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Determination of Blood Vessels Expandability; Multichannel Photoplethysmography
14th IFAC Conference on Programmable Devices and Embedded 14th 14th IFAC IFAC Conference Conference on on Programmable Programmable Devices Devices and and Embedded Embedded Systems 14th IFAC Conference on Programmable Devices and Embedded Systems Available online at www.sciencedirect.com Systems October 5-7, 2016. Brno, Czech Republic Systems October 5-7, 2016. Brno, Czech Republic October October 5-7, 5-7, 2016. 2016. Brno, Brno, Czech Czech Republic Republic
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IFAC-PapersOnLine 49-25 (2016) 284–288
Determination of Blood Vessels Determination of Blood Determination ofMultichannel Blood Vessels Vessels Expandability; Expandability; Multichannel Expandability; Multichannel Photoplethysmography Photoplethysmography Photoplethysmography
Vorek ∗∗ Bertrand Massot ∗∗ Iveta Bryjova ∗ ∗∗ ∗ Bertrand Massot ∗∗ Iveta Bryjova ∗ ∗ Vorek ∗ ∗ Bertrand ∗∗ Iveta Bryjova ∗ Vorek Massot Tomas Urbanczyk Vorek Bertrand Massot Iveta Bryjova ∗ ∗ Tomas Urbanczyk ∗ Tomas Urbanczyk Tomas Urbanczyk ∗ VSB-Technical University of Ostrava, Department of Cybernetics and ∗ ∗ VSB-Technical University of Ostrava, Department of Cybernetics and ∗ VSB-Technical of of BiomedicalUniversity Engineering, Ostrava,Department Czech Republic (e-mail: and VSB-Technical University of Ostrava, Ostrava, Department of Cybernetics Cybernetics and Biomedical Engineering, Ostrava, Czech Republic (e-mail: Biomedical Engineering, Ostrava, Czech Republic (e-mail: [email protected]) Biomedical Engineering, Ostrava, Czech Republic (e-mail: [email protected]) ∗∗ [email protected]) INL, UMR5270 CNRS-INSA Lyon, University of Lyon, 69621 [email protected]) ∗∗ ∗∗ INL, UMR5270 CNRS-INSA Lyon, University of Lyon, 69621 ∗∗ INL, UMR5270 CNRS-INSA Lyon, of Cedex, France INL, UMR5270 Villeurbanne CNRS-INSA Lyon, University University of Lyon, Lyon, 69621 69621 Villeurbanne Cedex, France Villeurbanne Cedex, France Villeurbanne Cedex, France Abstract: Clinicians would benefit from the evaluation of the state of the cardiovascular system, Abstract: from the evaluation the state of theusing cardiovascular system, Abstract: Clinicians would benefit from the of the of cardiovascular system, because it isClinicians one of thewould most benefit important system in humanof body, without invasive entrance. Abstract: Clinicians would benefit fromsystem the evaluation evaluation ofbody, the state state of the theusing cardiovascular system, because it is one of the most important in human without invasive entrance. because it is one of the most important system in human body, without using invasive entrance. Multichannel pulse wave measurement could be one possibility of how to noninvasively evaluate because it is one of the most important system in human body, without using invasive entrance. Multichannel pulse wave wave measurement could beproperties one possibility possibility of how how by to the noninvasively evaluate Multichannel pulse be one of to noninvasively the state of cardiovascular system. Pulsecould wave’s are affected properties evaluate of blood Multichannel pulse wave measurement measurement could beproperties one possibility of how by to the noninvasively evaluate the state of cardiovascular system. Pulse wave’s are affected properties of blood the state of cardiovascular system. Pulse wave’s properties are affected by the properties of blood vessels, like stiffness and diameter, and wave’s also byproperties heart function. An analysis of shape and time the state of cardiovascular system. Pulse are affected by the properties of blood vessels, like stiffness and diameter, and also by heart function. An analysis of shape and time vessels, like stiffness and diameter, and also by heart function. An analysis of shape and time properties of pulse wave propagation provides information about the state of the cardiovascular vessels, like stiffness and diameter, and also by heart function. An analysis of shape and time properties of pulse wave propagation about the of the cardiovascular properties of wave provides information about state of cardiovascular system. The measurement of pulse provides wave on information different locations of state human body could bring properties of pulse pulse wave propagation propagation provides information about the the state of the the cardiovascular system. The measurement of pulse wave on different locations of human body could bring system. The measurement of pulse wave on different locations of human body bring information about the different subparts of on thedifferent cardiovascular system. For this kindcould of purpose system. The measurement of pulse wave locations of human body could bring information about the different subparts of the cardiovascular system. For this kind of purpose information about the different subparts of the cardiovascular system. For this kind of purpose it is necessary to developed measurement system which can achieve pulse wave measurement on information about the different subparts of the cardiovascular system. For this kind of purpose it is necessary to developed measurement system which can achieve pulse wave measurement on it is necessary to developed measurement system which can achieve pulse wave measurement on several locations of the human body simultaneously and synchronously. The paper introduces it is necessary to developed measurement system which can achieve pulse wave measurement on several locations of the human body simultaneously and synchronously. The paper introduces several locations of the human body simultaneously and synchronously. The paper introduces developing of such system which is able to measure and displayed several PPG signals and several locations of the human body simultaneously and synchronously. The paper introduces developing ofone such system which istime ablereference to measure measure and displayed displayed several PPGand signals and developing such system which able to and PPG signals and also includeof ECG channel as ais for synchronization. Hardware software developing ofone such system which istime ablereference to measure and displayed several several PPGand signals and also