PHOTODETECTOR SIZE CONSIDERATIONS IN THE DESIGN OF A
`NONINVASIVE REFLECTANCE PULSE OXIMETER FOR TELEMEDICINE
`APPLICATIONS
`C. Pujary, M. Savage, Y. Mendelson
`Department of Biomedical Engmeemg, and Bioengmeenng Inshtute
`Worcester Polytechnic Institute, Worcester, MA 01609.
`
`Abstract-Low power management without compromising
`signal quality is a key requirement in the optimal design of a
`wearable pulse oximeter.
`This paper investigates the
`advantage gained by using a photodetector mtb a larger active
`area. Preliminary in vivo experiments have demonstrated that
`by increasing the area of the photodetector it is possible to
`reduce the overall power requirement of a wireless sensor
`intended for telemedicine applications.
`Keywordspuise oximeter, telemedicine, wearable sensor
`
`provided an easy way to simulate various PD areas by
`connecting in parallel multiple PDs to the common
`summing
`input of a current-to-voltage converter.
`Additional circuitry, consisting of amplifiers and band pass
`filters, were used to produce two different signals (a
`pulsatile AC component and a non-pulsatile DC cqmponent)
`from each PPG. Analog data streams were digitized at
`50Hz for 30s periods by a National Instnunent DAQ card
`installed in a PC running L A B P W 6.0 software.
`
`I. INTRODUCTION
`Commercially available sensors used in reflectance
`mode pulse oximeters employ a single photodetector (PD)
`element typically with an active area of about 12-IS"*.
`The light intensity detected by the photodetector in a
`reflectance pulse oximeter depends on the incident light
`intensity, absorption by skin, reflection by bones, tissue
`backscattering and the amount of blood in the vascular bed
`[I]. Compared to transmittance mode pulse oximetry,
`reflected photoplethysmogiami (PPGs) have generally
`lower amplitudes.
`
`Low power management without compromising signal
`quality is a key requirement in optimizing the design of a
`wearable telesensor. One approach to lowering the power
`consumption of a wireless pulse oximeter, which is
`dominated by the current required to drive the LEDs, is to
`reduce the LED duty cycle [2]. Alternatively, lowering the
`current supplied to the red and Mared LEDs can also
`reduce power consumption. However, with reduced current
`drive, there is a direct impact on the quality of the detected
`PPGs.. Mendelson et al. [3] showed that a concentric array
`of PDs could be used to
`increase the amount of
`backscattered light detected-by a reflectance type pulse
`oximeter sensor. In this paper we investigate the advantages
`of increasing the PD area and minimizing LED driving
`currents to optimize the overall power requirement of a
`reflectance mode pulse oximeter intended for .future
`applications in telemedicine.
`II. METHODOLOGY
`Experimental setup
`To study the effect of different PD areas, we constructed
`and tested a prototype reflectance sensor employing 6
`identical (3"
`PDs. The'equally spaced PDs were
`x 4")
`- concentrically arranged
`in a 18mm diameter planar
`configuration arovd a pair of red and infrared LEDs. Each
`PD was individually connected to'a central hub. The hub
`
`In- Vifro Experiments
`Durk Tests: To test the background noise level generated
`by each PD, we performed a series of dark measurements by
`switching off the LEDs in the sensor and blocking ambient
`light from reaching the six PDs.
`Muhipre Phofodefecfors Tests: Each PD was randomly
`connected through the hub:to
`determine the spatial
`uniformity of the illuminating field incident on the PDs. To
`produce a constant level background illumination, a signal
`composed of a DC bias voltage modulated by a small 1Hz
`AC sine wave was generated by a programmable function
`generator. The signal from the function-generator was
`applied to a separate LED that was used to simulate a
`typical PPG signal. The extemal LED was attached to a
`translucent flat medium serving as an optical diffuser. The
`surface of the diffuser was positioned at a distance of 30cm
`away from the planar surface of the sensor.
`
`In- Vivo Experiments
`A series of in vivo experiments were performed to
`determine the signal improvement gained by using different
`PD areas. The prototype sensor was attachedto the base of
`supplied to the &I
`a volunteer's finger cd,the peak c&&
`and infrared LEDs were adjusted to 3niA and 1.911~4,
`respectively. As the driving currents Were adjusted,.ththg . .
`output of each amplifier was monitored to assure that '(i) a
`distinguishable and stahle PPG was observed when a single
`PD was employed, and (ii) maximal PPG signals were
`produced without causing amplifier saturation when all 6
`PDs were connected in parallel through the hub.
