Available online at www.sciencedirect.com
`
`ScienceDirect
`
`ICT Express 2 (2016) 195–198
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`www.elsevier.com/locate/icte
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`Reflectance pulse oximetry: Practical issues and limitations✩
`Hooseok Lee, Hoon Ko, Jinseok Lee∗
`
`Department of Biomedical Engineering, Wonkwang University College of Medicine, Iksan, Republic of Korea
`Received 14 August 2016; received in revised form 10 October 2016; accepted 11 October 2016
`Available online 9 November 2016
`
`Abstract
`
`The demand for reflective-mode pulse oximetry to monitor oxygen saturation has been continuously increasing because it can be used at
`diverse measurement sites such as the feet, forehead, chest, and wrists. For the wrists, in particular, pulse oximeters are easily available in the form
`of a band or watch. In this study, we developed a reflectance pulse oximeter and used it to measure oxygen saturation levels at the fingertips and
`the wrist. We analyzed the performance of this oximeter to address the challenges and limitations associated with using reflective-mode oximeters
`at the wrist for clinical purposes.
`c⃝ 2016 The Korean Institute of Communications Information Sciences. Publishing Services by Elsevier B.V. This is an open access article under
`the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
`
`Keywords: Pulse oximetry; Reflective mode; Wrist; Wearable sensor
`
`1. Introduction
`
`Pulse oximetry is a noninvasive method for accurately es-
`timating oxygen saturation (SaO2) by reading the peripheral
`oxygen saturation (SpO2). As SaO2 and SpO2 are sufficiently
`correlated and pulse oximetry has the advantages of being safe,
`convenient, inexpensive, and noninvasive, this approach is clin-
`ically accepted for monitoring oxygen saturation [1,2].
`Pulse oximetry is simple to carry out; it only uses two dif-
`ferent light sources and a photodiode [3–5]. Depending on the
`measurement site, either the transmissive or the reflective mode
`can be used. In the transmissive mode, the light sources and
`photodiode are opposite to each other with the measurement
`site between them. Light then passes through the site. In the re-
`flective mode, the light sources and photodiode are on the same
`side, and light is reflected to the photodiode across the measure-
`ment site.
`Currently, the transmissive mode is the most commonly used
`method because of its high accuracy and stability. Nevertheless,
`the demand for reflective-mode oximetry is continuously in-
`creasing because it does not require a thin measurement site. It
`
`∗ Corresponding author.
`E-mail addresses: gntjr2@naver.com (H. Lee), idayfly8710@gmail.com
`(H. Ko), gonasago@wku.ac.kr (J. Lee).
`Peer review under responsibility of The Korean Institute of Communica-
`tions Information Sciences.
`✩ This paper is part of a special issue titled Special Issue on Emerging
`Technologies for Medical Diagnostics guest edited by Ki H. Chon, Sangho Ha,
`Jinseok Lee, Yunyoung Nam, Jo Woon Chong and Marco DiRienzo.
`
`can be used at diverse measurement sites such as the feet, fore-
`head, chest, and wrists. In particular, if the wrist is the available
`measurement site, pulse oximeters can be conveniently used in
`the form of a band or watch. To the best of our knowledge,
`reflectance pulse oximetry, specifically for monitoring oxygen
`saturation at the wrist, is currently not practiced clinically. In re-
`cent years, many reflectance pulse oximeters have become com-
`mercially available, but they are only for personal monitoring of
`oxygen saturation and for entertainment purposes. Obviously,
`the focus is on developing oximeters for medical purposes, and
`research is being carried out in this regard. Furthermore, most
`research papers discuss only the basic principle of pulse oxime-
`try and its utilization in smart devices; only a few papers discuss
`the challenges associated with reflective pulse oximetry.
`In this study, we developed a reflectance pulse oximeter and
`addressed the practical issues and limitations associated with
`its use. Using this oximeter, we investigated the results of AC
`amplitudes and DC offsets of the red and infrared signals, in-
`cluding modulation ratio, which are critical factors to estimate
`SpO2. We first tested our device by measuring oxygen satura-
`tion at a fingertip and wrist and analyzed its performance. Fi-
`nally, we summarized all issues and discussed the feasibility of
`using the device for clinical purposes.
`
`2. Methods
`
`2.1. Principle of SpO2 estimation
`
`Pulse oximetry measures arterial oxygen saturation based on
`the light absorption properties of blood. When it combines with
`
`http://dx.doi.org/10.1016/j.icte.2016.10.004
`2405-9595/ c⃝ 2016 The Korean Institute of Communications Information Sciences. Publishing Services by Elsevier B.V. This is an open access article under the
`CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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`H. Lee et al. / ICT Express 2 (2016) 195–198
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`Fig. 1. (a) Changes in AC R, ACIR, DC R, and DCIR with decreased SaO2. (b)
`Empirical relationship between modulation ratio R and SaO2.
`
`oxygen, deoxyhemoglobin (Hb) changes its light absorption
`characteristics. Pulse oximetry exploits the light absorption
`difference between Hb and oxygenated hemoglobin (HbO2).
`(660 nm wavelength)
`HbO2 absorbs more infrared light
`and lesser red light (940 nm wavelength) than Hb. In the
`transmissive mode, light from a pair of red and infrared light-
`emitting diodes (LEDs) is transmitted through a fingertip.
`Then, the transmitted light is received by a photodiode on the
`opposite side. The transmitted light signals consist of a direct
`current (DC) component and pulsatile alternating current (AC)
`component. Pulse oximetry calculates the modulation ratio R
`by using the DC and AC components of the red and infrared
`signals as follows:
`R = AC R /DC R
`ACIR/DCIR
`where AC R and ACIR are the AC amplitudes of the red and
`infrared signals, respectively [6,7]. DC R and DCIR are the
`DC offsets of the red and infrared signals, respectively. Then,
`empirically derived calibration curves are used to estimate
`SaO2 based on the modulation ratio R, as seen in Fig. 1(a). AC R
`and DCIR increase with decreasing SaO2, as seen in Fig. 1(b).
