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

Slow-Wave Activity in Sleep Apnea Patients Before and After Continuous Positive Airway Pressure Treatment: MSLTThe MSLT consists of five opportunities to nap administered at 10:00 am, 12:00 noon, 2:00 pm, 4:00 pm, and 6:00 pm.18 As for PSG, sleep onset in the modified MSLT was defined as three consecutive epochs (1 min) of stage 1 sleep or one epoch (20 s) of any other sleep stage. Participants were awakened after 10 min of sleep, or the test was stopped after 20 min if they did not fall asleep. Participants were not allowed to drink alcohol or beverages containing caffeine, nor were they allowed to sleep between the five tests.
EEG Spectral Analysis
EEGs were low-pass filtered and digitized on-line at a sampling rate of 128 Hz. Quantitative analysis of the EEG was performed by fast Fourier transform calculated on 4-s miniepochs for the nights preceding the MSLT. SWA was defined as the power (in microvolts squared) in the 0.75- to 4.5-Hz frequency band. The 4-s mini-epochs containing an artifact were rejected and were considered as missing data to preserve sleep continuity. Two visual inspections were performed according to two different criteria. First, “classical artifacts” such as movement, ocular, or muscle artifacts were removed. The second time, “prearousal” slow waves distinguishable from the background activity that occurred from 4 s prior, to 8 s after the end of the respiratory events were also removed. An example of these prearousal slow waves is shown in Figure 1. Interrater reliability between two experienced scorers was tested for the two patients with the highest AHI. To do so, a homemade computer program compares the scorings of the two scorers epoch by epoch and determines the percentage of similarly scored epoch. Between-scorer correlation rates of 98% and 93% were obtained for the first and the second patients, respectively.
Total SWA was calculated by adding the power of all valid NREM sleep epochs for the entire night. The time course of SWA was standardized for each subject by subdividing each NREM episode into 20 equal intervals and each REM episode into 5 intervals. Data were then averaged per subject to also obtain the mean SWA per sleep cycle. Sleep cycles were scored according to the criteria of Feinberg and Floyd. A cycle was defined by the succession of a NREM sleep episode lasting at least 15 min followed by a REM episode of at least 5-min duration. A NREM episode was defined as the time interval between the first occurrence of stage 1 sleep and the first occurrence of REM within a cycle. It has to be followed by a REM episode to be considered complete. Only the first three completed NREM episodes were included in the calculations.
Figure 1. Examples of prearousal slow waves following an apnea (marked by a box) rejected as artifacts before computation of SWA. EMG = electromyogram; ROC = right outer canthus; LOC = left outer canthus; Therm = thermistor.