Odds ratio product of sleep EEG as a continuous measure of sleep state

Abstract

Study objectives: To develop and validate an algorithm that provides a continuous estimate of sleep depth from the electroencephalogram (EEG).

Design: Retrospective analysis of polysomnograms.

Setting: Research laboratory.

Participants: 114 patients who underwent clinical polysomnography in sleep centers at the University of Manitoba (n = 58) and the University of Calgary (n = 56).

Interventions: None.

Measurements and results: Power spectrum of EEG was determined in 3-second epochs and divided into delta, theta, alpha-sigma, and beta frequency bands. The range of powers in each band was divided into 10 aliquots. EEG patterns were assigned a 4-digit number that reflects the relative power in the 4 frequency ranges (10,000 possible patterns). Probability of each pattern occurring in 30-s epochs staged awake was determined, resulting in a continuous probability value from 0% to 100%. This was divided by 40 (% of epochs staged awake) producing the odds ratio product (ORP), with a range of 0-2.5. In validation testing, average ORP decreased progressively as EEG progressed from wakefulness (2.19 ± 0.29) to stage N3 (0.13 ± 0.05). ORP < 1.0 predicted sleep and ORP > 2.0 predicted wakefulness in > 95% of 30-s epochs. Epochs with intermediate ORP occurred in unstable sleep with a high arousal index (> 70/h) and were subject to much interrater scoring variability. There was an excellent correlation (r(2) = 0.98) between ORP in current 30-s epochs and the likelihood of arousal or awakening occurring in the next 30-s epoch.

Conclusions: Our results support the use of the odds ratio product (ORP) as a continuous measure of sleep depth.

Keywords: ORP; Sleep quality; odds ratio product; sleep depth; sleep staging.

© 2015 Associated Professional Sleep Societies, LLC.

Figures

Figure 1

Percent of 3-s epochs falling within 30-s epochs staged manually as awake (% Awake) as a function of relative power in different frequency ranges. (A) % Awake decreases as theta power increases but the relationship is markedly affected by the concurrent beta power. A high delta power increases % Awake when % Awake is high according to theta/beta powers and decreases it when theta/beta powers are in a low % Awake (Block arrows). (B,C) Impact of alpha power on % awake at different theta-beta combinations. Note that the same % Awake is observed at highly varied EEG spectral patterns.

Figure 2

Frequency distribution of 30-s epochs with different average odds ratio product (ORP) in the entire validation set. Within each bar white and black segments are epochs staged asleep and awake, respectively, by both technologists while hatched segments are epochs receiving a split awake/asleep decision.

Figure 3

Top panel: Sleep histogram from a representative patient spanning the first 3 hours of a study. Abscissa, epoch #. Arousals are indicated with a plus sign (+). The second panel gives the corresponding odds ratio product (ORP) values. Each dot is a 30-s epoch. Note that ORP rises to near 2.5 with every awakening and then declines gradually. Note also that stage N3 began appearing as ORP approached 0.5 and arousals disappeared simultaneously. A to F are the points from which the tracings below were obtained. Tracings A and B were staged awake, but in A the patient was fully awake, while in B he was having short periods of sleep. Note the marked difference in ORP distribution within the two awake panels and the transient reduction in ORP during the unstable state of tracing B. Tracing C was staged N1 but it is visually substantially similar to B and has a similar average ORP. D and E were both staged N2. Their visual appearances are quite different, with D looking more like N1, but with spindles, while E looks more like N3 (F). ORP values are quite different between the two N2 epochs. Panel F was staged N3. ORP at this point was 0.34 while it was similar (0.33) in the previous tracing (E), which was staged N2. The difference was only that delta waves exceeded 6 seconds in F but not in E.

Figure 4

Average ORP values in different sleep stages. Each solid dot is a different patient. Open circles and horizontal bars are means ± standard deviation. W2, N1, N2, N3, and REM are standard Rechtschaffen and Kales stages, with W2 representing awake epochs. W1 is average of the highest 5 ORP values within 30-s epochs staged awake (see text). W/N1, N1/N2 and N2/N3 are epochs that received a split score by the 2 technologists. n = 56 for W1 to N2, 52 in N2/N3, 36 in N3 and 51 in REM. Note that average ORP in N3 was

Figure 5

Four examples of atypical awake EEG patterns that cause the odds ratio product (ORP) to be underestimated during wakefulness. C3/A2 and C4/A1 are the two central electrodes. R&L EOG, are the eye electrodes. EKG, electrocardiogram. Calibration of EEG is shown in the top panel and is the same in all panels. (A) Predominant wave frequency (6.0 Hz) is in the theta and not the alpha range. (B) EEG is very similar to N1 but there are rapid eye movements and high chin EMG activity (not shown). (C) Very low power EEG. (D) A very large T wave cardiac artifact in C3/A2 not removed by the EKG removal algorithm because of its unusual pattern and distance from the R wave, resulting in high theta power in one electrode.

Figure 6

Relationship between average ORP in current 30-s epochs and the likelihood of an arousal or awakening occurring in the next 30-s epoch (arousability index). Vertical bars are the confidence interval of the probability.

Figure 7

(A) Relationship between average ORP in different sleep stages (see legend in panel B) and the corresponding arousal/awakening index. Each dot is the average ORP in all epochs having the same sleep stage in one patient. Note the highly variable arousal/awakening index among patients in stages N1, N2 and N1/N2. Straight line, linear regression line (see inset for regression data). Dashed line is moving average showing a sigmoid relation between ORP and the index. Arrows indicate outliers that were examined in detail (Figure 8). (B) Relationship between delta power and arousal + awakening index.

Figure 8

(A) tracing showing an example from a patient in whom arousal and awakening index was low relative to ORP in stage N2 (heavy right arrow in Figure 7). ORP, odds ratio product. C3/A2 and C4/A1 are 2 central EEG electrodes. Note the frequent alpha/beta intrusions that did not meet arousal criteria because they were too brief or were not preceded by 10 s of continuous sleep. ORP was high, reflecting these intrusions. (B) Tracing showing an example from a patient in whom arousal and awakening index was high relative to ORP (heavy left arrow in Figure 7). Note the extensive intrusion by waves that appear like alpha waves but are in reality in the theta range (predominant frequency = 6.3 Hz). This causes underestimation of ORP.

Figure 9

Odds ratio product (ORP) in different sleep stages and for total sleep time and total recording time in patients with no identified pathology during the sleep study (NO) and in patients with various disorders. NA, narcolepsy; PLM, periodic limb movement disorder; INS, insomnia; OMILD, OMOD, and OSEV are patients with mild, moderate and severe obstructive sleep apnea; OHS, obesity hypoventilation syndrome; CSA, central sleep apnea. Note the wide range of ORP within each group and the considerable overlap between groups. See Table 1.