The 23rd International Technical Conference on Circuits/Systems,
`Computers and Communications (ITC-CSCC 2008)
`
`Measurement Site and Applied Pressure Consideration in Wrist
`Photoplethysmography
`Eun Geun1, Hyun Heo2, Ki Chang Nam3 and Young Huh4
`1, 2, 3, 4Korea Electrotechnology Research Institute (KERI)
`1271-19, Sa-dong, Sangnok-gu, Ansan, Gyeonggi-do, Korea
`E-mail: 1kimeg917@keri.re.kr, 2gjgus1@nate.com, 3chadol@keri.re.kr, 4yhuh@keri.re.kr
`
`Abstract: The objective of this study is to describe
`preliminary evaluation of a new optical reflectance sensor
`module with air pressure cuff. In order to improve PPG
`signals from the wrist as an example of wearable PPG, the
`optical reflectance sensor module includes two identical
`photodiodes and a pair of red and infrared LED. The sensor
`module was packaged inside of a cuff to demonstrate the
`effects of pressure applied to reflectance probe at the radial
`artery in the wrist. PPG signals with large amplitude were
`measured when the induced cuff pressure was close to
`mean blood pressure. This result will be applied to
`development of a wrist type healthcare device.
`
`1. Introduction
`It is well established that recordings of the beat-to-beat
`variations
`in heart rate can be a useful diagnostic
`technology in cardiovascular medicine [1]. Although the
`majority of applications use ECG as the sensing mechanism
`or cardiac dynamics, it has been noted that photoelectric
`plethysmography [2], also known as photoplethysmography
`(PPG), can also be employed to derive essentially the same
`information [3]. PPG is a non-invasive method of detecting
`the cardio-vascular pulse wave that propagates through the
`body. The cardiovascular wave is stimulated by each
`cardiac contraction, and detected in the periphery using a
`light source and a detector. Clinical applications of PPG
`technology include the measurement of heart rate [4, 5],
`peripheral vascular properties [6-8], blood pressure [9, 10],
`and autonomous nervous activity [11, 12].
`Photoplethysmography(PPG) is measured by analyzing
`the pulsatile components of the detected red and infrared
`plethysmograms which make use of
`transmitted or
`reflected light intensities. In reflectance , the light from the
`LEDs enters the tissue, is scattered by both the moving red
`blood cells and the non-moving tissue, and a part of this
`back scattered light is detected by the photodiode(PD)[13].
`Reflectance(back-scatter) PPG has potential advantages
`compare with transmission PPG. The probe can be placed
`at virtually any site on the skin, because the light sources
`and the detector(s) are integrated side by side in the probe.
`Therefore, the probe can even be used on locations where
`light cannot be transmitted[14].
`However, there are some problems in the reflectance
`PPG method. The major practical
`limitation
`is
`the
`comparatively low-level photoplethysmograms recorded
`from low-density vascular areas of the skin. Therefore, the
`feasibility of reflectance PPG depends on the ability to
`design an optical reflectance sensor that can reliably detect
`sufficiently strong reflectance photoplethysmograms from
`various locations on the skin[15]. In order to partially
`
`overcome these limitations Dassel et al. applied pressure on
`the probe to increase the accuracy of reflectance pulse
`oximetry at the forehead[14].
`Measuring with high-quality data on the wrist is also
`difficult and often requiring the application of pressure to
`the sensor in order to reduce the physical contact between
`the sensor and
`the wrist[16]. Furthermore, sensor
`configuration and power consumption play a important role
`in the design of a wearable bio -instrumentation device[17].
`To implement wrist wearable healthcare device, we
`have developed an optical reflectance sensor module that
`includes an array of two identical photodiodes and a pair of
`red and infrared LED. The sensor module was packaged
`inside of a cuff to demonstrate the effects of pressure
`applied to reflectance probe at the radial artery in the wrist.
`The objective of this study is to describe preliminary
`evaluation of a new optical reflectance sensor module with
`air pressure cuff, in order to improve PPG signals from the
`wrist area as an example of wearable PPG.
`
`2. Method
`2. 1 Experimental Setup
`Fig. 1 shows the general PPG block diagram. The system
`was designed to detect red and infrared (IR) signal. The
`photodiodes(PDs) detected radiation back scattered by the
`tissue from both infrared and red LED and delivered output
`current proportional to the detected radiation level. The
`analog signals from the common current to voltage
`converter were subsequently separated into IR and red
`components by
`signal
`conditioning
`circuitry(Fig.2)
`Amplified and filtered analog signal components were
`measured by a oscilloscope (TDS3014, Tektronix).
