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

Between-group differences in sleep variables and in total SWA were assessed by either Mann-Whitney U tests (control subjects vs SAS patients) or Wilcoxon matched-pair tests (treated vs untreated SAS patients). A two-way analysis of variance (ANOVA) with one independent and one repeated measure was used to compare SWA between SAS patients and control subjects for three successive NREM episodes. A two-way ANOVA with two repeated measures was used to compare SAS patients before and after treatment for three successive NREM episodes. The degrees of freedom were corrected according to Huynh-Feldt adjustments for sphericity violation. Post hoc comparisons were performed for the three episodes. Because 3 untreated patients did not complete their third cycle, the ANOVAs were performed using only 7 patients (before and after treatment and for all cycles) and 10 control subjects. emergency severe sepsis
In order to assess the relationship between the MSLT and different sleep parameters including SWA, Pearson product-moment correlations (unilateral) were used. Wilcoxon matched-pair tests were performed to compare sleep parameters and MSLT results before and after CPAP treatment, and Mann-Whitney U tests to compare control subjects with SAS patients before treatment and with patients after CPAP treatment. Data are presented as mean ± SEM. All statistic analyses have been performed using a software package (Statistica 5.1; StatSoft; Tulsa, OK).
Results
Results of PSG recordings are shown in Table 1. A shorter total sleep time was seen in SAS patients before treatment, compared to normal control subjects. SAS patients also had more stage 1 sleep and less stage REM sleep; they also fell asleep more rapidly on the MSLT. Stage 2 sleep, stages 3 and 4 sleep, and sleep efficiency were not statistically different between groups. SAS patients presented respiratory impairments (a mean AHI > 50 events/h, a mean time spent with Sa02 < 90% of 115 min, and a mean minimum of Sa02 of 63.9%) obviously not present in the control group. After CPAP treatment, both respiratory and sleep variables returned to normal values; there was no significant difference between posttreatment values and those of control subjects (Table 1).
The accumulation of all SWA during NREM sleep for the entire night was not statistically different for controls and untreated SAS patients. A statistically significant difference in total SWA was found, however, between pretreatment and posttreatment values in SAS patients (1,164,390 ± 528,219 ^V2 vs 1,425.031 ± 591.825 ^V2; Wilcoxon, p = 0.05).
Table 1—Sleep Parameters for SAS Patients (Pretreatment), CPAP-Treated SAS Patients, and Control Subjects

Parameters SAS CPAP ControlSubjects SAS vs CPAP, Wilcoxon SAS vs Control Subjects, Mann-Whitney CPAP vs Control Subjects, Mann-Whitney
TST, min 426 ± 11.8 420 ± 12.0 465 ± 9.7 NS 0.02 0.02
Sleep efficiency, % 88.68 ± 1.58 91.22 ± 0.92 90.3 ± 1.43 NS NS NS
Stage 1, % 21.71 ± 2.88 10.42 ± 0.85 11.9 ± 1.19 0.005 0.008 NS
Stage 2, % 64.40 ± 2.15 65.43 ± 1.64 63.3 ± 2.56 NS NS NS
Stage 3 and 4, % 3.21 ± 1.05 4.82 ± 1.22 4.86 ± 1.88 NS NS NS
Stage REM, % 10.66 ± 1.06 19.33 ± 1.36 19.39 ± 1.41 0.005 0.0003 NS
Microarousals index 43.50 ± 7.34 9.0 ± 1.35 10.6 ± 1.82 0.005 0.0007 NS
AHI, events/h 54.65 ± 7.18 1.68 ± 0.81 0.66 ± 0.33 0.005 0.0004 NS
Sao2 < 90%, min 115.6 ± 33.8 0.12 ± 0.12 0.07 ± 0.05 0.005 0.001 NS
Minimum Sao2, % 63.8 ± 4.4 90.8 ± 0.5 91.8 ± 0.6 0.005 0.001 NS
MSLT, min 3.99 ± 0.54 9.97 ± 1.43 12.71 ± 0.79 0.005 0.0006 NS