Reflective oxygen saturation monitoring at hypothenar and its validation by human hypoxia experiment

Abstract

Background: Pulse oxygen saturation (SpO2) is an important parameter for healthcare, and wearable sensors and systems for SpO2 monitoring have become increasingly popular. The aim of this paper is to develop a novel SpO2 monitoring system, which detects photoplethysmographic (PPG) signals at hypothenar with a reflection-mode sensor embedded into a glove.

Methods: A special photo-detector section was designed with two photodiodes arranged symmetrically to the red and infrared light-emitting diodes (LED) to enhance the signal quality. The reflective sensor was placed in a soft silicon substrate sewn in a glove to fit the surface of the hypothenar. To lower the power consumption, the LED driving current was reduced and energy-efficient electronic components were applied. The performance for PPG signal detection and SpO2 monitoring was evaluated by human hypoxia experiments. Accelerometer-based adaptive noise cancellation (ANC) methods applying the least mean squares (LMS) and recursive least squares (RLS) algorithms were studied to suppress motion artifact.

Results: A total of 20 subjects participated in the hypoxia experiment. The degree of comfort for wearing this system was accepted by them. The PPG signals were detected effectively at SpO2 levels from about 100-70%. The experiment validated the accuracy of the system was 2.34%, compared to the invasive measurements. Both the LMS and RLS algorithms improved the performance during motion. The total current consumed by the system was only 8 mA.

Conclusions: It is feasible to detect PPG signal and monitor SpO2 at the location of hypothenar. This novel system can achieve reliable SpO2 measurements at different SpO2 levels and on different individuals. The system is light-weighted, easy to wear and power-saving. It has the potential to be a solution for wearable monitoring, although more work should be conducted to improve the motion-resistant performance significantly.

Fig. 1

Overview of the system. a The sensor node. b Inside view of the glove and position of the sensor. c Wearing appearance of the system.

Fig. 2

Characteristic PPG curve due to variations in dermal blood volume. I_AC represents the alternating component of the signal and I_DC the constant component.

Fig. 3

Structure of the sensor and photon migration. Photons ejected from LED get to PD through a parabolic trace.

Fig. 4

Circuit diagram of system. The system contains five main parts—LED driving unit (H-bridge), MCU, Bluetooth, power management and accelerometer.

Fig. 5

Control flow diagram. The red and infrared LEDs are activated alternatively. When each LED is on, the corresponding PD signal is sampled and the DC component in it is subtracted by DC tracking. The left red and infrared AC components are used to calculate SpO₂.

Fig. 6

Timing and duty cycle of the LEDs. The red and purple pulses represent the PD signals when the red and infrared LEDs are switched on, respectively. The blue pulse indicates the activation of ADC.

Fig. 7

Hypoxia experiment design. a Cabin to make low oxygen conditions. b Design of the hypoxia study. c Protocol to obtain the relationship between the R value and the SaO₂.

Fig. 8

Block diagram of adaptive filter. The reference noise x(n) is passed through a delay line (represented by Z blocks). The tap-weights (w _i (n)) multiply the delayed x(n − i), which are summed to form y(n). Because y(n) approaches the true noise in d(n), the difference between them ε(n) form the output with the noise cancelled. The adaptive algorithm (LMS or RLS) regulates w _i (n) based on ε(n).

Fig. 9

Two periods of pulse wave. a Red and infrared pulse waves when SaO₂ is 98%. b SaO₂ is 72%.

Fig. 10

R curve of one subject in a calibration experiment. For one subject, SaO₂ was sampled 25 times for the full range while the R curve was recorded simultaneously.

Fig. 11

An individual calibration curve of SpO₂ on the R values. For this subject, the function of SpO₂ about R was got by fit the reference SaO₂ on R using second-order polynomial (RMSE = 1.07%, ρ ² = 0.99).

Fig. 12

Individual calibration curves of all the participants in the calibration experiment. All the subjects’ calibration curves were consistent with each other for the full SaO₂ range, except less than 70%.

Fig. 13

Curve of SpO₂ on the R values of the calibration experiment. All the 250 SaO₂ and R values were utilized to fit (RMSE = 2.27%, ρ ² = 0.95).

Fig. 14

Correlation and regression line of SpO₂ with SaO₂ in the calibration experiment. The Pearson correlation coefficient of SpO₂ with SaO₂ was 0.97.

Fig. 15

Errors between SpO₂ and SaO₂ for different intervals covering 60–100%. Each column’s width means an interval of 10% SaO₂. Error bars indicate ±1 SD of the deviations for corresponding intervals.

Fig. 16

Bland–Altman graph of SpO₂ versus SaO₂ of the validation experiment. The dots represent all the deviations. The middle solid line represents the mean of deviations, while the upper and under lines indicate +2SD and −2SD, respectively.

Fig. 17

Comparison to the reference Masimo equipment. A complete comparision between the system’s and Masimo’s measurement traces for one subject in the validation experiment.

Fig. 18

Representative raw PPG, adaptively filtered PPG and hand acceleration signals. a Typical infrared PPG signals before and b after processing by the ANC algorithm (LMS, M = 24). c Typical red PPG signal before and d after processing by the ANC algorithm (LMS, M = 24). e The corresponding reference noise obtained simultaneously from the accelerometer.

Fig. 19

SpO₂ measurements processed by ANC compared to Masimo. The filter order M of LMS and RLS are both 24.

Fig. 20

SpO₂ RMSEs for different filter orders. Error bars indicate ±1 SD of RMSEs for 37 segments’ errors between the glove pulse oximeter and Masimo measurements processed with varying filter orders M. M = 0 means the error obtained without ANC.

Fig. 21

Drive current of the LEDs measured at work. Red and infrared LEDs were turned on alternately to save power.