`It is
`important to note that the fmal currents selected were
`significantly lower compared to the typical driving currents
`employed in commercial pulse oximeters.
`
`111. Rl3SUL.TS
`.
`The Root Mean S q y e (RMS) values corresponding to
`the amplitude of the AC components measured from each
`
`PD are plotted in Fig. 1.. k i n g cess the average noise
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`pD6
`
`PDI
`
`PDZ
`PD5
`PO3
`pI1(
`INDIVmUAL PEOTODETECTORS
`
`I
`I
`Fig. I. Individual photodnenm performance under complete darkness and
`a constant light illumination.
`In
`generated by the
`individual PDs was 0.114V.
`comparison, the average PPG amplitude measured in vitro
`by the 6 PDs under a spatially uniform illumination field
`produced by the external LED source was 0.647V.
`Fig. 2 shows the signals detected in vitro with multiple
`combinations of PDs using
`the simulated uniform
`illumination produced by the extemal LED. The right-side
`bars represent the measured RMS values corresponding to
`different PD areas. For comparison, the left-side bars were
`computed using
`the
`relationship 0.647n, where n
`corresponds to the number of PDs connected in parallel. The
`trend observed provide sufficient evidence of a linear
`improvement in signal intensity as a h c t i o n of an increase
`in the active area of the PDs.
`Fig. 3 shows the magnitude of the pulsatile red and
`infrared PPGs measured
`for different PD
`in-vivo
`combinations.
`rV. DISCUSSION
`Minimizing the current required to drive the LEDs is a
`critical design consideration in optimizing the power
`consumption of a wearable pulse oximeter. However,
`reduced LED driving currents has a direct impact on the
`incident light intensity produced by the sensor and could
`the quality of the PPGs.
`in
`to deterioration
`lead
`Consequently, it could result in unreliable and therefore
`inaccurate calculation of oxygen saturation.
`
`I-PD
`
`Eph
`Q P h
`3-PDa
`2-Ph
`MIJL.TPLE PHOTODETECTORS
`
`bPDa
`
`~
`
`Fig. 2. Signal improvement observed in-vim with multiple photodetectors.
`
`I-PD
`
`5 - P h
`3-PDa
`2-PLb
`4pDs
`MULTIPLE PHOTODETECTORS
`
`bph
`
`i
`
`ig. 3. Signal improvement obsened in vivo with multiple photodetectors.
`quality of the PPGs simply by increasing the overall sue of
`the PD in the sensor. Hence, with reduced LED driving
`currents, maximizing the backscattered light collected by
`the sensor and optimized digital switching techniques, a
`very low power consuming sensor can be developed thereby
`extending the overall battery life of a pulse oximeter
`intended for telemedicine applications.
`
`A C K N O W ~ ”
`This research was supported in part by Department of
`Defense Cooperative Agreement DAMD17-03-2-0006.
`
`REFERE”
`[I] Y. Mendelson, B. Ochs, ‘‘Noninvasive Pulse Oximetry
`Utilizing Skin Reflectance Photoplethysmography,” IEEE
`Transactions on Biomedical Engineering, vol. 35, no. 10,
`pp. 798-805, Oct. 1988.
`[2] S. Rhee, B.H. Yang and H. Asada,” The Ring Sensor: A
`new Ambulatory Wearable Sensor for twenty Four Hour
`Patient Monitoring,” Proc. of the 20th Annual International
`Confeence of the IEE Engineering in Medicine and Biology
`Society. Hong Kong, Oct. 1998.
`[3] Y. Mendelson, J.C. Kent, B.L. Yocnm and M.J. Bide,”
`Design and Evaluation of new reflectance pulse oximeter
`sensor,” Medical Instrumentation, Vo1..22, No. 4, pp. 167-
`173, Aug. 1988.
`
`From the data presented in Fig. 3, it can be observed that
`the overall increase in the reflected PPG signals achieved in
`vivo when all 6 PDs were connected in parallel is smaller
`and does not follow the same linear relationship observed in
`vitro as shown in Fig. 2. This deviation was most likely
`caused by the non-uniform backscattered light distribution
`emanating from the finger.
`
`V. CONCLUSION
`The data presented in this study demonstrate that the
`driving currents of the LEDs in a reflectance pulse oximeter
`can be lowered significantly without compromising the
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