Reflective oxygen saturation monitoring at hypothenar and its validation by human hypoxia experiment
© Guo et al. 2015
Received: 28 January 2015
Accepted: 27 July 2015
Published: 5 August 2015
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.
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.
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.
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.
As claimed by American Heart Association, cardiovascular diseases have been the leading cause of death . It is very critical to monitor patient’s physiological signals [such as pulse oxygen saturation (SpO2)] continuously and noninvasively before diagnosis or treatment of cardiovascular diseases. However, traditional monitoring methods are usually conducted in a short time window, which are likely to lose signals of transient events that may be of profound prognostic or therapeutic importance. To solve this problem, some wearable health monitoring devices (WHMDs) have been developed to monitor specific signals continuously and automatically .
In recent years, pulse oximetry has been extensively used in WHMDs to measure SpO2 and heart rate noninvasively . It was invented in the 1970s  and has been continuously improved since then, which is based on the detection of subcutaneous blood perfusion by irradiating light into the skin. The subdermal blood volume changes due to arterial pulsations modify the absorption, reflection or scattering of the incident light. Consequently, the fluctuation of resultant reflective/transmittal light intensity, i.e. photoplethysmograph (PPG), can indicate heart rate and other hemodynamic parameters that are related to local blood volume changes. Through the different spectral absorption coefficients of oxygenated and non-oxygenated blood, SpO2 can also be measured by using multiple wavelengths (pulse oximetry) .
Commercial PPG sensors usually work in transmission mode that needs the incident light sent by their emitters, red and infrared light-emitting diodes (LED), to penetrate the tissue at measuring sites to reach their detectors, photodiodes (PD). This work mode has its limitations—it can only be used in body regions which are not opaque to the light, such as the fingertip and earlobe. Unfortunately, both of these two regions are not ideal sensor locations for continuous monitoring. The former is occupied in most daily activities and the latter usually requires a clip which may cause discomfort during long time measurements . While a reflective PPG sensor uses the back-scattered or reflected light to measure, it can be chosen to overcome those limitations. It allows the LED and PD to be mounted next to each other on the same planar surface and thus enables monitoring SpO2 at multiple locations of the body where transmission measurements are not practical.
Based on the reflective technology, a number of wearable pulse oximeters (WPOs) have been developed during the past decade. One challenge to WPO design is how to balance comfortable wearing and reliable attachment. Patterson et al.  developed a multichannel PPG sensor placing around the ear, which needed a wired connection with processing module carried somewhere else on the body and thus is not convenient for wearing. This problem also exists in the design of Buschmann et al. , which embedded a SpO2 sensor inside an ear mould for measurement in the external auditory canal. Mendelson et al.  reported a more integrated solution that assembled the senor, power source and data handling in one module fixed on the forehead by a bandage around the head. To ensure good signal quality and reliable sensor attachment, the bandage should not be tied too loose, limiting the wearing comfort. Haahr et al.  devised a WPO in the form of an electronic patch, which contains an adhesive material that can unite it to the body. This patch is a single unit without wires and does not limit movements. However, confined to a small size, it only carries a coin size battery and thereby its duration of wireless data transmission is restricted.
This study presents a new integrated solution for WPO, which incorporated the sensor, power supply, data processing and wireless transmitting into a glove, realizing SpO2 measurement at the hypothenar. By this form, the sensor can be attached reliably during daily activities without limiting the movements of hand and with little discomfort for long-time wearing. Methods to decrease power consumption, such as decreasing the LED driving current, were applied in this system to achieve a long period of measurement with wireless communication. The device was evaluated under various levels of hypoxia and the results demonstrated that determination of oxygen saturation at hypothenar by this system is feasible. However, monitoring SpO2 at this location is vulnerable to motion interference. Therefore, accelerometer-based adaptive noise cancellation (ANC) methods using least mean squares (LMS) and recursive least squares (RLS) algorithms were adopted and evaluated on their effectiveness of suppressing motion artifacts.
Configuration of the system
Principle of measurement
The system employs a dual LED with two wavelengths—one (λ1 = 660 nm) is below the isosbestic point (λiso = 805 nm) and the other (λ2 = 905 nm) above. In this way, a considerable contrast can be achieved between oxygenated and non-oxygenated blood .
In theory, the function about R can be supposed to be a linear function . However, in practice, the relation between the R value and SpO2 must be acquired from “real” calibration measurements, because the theoretical hypotheses are relatively simple.
