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LED Based Sensor System for Non-Invasive Measurement of the Hemoglobin Concentration in Human Blood U. Timm1, E. Lewis1, D. McGrath2, J. Kraitl3 and H. Ewald3 1

University of Limerick, Optical Fibre Sensors Research Center, Limerick, Ireland 2 University of Limerick, Graduate Medical School, Limerick, Ireland 3 University of Rostock, Institute of General Electrical Engineering, Rostock, Germany Abstract — In the perioperative area, the period before and after surgery, it is essential to measure diagnostic parameters such as oxygen saturation, hemoglobin (Hb) concentration and pulse. The Hb concentration in human blood is an important parameter to evaluate the physiological condition. By determining the Hb concentration it is possible to observe imminent postoperative bleeding and autologous retransfusions. Currently, invasive methods are used to measure the Hb concentration. For this purpose blood is taken and analyzed. The disadvantage of this method is the delay between the blood collection and its analysis, which doesn't permit a real-time patient monitoring in critical situations. A non-invasive method allows pain free online patient monitoring with minimum risk of infection and facilitates real time data monitoring allowing immediate clinical reaction to the measured data. Keywords — hemoglobin, non-invasive, photoplethysmography

I. INTRODUCTION In the perioperative area, the period before and after surgery, it is essential to measure diagnostic parameters such as oxygen saturation, hemoglobin (Hb) concentration and pulse[1]. The Hb concentration in human blood is an important parameter to evaluate the physiological condition. By determining the Hb concentration it is possible to observe imminent postoperative bleeding and autologous retransfusions. Currently, invasive methods are used to measure the Hb concentration. For this purpose blood is taken and analyzed. The disadvantage of this method is the delay between the blood collection and its analysis, which does not a permit a real-time patient monitoring in critical situations. A non-invasive method allows pain free online patient monitoring with minimum risk of infection and facilitates real time data monitoring allowing immediate clinical reaction to the measured data. The absorption of whole blood in the visible and near infrared range is dominated by the different hemoglobin derivatives and the blood plasma that consists mainly of water.[2] It is well known that pulsatile changes of blood volume in tissue can be observed by measuring the transmission or reflection of light through it. This diagnostic method is called photo-plethysmography (PPG) The newly

developed optical sensor system uses three wavelengths for the measurement of the Hb concentration, oxygenation and pulse. This non-invasive multi-spectral measurement method is based on radiation of near monochromatic light, emitted by light emitting diodes (LED) in the range of 600nm to 1400nm, through an area of skin on the finger. The sensor assembled in this investigation is fully integrated into a wearable finger clip and allows full wireless operation through on board miniature wireless enabled microcontroller. II. MEASUREMENT METHOD The new developed sensor device allows a non-invasive continuous measurement of hemoglobin concentration, oxygen saturation and pulse which is based on a multispectral measurement method. Thereby the area of skin on the finger is transilluminated by monochromatic light which is emitted by LEDs in the range from 600nm-1400nm. The arteries contain more blood during the systolic phase of the heart than during the diastolic phase, due to an increased diameter of the arteries during the systolic phase. This effect occurs only in arteries but not in veins.[3] For this reason the absorbance of light in tissues with arteries increases during systole because the amount of hemoglobin (absorber) is higher and the light passes through a longer optical path length d in the arteries. These intensity changes are the so called PPG-waves.[4] The time varying part allows the distinction between the absorbance due to venous blood (DC part) and that due to the pulsatile component of the total absorbance (AC part). Figure 1 shows the absorption model for light penetrating tissue to sufficient depth to encounter arterial blood. Upon interaction with the tissue the transmitted light is detected non-invasively by photo diodes. Suitable wavelengths were selected for the analyses of relative hemoglobin concentration change and SpO2 measurement. During the measurement of hemoglobin the absorption should not be dependent on the oxygen saturation. That means that the measurement is only practicable at so called isobestic points where the extinction coefficients of HHb and HbO2 are identical. One such point is