`On the other hand, ACIR and DC R decrease.
`
`(1)
`
`,
`
`2.2. Reflectance pulse oximeter
`
`We developed a reflective pulse oximeter where the LED
`transmission unit and photodiode receiving units are on the
`
`Fig. 2. (a) Control and data flow. (b) Developed reflectance pulse oximeter.
`
`same side. To switch between red and infrared LED lights,
`the duty cycle was set to 50% with a switching frequency
`of 500 Hz, which generated a pulse width modulation signal
`through the microcontroller unit (MCU). Then, each reflected
`light beam was converted to an electrical signal, which was
`subsequently amplified with a gain of 100. A sample-and-hold
`circuit consisting of an analog switch and operational amplifier
`then separated each amplified signal with a sampling rate of
`500 Hz from red and infrared LEDs. Each separated signal was
`then filtered using a low-pass filter with a cutoff frequency of
`10 Hz and an additional gain of 10. For this study, we used
`an MCP6004 operational amplifier, a TM4C123GHPM MCU,
`an NJL5310R photodiode, an SML-LX0805SRC-TR red LED,
`and a KP-2012F3C infrared LED. Fig. 2 shows our design and
`its implementation in reflectance pulse oximetry. The maximum
`LED current was 20 mA, and the light intensity was set to the
`maximum level of the LED capability.
`To determine AC R, ACIR, DC R, and DCIR, we first detected
`the pulse peak by incorporating a filter bank with variable cutoff
`frequencies, spectral estimates of the heart rate, a rank-order
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`Fig. 3. Results for fingertip: (a) Reference SpO2 (%), (b) reflected infrared raw
`signal, (c) reflected red raw signal, and (d) the modulation ratio R.
`
`nonlinear filter, and decision logic [8]. Next, we calculated the
`average values of a 1-s segment signal with each detected peak
`as its center. Finally, DC was calculated from each average
`value, and AC was calculated by subtracting DC from each peak
`value. This method was applied to both red and infrared signals
`to determine AC R, ACIR, DC R, and DCIR.
`
`2.3. Performance evaluation
`
`We performed two experiments: one at a fingertip and the
`other at a wrist. In each experiment, the subjects breathed
`regularly for 40 s and then held their breath for as long as
`they could. Subsequently, an additional 1 min was given for
`their SpO2 levels to return to the normal state. PowerLab
`8/35 (ADINSTURMENTS, Sydney, Australia) was used with
`an oximeter pod (ADINSTRUMENTS, Sydney, Australia) to
`measure the reference SpO2.
`
`3. Results and discussion
`
`3.1. Results for the fingertip
`
`Fig. 4. Results from fingertip: (a) Reference SpO2 (%) measured at fingertip,
`(b) resultant ACIR, (c) resultant AC R, (d) resultant DCIR, and (e) resultant
`DC R.
`
`AC R increased, which sufficiently match the light absorption
`characteristics described in Fig. 1(a) and (b). On the other
`hand, the reflected DC components partially reflected these
`characteristics. As SaO2 decreases, DCIR should increase and
`DC R should decrease. However, in the results, only DCIR
`followed this pattern. In fact, DC R showed an increase. This
`behavior can be attributed to the fact that the reflective mode
`is more sensitive to pressure and ambient light sources, which
`leads to DC instability.
`
`3.2. Results for the wrist
`
`Fig. 3 shows the results for the fingertip: the reference
`SpO2, reflected infrared raw signal, reflected red raw signal,
`and modulation ratio R. As expected, the ratio R increased
`with decreasing SaO2. Fig. 4 shows the resultant ACIR, AC R,
`DCIR, and DC R. When SpO2 decreased, ACIR decreased and
`
`Fig. 5 shows the results for the wrist: the reference SpO2,
`the modulation ratio R, ACIR, AC R, DCIR, and DC R. As
`SpO2 decreased, the modulation ratio R increased as expected.
`However, the modulation ratio R was less stable than that for
`the fingertip. More specifically, as SpO2 decreased, ACIR did
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`the fingertip. Similarly, the reflected DC components did not
`reflect the characteristics well. This variation may be because
`the reflective mode has a low signal-to-noise ratio (SNR) and is
`sensitive to pressure and ambient light sources.
`
`3.3. Discussion
`
`Based on the performance of the oximeter, monitoring SpO2
`at the wrist using the reflective mode presents challenges with
`regard to clinical use. Another limitation is that the reflected red
`and infrared pulses can only be used for specific areas, such as
`a radial artery; thus, most areas of the wrist are not available for
`monitoring. In addition, a slight position change at the measure-
`ment site significantly affects the performance of the oximeter.
`Thus, the focus of research studies involving oximetry should
`be on choosing appropriate measurement sites, and optimizing
`pressure, ambient light, and SNR.
`
`Acknowledgment
`
`This study was supported by a Basic Science Research
`Program through the National Research Foundation of Korea
`(NRF) funded by the Ministry of Science, ICT & Future
`Planning: NRF-2015M3A9D7067215.
`
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`Fig. 5. Results for wrist: (a) Reference SpO2, (b) modulation ratio R, (c)
`resultant ACIR, (d) resultant AC R, (e) resultant DCIR, and (f) resultant DC R.
`
`not reflect the light absorption characteristics. Furthermore, the
`change in AC R was less pronounced than that observed for
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