Characterization of Sleep Using Bispectral Analysis

This study investigated the relationship between a Bispectral Index (BIS), as measured by Aspect Medical Systems’ A-1000 EEG monitor, and clinical sleep staging based on the Rechtschaffen and Kales (RK) scoring procedure. Eleven subjects – with no known sleep, respiratory, or cardiovascular disease were tested using full polysomnography and respiratory monitoring with simultaneous recording of BIS, in an accredited sleep laboratory in Ft. Worth, Texas. An ordinal scale was assigned to RK-based sleep stages: Wake = 1, REM = 2, Stage 1 = 3, Stage 2 = 4, Stage 3 = 5 and Stage 4= 6. By performing linear regression between average BIS values and assigned scales, it was found that average BIS values were highly correlated with non-rapid eye movement (NREM) sleep stages and wakefulness, both within (average R ± one standard deviation = 0.9376 ± 0.0264) and across subjects (R = 0.896). However, there was a considerable amount of inter-subject variability about the average BIS value for each sleep stage. The mean BIS values for each sleep stage across all subjects were: Wake: 87.057 ± 12.967 (97.7 – 17.4); REM: 78.081 ± 10.690 (95.2 27.5); Stage 1: 79.452 ± 13.337 (97.7 25.5); Stage 2: 66.771 ± 15.493 (97.6 24.4); Stage 3: 41.277 ± 9.280 (87.6 16.9); and Stage 4: 33.832 ± 8.021 (72.3 13.1). Across all subjects, only the mean BIS values for REM and Stage 1 were not significantly (P < 0.05) different from each other. These results indicate that BIS alone may not be adequate for automated sleep staging.