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III. SENSOR DEVICE The sensor system being developed consists of hardware modules including appropriate light sources and receivers, a microcontroller and a wireless interface. A key component of the sensor system is the low power microcontroller MSP430F1611.[8]

Figure 1: Absorption Spectra of Hemoglobin and Water

known to exist around a wavelength of 810nm.[5] According to the assumption that red blood cells are mainly filled with water, the absorption coefficient of blood is similar to a solution consisting HHb, HbO2 and water (H2O) and the absorption of HHb and Hb02 is indistinguishable to the absorption of H2O above 1200nm, it is necessary to select a wavelength value in this region above the diagnostic window.[6] (Figure 2)

Figure 2: Model of Tissue Layer

Finally the determination of hemoglobin concentration will be performed at the wavelengths O1 = 810nm and O2 = 1300nm. The AC/DC Values of both wavelengths leads to the quotient H (equation 1): I AC  DC 810nm

H

I DC 810nm I AC  DC 1300nm I DC 1300nm

ln 10





H Hb O 810nm cHb





P H 20 O 1300nm 64500g / I

This enables software controlled and time multiplexed operation of the light sources and receiver channels. The mean value is calculated and the dark current subtracted in software, and the data is transmitted via serial RS232 or wireless interface. With application software programmed in LabView it is possible to handle the data on a Laptop or PC. The light sources, three LEDs, with centre wavelengths of O1= 670nm, O2= 810nm and O3= 1300nm are installed in the upper part of the clip. The pulsed LED currents are controlled by the microcontroller that allows a change of the light source intensity. To detect the transmission signals of the LEDs a silicon photodiode with a spectral range of 400nm-1100nm was used. For measuring at 1300nm an additional Indium Gallium Arsenide (InGaAs) photodiode with a spectral of 1000nm-1700nm is selected. Figure 4 shows a photo of the sensor device.

(1)

This theoretical equation results in incorrect values in practise but the fact f(H) is true.[7] Thus, the hemoglobin sensor needs calibration using an external blood stream model including real blood.

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Figure 3: Functional Diagram Sensor Device

Figure 4: Sensor Head with Photodiodes and Light Sources

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LED Based Sensor System for Non-Invasive Measurement of the Hemoglobin Concentration in Human Blood

IV. BLOOD STREAM MODEL The human circulatory system is a fast regulatory transport system. This closed circuit consist of parallel and serially connected blood vessels for which the heart acts as a circulation pump. The mandatory blood circulation is maintained by the contraction of the muscular cardiac wall. The heart is made up of a left and right cardiac half, each of them being a muscular hollow organ. In functional and morphologic terms the circulatory system consists of two serial sections. The heart also can be considered as two serial pumps. The right cardiac side absorbs the deoxygenated blood from the body and transports it to the lungs (lung circuit), where it is re-oxygenated. The oxygenated blood arrives at the left cardiac side, wherefrom the dispensation to various organs takes place (body circuit). The oxygenated blood in the body circuit is pumped from the left ventricle to the aorta and main arteries whose sub branches lead to the tissue and organs. Following the next stage of branching into arteriole and capillary, the oxygen and nutrients and the ingestion of carbon monoxide and intermediate catabolic products are distributed to the tissue and organs of the body. Finally the capillaries terminate into venules and veins which transport the deoxygenated blood back to the right atrium. Figure 5 shows the measuring point for the developed hemoglobin sensor which is located on the finger terminal element.

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the typical physiological state. Therefore a setup for optical measurement on circlulating blood is necessary. By using a circulation system based on a roller pump the blood is circulated through the closed circuit and a pulsatile blood volume is generated. The schematic diagram of the system is shown in Fig. 6.