`
`Fig.1. System block diagram of PPG.
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`centered at three points as shown in Fig.5 and the measured
`amplitudes of red and IR signal were compared. In the
`second experiment, the probe was mounted on the finger,
`the palm of the hand and palmer side of the wrist. In the
`third experiment, sensor was located at ‘2’ in Fig.5 and the
`amplitudes of PPG were compared according to pressure
`application by air cuff.
`
`Fig.2. Developed PPG circuit board.
`
`The constructed reflectance sensor consists of 2 silicon
`PD chips(active chip area : 3mm x 3mm) and a pair of red
`(660nm)and IR(940nm) LEDs. LEDs were positioned
`between identical two PDs as shown in Fig. 3,4. The
`distance of PDs was set to 6 mm. The optical sensor
`module was packaged inside of a air pressure cuff.
`
`Fig.3. Air pressure cuff and reflectance PPG probe; side
`(top) and bottom (bottom) view.
`
`Fig.4. Constructed sensor module (Air pressure cuff and
`reflectance PPG probe)
`
`2. 2 Experiment
`One volunteer was participated in preliminary measurement.
`Subject was informed about the purpose and procedure of
`the study and asked written consent. During
`the
`measurement of PPG with wrist cuff, air pressure was
`induced by gradually from 0 to 140 mmHg occluding the
`radial artery at increment of 20 mmHg. Pressure was
`monitored with a analog sphygmomanometer. In the event
`the volunteer felt uncomfortable, the measurement process
`was stopped.
`Three experiments performed to compare the PPG
`amplitudes measured by the reflectance sensor probe. In
`sensor positioning experiment, the probe was mounted on
`the palmer side of the wrist. The sensor module was
`
`1
`A
`
`2
`B
`
`C
`3
`
`Radial artery
`
`Fig.5. Sensor positioning on the wrist (top) and anatomical
`position in bone (bottom)
`
`3. Results & Discussion
`Fig.6, 7, and 8 shows the measured PPG at three point ‘1, 2,
`and 3’ in the wrist described in Fig. 5, respectively. There
`were no sigficant amplitude difference among
`three
`measurement positions except that it seems to be less
`amplitude at ‘3’. It is thought that sensor attachment on the
`wrist is affected by radius bone structure and radial artery
`location.
`The PPG signal from widely distributed portion of the
`capillaries parts such as finger or palm was relatively stable.
`But the results measured at another location of the wrist got
`a weak signal. Even thougth the PPG is easily measurable
`at finger, extensional finger probe is necessary. To
`implement a wrist type device, other possible measurement
`positions which is necessary of extentional probe option
`was exclude from the experiment.
`Fig. 9 shows the measured red LED reflectance signal
`by applying pressure on the wrist. Naturally, maximum
`amplitude changes were shown when the induced pressure
`was close to the mean arterial blood pressure of the subject.
`It is considered that air cuff module can be used for stable
`attachment between sensor and skin.
`PPG signal from radial artery might different from other
`subject because every person have different radial artery
`location and depth. Furthermore, PPG signal greatly
`distorted depending on applied pressure. Through
`experiments, it was verified that proper location of the
`sensors and pressure on the the skin surface are important
`element on stable PPG measurement.
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`4. Conclusion
`The objective of this study is to describe reflectance sensor
`module configuration with air pressure cuff, in order to
`improve PPG signals from the wrist area as an example of
`wearable PPG.
`Measuring PPG on the wrist is difficult. Because people
`have different position and depth of radial artery each other.
`The position of sensor and application of pressure to the
`sensor in order to reduce the physical contact between the
`sensor and the wrist affect on the magitude of PPG. As
`preliminary study to implement wrist wearable device, we
`have developed an sensor module that includes an optical
`reflectance sensor and air cuff. Further study is necessary to
`find optimal LED and PDs array
`to
`increase
`the
`measurement efficiency. This result will be applied to
`development of a wrist type healthcare device.
`
`Fig.6. PPG signal measured at ‘1’ in Fig. 5
`
`Fig.7. PPG signal measured at ‘2’ in Fig. 5
`
`Fig.8. PPG signal measured at ‘3’ in Fig. 5
`
`Fig.9. PPG amplitude versus pressure applied to the probe
`
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