Control flow of the system
The current is converted into a proportional voltage by the trans-impedance amplifier OA0. The output of OA0 is sampled by ADC12 with a sample rate of 200 Hz. As a result, the PD signal for each LED is sampled at 100 Hz (Fig. 6). Firstly, this raw signal is fed back to control the brightness of the LEDs such that the PD outputs corresponding to both LEDs match each other with a small tolerance. Because DC component makes up more than 98% of the total output strength (AC + DC) , the red and infrared DC components will be approximately equal to each other and can be neglected in the calculation of SpO2 (Eq. 1). Secondly, the raw signal is fed into a DC tracking unit using a digital tracking algorithm to pick out its DC component. This output is connected with the negative terminal of the amplifier OA1, while the raw signal connected with the positive one. As OA1 would only amplify the difference between its two terminals, the DC portion of the signal is subtracted and only the AC portion, i.e., the pulse wave is extracted and amplified. The pulse wave is sampled by ADC12 and utilized to compute SpO2 in the next step. At the same time, the acceleration signals at three orthogonal directions measured by the tri-axial accelerometer are sampled by another three ADC12 s. The processed data are transmitted to an upper computer via the Bluetooth module.
In vivo calibration
Because the function of SpO2 about R value can not be precisely analyzed in theory, pulse oximeters have to be calibrated with the help of an in vivo experiment nowadays . In that process, the pulse oximeter is applied to the participants and their arterial oxygen saturation (SaO2) is analyzed by blood-gas analysis (BGA). The participants’ blood oxygen levels are decreased by reducing the amount of their oxygen inhalation. The R value (Eq. 1) of the pulse oximeter and the corresponding SaO2 are recorded at the same time, and the relationship between them can then be determined. The human experiment also can be employed to examine the accuracy of a pulse oximeter by comparing its measurement (SpO2) to the SaO2.
We performed two human hypoxia experiments for the glove pulse oximeter at the People’s Liberation Army (PLA) General Hospital (Beijing, China). The first one calibrated the function of SpO2 about the R value and the second validated the accuracy. Twenty healthy participants (nonsmokers) were recruited for this study according to the recommendation of the International Standardization Organization . The calibration group was comprised of three females and seven males ranging in age from 18 to 31 years old, while the validation group comprised of four females and six males from 20 to 28 years old. The study was approved by the hospital’s Ethics Committee and written informed consent was obtained from all participants.
By fitting the reference SaO2s on the corresponding R values, the function of SpO2 about R of the system was calibrated and then implemented in the firmware. After calibrated, the system was employed to measure the SpO2s of the participants in the following validation experiment, while the SaO2s were also sampled at the same time. The relationship between the SpO2 and SaO2 values (n = 250) was analyzed by Pearson correlation and linear regression. The accuracy was calculated based on the deviation between them.
Accelerometer-based adaptive motion artifact cancellation
PPG signals are susceptible to motion interferences, leading to damaged accuracy of SpO2 measurement. The most promising approach that can be realized in real-time to reduce motion artifacts is adaptive noise cancellation (ANC) . The method utilizes acceleration signals as a reference to the motion artifact components present in the corrupted PPG signals.
A primary advantage of the LMS algorithm is that a lower number of computations are required relative to RLS. However, RLS algorithm provides a faster learning rate and can obtain a smaller error at the cost of longer execution time . To compare these two algorithms, they were implemented offline in Matlab to process the corrupted PPG data. The signals from the accelerometer (Fig. 4) at three orthogonal axes were summed and filtered by a 6th order Butterworth band-pass filter (fc1 = 0.5 Hz, fc2 = 5.0 Hz) to obtain the AC component. This signal was provided as the reference noise signal x (n) to the LMS and RLS ANC, while the corrupted PPG signal given as the desired signal d(n). The filter tap-weight vector w (n) of both algorithms were initialized to 0. The initial inverse covariance matrix P(0) of the RLS was set as 0.1I, where I is an identity matrix. The step-size μ of the LMS algorithm and the forgetting factor τ of the RLS algorithm were chosen to be 0.016 and 0.99, respectively. Thirty-seven segments of corrupted PPG signals taken from the validation experiment were processed by the two algorithms. For each segment, the SpO2 root mean square error (RMSE) was quantified based on the differences between the processed results and the measurements of the reference Masimo pulse oximeter. Then the mean and stand deviation of the RMSEs for all segments were calculated to evaluate the performance of motion artifact cancellation. The efficiencies in motion resistance of these two algorithms with different filter order M were evaluated by the trade-off between their performance and complexity of computation.
