Multi-wavelength photoplethysmography method for skin arterial pulse extraction

Abstract

In this work, we present a multi-wavelength (MW) PPG method exploiting the wavelength dependence of light penetration in skin tissue to provide depth resolution of skin blood pulsation. The MW PPG system requires two to three light sources in different wavelengths and extracts the arterial blood pulsation through a multi-wavelength multi-layer light-skin interaction model, which removes the capillary pulsation (determined from the short-wavelength PPG signal) from the long-wavelength PPG signal using absorption weighting factors that are quasi-analytically calibrated. The extracted pulsations are used to calculate blood pressure (BP) through pulse transit time (PTT), and the results are compared with those obtained from the single wavelength PPG method. The comparative study is clinically performed on 20 subjects including 10 patients diagnosed with cardiovascular diseases and 10 healthy subjects. The result demonstrates that the MW PPG method significantly improves the measurement accuracy of systolic BP (SBP), reducing the mean absolute difference between the reference and the estimated SBP values from 5.7 mmHg (for single-wavelength PPG) to 2.9 mmHg (for three-wavelength PPG).

Keywords: (070.6020) Continuous optical signal processing; (170.1470) Blood or tissue constituent monitoring.

Figures

Fig. 1

Schematic of skin vasculature and MW reflectance PPG measurement.

Fig. 2

Illustration on the inaccurate determination of arterial PTT by the peripheral PTT measured by peripheral PPG.

Fig. 3

(a) Two-layer and two-wavelength light tissue interaction model; and (b) Three-layer and three-wavelength light tissue interaction model.

Fig. 4

Schematic of the MW PPG measurement in reflection mode and the component alignment on the sensor probe.

Fig. 5

Illustration on key parameters extraction from a set of sample signals.

Fig. 6

The workflow for calculating D_PTT from ECG and MW PPG.

Fig. 7

(a) and (b) Calibration of the two-layer and two-wavelength (IR and green) light tissue interaction model; the high and (c) and (d) Calibration of three-layer and three-wavelength (IR, yellow and green) light tissue interaction model. The white circles in dotted line indicate the stable states corresponding to the stage that removes capillary pulsation in Layer 1 and the arteriole pulsation in Layer 2.

Fig. 8

Bar plot of the average absolute correlation coeffcients between reference SBP and IR_PTT, reference SBP and D2_PTT, and reference SBP and D3_PTT on diseased subjects, healthy subjects and all subjects. ‘*’ represents the significance in difference is p<0.05 and ‘**’ represents the significance in difference is p<0.01.

Fig. 9

(a)-(c): Scatter plots of the reference SBP measured by Finapres versus the estimated SBP derived from IR_PTT, D2_PTT and D3_PTT respectively; (d)-(f) Bland-Altman plots for the reference SBP and estimated SBP. The figures include the data obtained for all the subjects. In (a)-(c), R-square is the square of the Pearson’s correlation coefficient between the reference SBP and estimated SBP, and MAD is the mean absolute difference of the reference SBP and estimated SBP. In (d)-(e), the grey solid line indicates bias, or the mean of the differences (MD) between reference and estimated SBP, and the dotted grey line indicates the limits of agreement which are MD ± 1.96* SD(standard deviation of the differences between reference and estimated SBP).

Fig. 10

Comparison of the pulsation index P_idx(W) variations when ECG (black), IR PPG (red) and Green PPG (green) are used as the calibration reference signal, respectively.