreet gas exchange abnormalities, and may contribute to recommendations for treatment. An AHI of greater than one event per hour is out of the normal range in children,39,55–56 but, as discussed in the previous section, the AHI should also be used in context with other respiratory or arousal parameters since the clinical significance of a mildly abnormal AHI is unknown. Hypercapnia time, measured with end-tidal or transcutaneous CO2 levels of > 50 mm Hg in healthy children, has been reported to be < 10 to 20% of total sleep time in healthy children depending on the device used for measuring CO2,24–25,56 and the strictness of the exclusion criteria (excluding children who snored associated with lower expected CO2 values as in the study by Uliel et al,24). Oxygen saturation in nonsnoring healthy children, measured by pulse oximetry, rarely drops to < 95% from obstructive events in children,24 although occasionally drops into the 89% range can occur normally with central apnea.24 In another study57 that included 180 children aged 1 to 10 years (including snoring children or those with an AHI of < 5), the saturation nadir was 90.1%, with > 90% of time spent at a saturation of > 95.1%. However, the amount of time with intermittent desaturations between 90% and 95% might be clinically significant in these children.25 The expected arousal index (ie, the number of arousals per hour) in children has been not been standardized; however, a recent population study,25 of preschool children aged 3 to 7 years suggests that the average arousal index over total sleep time was less than eight arousals per hour; and less than one respiratory-related arousal (associated with apnea, hypopnea, or snore) per hour was noted. Arousal indexes in healthy children in the sleep laboratory tend to be < 14.25,58 Respiratory-related arousals are not recorded as standard diagnostic criteria in many clinical laboratories, but this value may improve the accuracy of the arousal index as a characteristic of SDB.