Comparison between Speckle Plethysmography and Photoplethysmography during Cold Pressor Test Referenced to Finger Arterial Pressure

Figures

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  Figure 1. Flow diagram of the experimental protocol and data analysis of the experiments at baseline and CPT phases.

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  Figure 2. Example from Subject 1 of a 15 s period of the 4 simultaneous signals extracted from the videos. The signals shown were low pass filtered (10 Hz) and inverted. Note the quasiperiodic nature of SPG and PPG. The fast pulse waves originate from the heartbeat (arterial component dominates), the slower baseline variations mainly originate from respiration (venous component dominates).

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  Figure 3. Processing pipeline for AC and SNR calculation.

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  Figure 4. Typical spectrum of SPG from a single 10 s window in blue, with superimposed binary spectral mask (orange) describing bands containing signal (“1”) and noise (“0”). Numbering of the binary spectral mask refers to: 1—Fundamental, 2—Second harmonic, 3—Third harmonic, 4—Fourth harmonic, 5—Fifth harmonic.

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  Figure 5. Processing pipeline for the selection of the representative beat.

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  Figure 6. Representative SPG, PPG, and fiAP beats per subject, measured during the baseline test phase. The illumination chosen for this analysis was a reflective 850 nm laser diode. The representative beats are not simultaneous between the different signals.

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  Figure 7. Inter-subject average values and 95% CI of AC-amplitude of the signals extracted from the videos during baseline and CPT phases.

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  Figure 8. Inter-subject average values and 95% CI of AC-amplitude of the fiAP signals during baseline and CPT phases.

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  Figure 9. Percentage of AC-amplitude change between baseline and CPT on each of the experimental and reference signals for all subjects. p-values from Wilcoxon test prove the significance of the rejection of the null-hypothesis, indicating that AC-amplitude is significantly higher at baseline than at CPT phase at 850 nm. The term “ns” refers to non-significant.

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  Figure 10. Example of SPG and PPG signals at different quality levels. From top to bottom: 18 dB, 10 dB, 5 dB, and 0 dB.

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  Figure 11. Average values and 95% CI of the SNR of the signals extracted from the videos during baseline and CPT.

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  Figure 12. All-subject overview of SPG and PPG SNR values at both wavelengths for baseline and CPT phases. p-values shown on paired Wilcoxon test, reveal significance of null-hypothesis rejection, showing that SNR SPG is significantly higher than SNR PPG.