In vivo calibration and evaluation of accuracy
Coefficients of all individual calibration curves
Motion artifact cancellation
A novel pulse oximetry system in form of a glove has been developed and clinically tested in a human hypoxia study. In contrast to classical transmissive mode measurement devices, the new system uses a reflective mode realizing measurement at hypothenar. This system was light-weighted and easy to wear. The feasibility of SpO2 measurement with the system has been demonstrated in two human hypoxia experiments (n = 20). First, the function of SpO2 on R value was calibrated using a second-order polynomial for 10 healthy participants in the calibration experiment. Because the PPG signal was properly detected at all hypoxia levels, each individual function was acquired with a small RMSE and strong correlation, except no 7. This was mainly because the glove’s size was not suitable for the subject’s hand and the sensor was not attached at hypothenar very well. If a series of gloves with different sizes were manufactured, the sensor would be guaranteed to fit different individuals and the calibrated accuracy would be improved further. Nevertheless, the deviation between those calibration functions was not significant and thus we can achieve a general calibrated function on all the points got in the calibration experiment. Second, this function was implemented in MCU and the accuracy was verified in the validation experiment for another 10 healthy participants. Because the SaO2 samples lower than 70% were difficult to be obtained in the calibration experiment, the function for this range was not calibrated precisely. Hence, when SaO2 was lower than 70%, the proved accuracy was not favorable. However, because the SaO2 samples higher than 70% accounted for the majority of the total ones and the accuracy for this range was relatively high, the system’s global accuracy was 2.34% for the full normal measurement range. In addition, the accuracy proved in the validation experiment was consistent with the RMSE (2.27%) of the general function obtained in the calibration experiment. This indicates that the custom pulse oximeter can achieve a repeatable SpO2 measurement with a good accuracy for different individuals and SpO2 levels. The accuracy of the system is acceptable according to the defined level in  (<4%). However, in addition to individual physiological differences (e.g., different thickness of each layer of skin), the light crosstalk between the LED and PD of the sensor may lead to the deviation of measurement among individuals. If methods to avoid this crosstalk, such as set a barrier, were applied, the accuracy of the system would be increased.
The total current needed at normal work mode was 8 mA. Since the capacity of the battery is 600 mAh, the system is able to work for at least 60 h on a single charge. To improve the performance of the system during motion, we investigated the effectiveness of the LMS and RLS ANC algorithms with a tri-axial accelerometer as the noise reference input. Analysis of the data processed by them showed that ANC implemented using the LMS and RLS algorithms can help to improve the accuracy, as shown in Fig. 17. We also found that the degree of improvement depends on the filter order M used to implement each adaptive algorithm (Fig. 18). The trade-off between computation complexity and performance is important since our goal is to implement ANC to resist motion artifact in real-time. For example, an implementation based on a 24th order filter would provide an acceptable error reduction, which implies that the LMS algorithm will require only 24 operations compared to 576 operations that will be required by an RLS algorithm. The motion-resistant effects of both algorithms were limited and especially damaged when the frequency and amplitude of movements varied in a dramatically stochastic way. This could be caused mainly by the unstable performances of both algorithms for treating various MAs with fixed parameters, i.e., the step size and forgetting factor. For better motion-resistant performance, deeper work should be done to study the effects on PPG imposed by different kinds of movements, such as translational and rotational motions, in addition with the algorithms’ parameters corresponding to these movements.
The glove pulse oximeter measures SpO2 at the hypothenar of hand and has to face a wide range of motion interferences, although some methods have been adopted to suppress MAs. However, the glove form can ensure a reliable sensor attachment with least discomfort for long-time monitoring and we think the system may be applicable for sleep monitoring.
We have developed a new pulse oximetry system measuring PPG and SpO2 at hypothenar based on a reflective sensor. The accuracy and repeatability of SpO2 measurement with this system has been proved in the human hypoxia experiment. With small size and low weight, the system was integrated into a glove for easy wearing. Furthermore, it can work for a long time in wireless communication mode. We applied the LMS and RLS adaptive algorithms with the acceleration signal as the reference noise to suppress the motion artifact. Although more work should be done to improve the performance for resisting motion interference in real-time, this novel system promises to be a wearable wireless solution for healthcare monitoring.
TG contributed to development of the system, experimental design, acquisition, analysis and interpretation of data and drafting the manuscript. ZC participated in coordination and helped to mathematic calculation and interpretation of data. ZZ has participated in the experiment, data analysis, and helped draft and revise the manuscript. DL and MY conceived of the study and participated in its design and coordination. All authors read and approved the final manuscript.
This work was partially supported by the Key Technologies Research and Development Program of China under the Grant of 2011BAI02B08 and 2013BAI03B02.
Compliance with ethical guidelines
Competing interests The authors declare that they have no competing interests.
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