Background: When considering supine position and REM dependence, many previous studies have referred to the traditional definition in which obstructive sleep apnea (OSA) severity is at least doubled in the corresponding position/sleep stage (Cartwright index). The lack of consideration of the time spent in the particular sleep situation could cause clinical bias.
Patients and methods: Two cohorts of patients with at least moderate OSA were analyzed retrospectively for anthropometric associations with OSA severity. One group consisted of 48 patients diagnosed using a polygraph and the other group of 222 patients underwent polysomnography. First, the conventional Cartwright index was used, and a modified index was later applied to integrate the relative time component for REM sleep and the supine position.
Results: Less than a fifth of the patients fulfilled the classic conditions for supine position or REM sleep dependency. There were no definitive cutoffs in the classic or modified Cartwright index with regard to daytime sleepiness. Both indices show there was a correlation between OSA severity and being overweight.
Conclusion: The modified Cartwright index allowed identification of borderline cases that were characterized by a very low or high amount of time spent in the supine position or REM sleep situation. Treatment effects that could have been caused only by other components, e.g., different times spent in the supine position, could be better controlled for. In future studies there will be various other ratios besides the previously accepted 2:1 ratio when different statistical parameters are considered, such as the reduction of OSA severity or adherence to treatment.
Keywords: Polysomnography; Sleep apnea syndromes; Sleep stages; Sleep-disordered breathing; Supine position.