Entropy Measures as Descriptors to Identify Apneas in Rheoencephalographic Signals

Figures

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  Figure 1. Filtered rheoencephalography (REG) signal collected during breath holding.

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  Figure 2. Corrected conditional entropy (CCE(N, m, ε)) values of apnea and baseline recordings as a function of (a) the quantification intervals (ε), (b) the embedding dimension (m) and (c) the signal length (N). The corresponding statistical significance (p-value) of the differences between apnea and baseline recordings is presented in (d), (e) and (f), respectively.

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  Figure 3. The influence of the signal length and the number of quantization levels in the regularity index (ρ) is analyzed in: (a) values of ρ as a function of the number of quantification intervals (ε) and (b) values of ρ as a function of the signal length (N). The results of the statistical analysis (p-value) comparing apnea and baseline signals using this entropy indexes are shown in: (c) p-values versus the number of quantification intervals and (d) p-values versus the signal length.

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  Figure 4. Values of the entropy CCE (ε = 20; N = 2000) as a function of the embedding dimension m for all apnea (a) and baseline (b) recordings, including their median values (thick black line).

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  Figure 5. Values of the entropies (a) ApEn, (b) SampEn and (c) FuzzyEn as a function of the number of samples (N) and the dimension (m) for apnea (solid line) and baseline segments (dashed line).

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  Figure 6. Entropy values of ApEn(2,r,2000), SampEn(2,r,2000) and FuzzyEn(2,2,r,2000) as a function of r for apnea and baseline recordings (a–c) and the corresponding p-values (d–f).

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  Figure 7. (a) Median FuzzyEn values as a function of n, including the 25–75 interquartile range (colored area); (b) standard deviation of FuzzyEn as a function of n; (c) p-value obtained comparing FuzzyEn values in apnea and baseline groups as a function of n.

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  Figure 8. Standard deviation of ApEn(2, r, 2000), SampEn(2, r, 2000) and FuzzyEn(2, 2, r, 2000) as a function of r comparing baseline and apnea segments.

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  Figure 9. Receiver operating characteristic (ROC) curves of all entropy metrics providing statistically significant differences between apnea and baseline recordings.

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  Figure 10. Boxplot of all selected entropy metrics, showing the median values (horizontal red lines) and outliers (red crosses).