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Slow-Wave Activity in Sleep Apnea Patients Before and After Continuous Positive Airway Pressure Treatment: Discussion

Results of the present study show that there is a lower amount of SWA across the night, and especially in the first two NREM episodes, before treatment compared to posttreatment values. Comparison between control subjects and untreated apneic patients did not reach the significance, even though control subjects had values that were even higher than those of treated apneic patients. This result can first be explained by the small sample size and large SDs in SWA values for each group. The latter could be related to the large age range of subjects selected for the study, since it is known that SWA varies greatly with age.> The lack of statistically significant differences is also due to the different statistical tests used to assess the differences between conditions in apneic patients (within-group test) on one hand and between control subjects and untreated patients (between-group test) on the other hand. Nonetheless, these results showed that SWA is a more sensitive index of change in slow-wave sleep organization throughout the night than is the proportion of stages 3 and 4 sleep, which was not different from pretreatment to posttreatment recordings. These results show that the general pattern of SWA distribution across the night is normal in CPAP-treated apneic patients. These results also suggest that the decrease in SWA found in untreated apneic patients is at least partly reversible with CPAP treatment. This is consistent with previous findings of an increase in SWS with CPAP treatment. Similarly, a slowing of the EEG during wakefulness had been found in untreated apneic subjects in frontal, central, parietal, occipital, and temporal regions, which was corrected after CPAP. buy asthma inhalers online

One question that is often raised with regard to SAS is the identification of factors responsible for EDS. Many studies on SAS patients have shown a correlation between the number of arousals due to respiratory events during the night and the severity of EDS measured with the MSLT.- One study (n = 1,146) showed that the AHI was positively correlated with daytime sleepiness, but AHI explained only 11% of the variance in MSLT results. A study of 466 patients showed that arousals resulting from respiratory disturbances were a good predictor of daytime sleepiness, explaining 13% of the variance in MSLT results. Daytime sleepiness was also positively correlated with oxygen desaturation, increased respiratory efforts, and parasympathetic activation. However, another study (n = 100) failed to show any correlation between MSLT and AHI, or oxygen desaturation. It has also been shown that oxygen desaturation induced experimentally by CO2 inhalation in apneic patients treated with CPAP did not decrease sleep latency at the MSLT. However, experimentally induced mi-croarousals in healthy subjects resulted in daytime sleepiness.” In the present study, although the microarousal index was highly correlated (negatively) with the total amount of SWA (r = —0.75; p = 0.007), it was not significantly correlated with the MSLT (r = —0.04; not significant [NS]). No correlation was found between the number of respiratory events (AHI) and the severity of EDS. The MSLT test could have indeed been more sensitive to drowsiness if 30 s instead of 1 min (three epochs of 20 s) would have been used as the sleep onset criterion and if subjects had not been allowed to sleep for 10 min when they fell asleep. These points may have had an effect on the lack of correlation between the MSLT and the microarousal index or the AHI. Despite the low percentage of REM sleep found in untreated patients, which is restored after CPAP treatment, it does not appear that REM sleep percent plays a role in EDS. On the other hand, a significant correlation was found between results of the MSLT and SWA during the first sleep cycle. These results suggest that SWA may have a major predictive value of EDS as measured by the MSLT in SAS patients. Since it was not possible to match the control subjects for BMI to the apneic patients, one should keep in mind that obesity might be a confounding factor. However, this had no bearing on the fact that both MSLT and SWA values improved after treatment compared to before treatment in the apneic group, irrespective of a weight change.
This study also shows the importance of the nocturnal distribution of SWA across the night. It is not the total amount of SWA that was best correlated with the daytime vigilance, but rather the peak of SWA noted in the first part of the night. Indeed, the first NREM episode probably has a special role in sleep physiology since it is the period most affected by age, sleep loss, or sleep extension.
Taken altogether, these results suggest that the occurrence of respiratory events at night, associated with repetitive microarousals, decreases the amount of SWA across the night in patients with SAS. As a consequence of the decreased SWA, patients experience more EDS during the day. However, there was a lack of correlation between SWA and the MSLT after treatment with nasal CPAP. The MSLT value increased remarkably after successful CPAP treatment to a near normal value (mean, 9.97 ± 1.43). This result suggests that SWA may not be a major determinant of the mean sleep latency on the MSL when there is no major residual somnolence.