Half of patients with obstructive sleep apnea have a higher NREM AHI than REM AHI.

BACKGROUND AND PURPOSE: To determine the percentage of sequential patients with obstructive sleep apnea with a higher non-rapid eye movement (NREM) apnea-hypopnea index (AHI) than rapid eye movement (REM) AHI and those with a higher REM AHI than NREM AHI, and to look for factors that might influence the AHI to be higher in one of these two groups versus the other and thus ascertain the factors that cause an AHI to be higher in NREM than REM. A high body mass index (BMI) and a supine body position are well known as exacerbating factors for obstructive sleep apnea (OSA). Males, as well as older individuals, are generally more predominantly affected with OSA than females. Usually OSA is worse in REM sleep than in NREM sleep, although this is not always true.

METHODS: A retrospective study of sequential patients from one month's admission to a single sleep laboratory was conducted. We determined the age, sex, BMI, body position, duration of apnea, amount of time spent in REM and oxygen desaturation in patients who had a higher NREM AHI than REM AHI versus those who had a higher REM AHI than NREM AHI. To minimize variability, the sleep studies were scored by a single individual.

RESULTS: A higher NREM AHI than REM AHI was found in 50% of the 66 patients with OSA. Males predominated in each group and there was no age difference between the groups. Although AHI for both groups tended to become higher with an increase in BMI, the BMI was not statistically different between the two experimental groups. OSA was worse in the supine position in both experimental groups consistent with previous literature. Percentage of time spent in REM or the duration of the apnea did not determine whether a patient fell into the NREM AHI>REM AHI group versus the REM AHI>NREM AHI. The severity of oxygen desaturation was not significantly different between the NREM AHI>REM AHI versus the REM AHI>NREM AHI group. However, when we combined and analyzed the two groups as a whole, the apneas were longer in REM, consistent with previous literature.

CONCLUSIONS: Although it is well known that OSA is generally worse in REM sleep because of the degree of desaturation and duration of apnea, a higher NREM AHI than REM AHI is found in up to one half of individuals. Most of the usual predictors for severity of OSA as a whole did not discriminate these groups. Further work needs to be done to determine the factors that discriminate these two groups and thus make AHI higher in NREM than REM.