Figure 6: Functional Diagram of the Blood Stream Model

Using this system measurement allowing the accurate assessment of the pulsatile blood portion (AC/DC signal ratio) is possible. The input point is at the same time the blood sample extraction point which will be needed for the reference point measurement. The blood gassing takes place in the oxygenator which allows the precisely controlled introduction of oxygen or carbon monoxide and a separate heated water circuit allows the elevation of the blood temperature to 36qC -37qC V. RESULTS

Figure 5: Measuring Point on Circulatory System

Based on human circulatory system a bloodstream model has been designed which is necessary for validation of the measurement method of hemoglobin concentration and the newly developed optical sensor system. With the help of the model a controlled variation of the blood parameter, hemoglobin concentration and oxygenation are feasible. Thereby both parameters can be changed separately or simultaneously on circulating blood is not representative of

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Initial measurements of the transmission signals on the finger terminal element have shown variations in light absorption due to the arterial pulse at all three wavelengths. The signal at 1300nm is especially weak and requires further effort in signal processing. However, the signal quality is sufficient to allow analysis of the signals and to calculate the relative attenuation coefficient of the arterial blood. In case of the signal components at 1300nm an evaluation of the relative portions of hemoglobin and water in blood are feasible.[9] On the basis of this measurement technique a pulsed signal for the calculation of the relative attenuation coefficient is required. Decreases of the signal amplitude and therefore the signal to noise ratio caused by vasoconstriction at the extremities is a potential problem. Small signal amplitudes can give rise to inaccurate results.[10] Figure 7 shows the signal at 1300nm and 810nm when light is transmitted through the finger.

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using an experimental blood stream model and optimisation of the sensor device.

REFERENCES 1.

Figure 7: PPG Wave at 810nm and 1300nm

The developed sensor device is suitable for non-invasive continuous online monitoring of one ore biological parameter. The advantage of this measuring technique is independent of blood samples and this allows a minimum risk of infection and it makes possible to react immediately on the measured data. VI. CONCLUSIONS In this paper a non invasive device for measuring hemoglobin concentration based on multi wavelength light absorption has been reported. The newly developed sensor device is able to measure PPG signals at three independent wavelengths continuously. Future work involves validation of the sensor measurement based on further measurements

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T. Ahrens and K.Rutherford. Essentials of Oxygenation: Implication for Clinical Practice. Jones & Bartlett Pub, 1993. 2. S. J. Matcher, M. Cope, and D. T. Delpy. Use of the water absorption spectrumto quantifies tissue chromospheres concentration changes in near-infrared spectroscopy. Phys. Med. Biol., 38:177–196, September 1993. 3. Woods AM, Queen, JS, Lawson D., 1991 Valsalva maneuver in obstetrics: The influence of peripheral circulatory changes on function of the pulse oximeter, Anesth. Analg. 73 pp. 765-71 4. Lock I, Jerov M, Scovith S (2003) Future of modeling and simulation, IFMBE Proc. vol. 4, World Congress on Med. Phys. & Biomed. Eng., Sydney, Australia, 2003, pp 789–792 5. Kamal AAR, Hatness JB, Irving G, Means AJ, 1989 Skin photoplethysmography – a review, Comp. Meth.Progr. Biomed., 28, pp 257-69 6. N. Kollias. Tabulated molar extinction coe_cient for hemoglobin in water. Wellman Laboratories, Harvard Medical School, Boston, 1999. 7. Kratil, J., Ewald H., Results of hemoglobin concentration measurements in whole blood with an optical non-invasive method”, Photon08, Optics and Photonics, IOP Conference, p 77, Edinburgh UK, 2008 8. U.Timm, E.Lewis, H.Ewald, "Non-Invasive Wireless Sensor System for Measurement of Hemoglobin Concentration in Human Blood", 6th International Forum Life Science Automation (LSA) 2008, September 10th-12th, Rostock, Germany, ISBN: 978-3-938042-17-54 9. Kraitl, J., Ewald H.: None-invasive measurement of hemoglobin concentration with a pulsephotometric method“ Proceedings, 5th International Forum Life Science Automation, Washington (DC), USA 10. Yosihida I, Shimada Y, Oka N, Hamaguri K, 1984, Effects of multiple scattering and pheripheral circulation on aterial oxygen saturation measured with pulse type oximeter, Med & Biol. Eng & Comput., 22, pp 475-78

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