include ECG channel as a for synchronization. Hardware software also include one ECG channel as a time reference for synchronization. Hardware and software materials developed forchannel the solution are described and preliminary results are presented. also include one ECG as a time reference for synchronization. Hardware and software materials developed for the solution are described and preliminary results are presented. materials developed for solution described and results are presented. materials developed for the the solutionofare are described and preliminary preliminary resultsLtd. areAll presented. © 2016, IFAC (International Federation Automatic Control) Hosting by Elsevier rights reserved. Keywords: ECG, PPG, Cardiovascular system, LabVIEW, PWV Keywords: ECG, PPG, Cardiovascular system, LabVIEW, PWV Keywords: ECG, ECG, PPG, PPG, Cardiovascular Cardiovascular system, system, LabVIEW, LabVIEW, PWV PWV Keywords: 1. INTRODUCTION 1. INTRODUCTION INTRODUCTION 1. 1. INTRODUCTION Pulse wave results from the work of the left part of heart. Pulse wave results from the work workduring of the the each left of Pulse wave results the of left part part of heart. heart. A single pulse wavefrom is generated cardio cycle, Pulse wave results from the workduring of the each left part of heart. A single pulse wave is generated cardio cycle, A single pulse wave is generated during each cardio cycle, defined by thewave interval between during two successive systoles. A single pulse is generated each cardio cycle, defined by the interval between two successive successive systoles. defined interval between two systoles. Most of by the the time, Pulse wave is measured from periphery defined by the interval between two successive systoles. Most of the time, Pulse wave is measured from periphery Most of the time, Pulse wave is measured measured from periphery like fingertip or from places showing good blood perfusion Most of the time, Pulse wave is from periphery like fingertip fingertip from showing like or from places showing good good blood blood perfusion perfusion ear lobe, or nose or places forehead. like from showing good blood perfusion like fingertip ear lobe, lobe, or nose or places forehead. like ear nose or forehead. like lobe, nose or forehead. The ear analysis of pulse wave provide relevant information The analysis of pulse wave provide provide relevant information The analysis of pulse wave relevant information aboutanalysis the state of cardiovascular system due to the influThe of pulse wave provide relevant information about the state of cardiovascular system due to the influabout the state of cardiovascular system due to the ence ofthe thestate different properties ofsystem blood due vessels andinflualso about of cardiovascular to the ence of the the different on properties of and blood vessels andinflualso ence of different properties of blood vessels and also of heart functioning the shape time properties of ence of the different properties of blood vessels and also of heart functioning onal., the2011). shape and and time time properties properties of of of heart functioning on the shape pulse wave (Nichols et of heart functioning onal., the2011). shape and time properties of pulse wave (Nichols et et pulse wave al., pulse wave (Nichols (Nichols al., 2011). 2011). Parameters of pulseetwave provide relevant information Parameters pulse relevant information Parameters ofrate pulse wave provide relevant about heart of andwave also provide about the good information functioning Parameters of pulse wave provide relevant information about heart rate and also about the good functioning about heart rate and also about the good of heart. Pulse wave velocity (PWV) which isfunctioning associated about heart rate and also about the good functioning of heart. Pulse wave waveof velocity velocity (PWV)elasticity which is is and associated of Pulse (PWV) which associated to heart. the calculation blood vessels blood of heart. Pulse wave velocity (PWV) which is associated to the calculation calculation of elasticity blood vessels elasticity and blood blood to the of blood vessels elasticity and pressure values. The of the blood vessels can to the calculation of blood vessels elasticity and blood pressure values. The elasticity elasticity of the the assessment blood vessels vessels can pressure values. The of blood can significantly contribute to the overall of the pressure values. The elasticity of the assessment blood vessels can significantly contribute to system the overall overall of the significantly contribute to the assessment the state of the cardiovascular (Borik and Cap, of 2013). significantly contribute to the overall assessment of the state of the theelasticity cardiovascular system (Borik (Borik and Cap, Cap, 2013). state of cardiovascular system and A reduced makes changes in pressure wave 2013). velocstate of theelasticity cardiovascular system (Borik and Cap, 2013). A reduced makes changes in pressure wave velocA elasticity makes changes wave velocityreduced and blood pressure value which in is pressure a complex indicator A reduced elasticity makes changes in pressure wave velocity andstate blood pressure value which aa complex indicator ity and blood value which is (Peter complex indicator about of pressure cardiovascular systemis et al., 2014). ity andstate blood value which a complex indicator Fig. 1. Different shape of pulse waves in each cardioabout of pressure cardiovascular systemis (Peter (Peter et al., al., 2014). about state of cardiovascular system et 2014). 1. shape pulse in about state of cardiovascular system (Peter et al., 2014). Fig. vascular 1. Different Different shape Aof of comparison pulse waves waveswith in each each cardiosegments. ECGcardiosignal The pulse wave velocity is equal to the distance between Fig. Fig. 1. Different shape of pulse waves in each cardiovascular segments. A comparison with ECG signal The pulse wave velocity is equal to the distance between vascular segments. A comparison with ECG signal The pulse wave velocity is equal the distance enable the evaluation of time dependency between two points of measurement (D) ofto the pulse wavebetween to the vascular segments. A comparison with ECG signal The pulse wave velocity is equal to the distance between enable the evaluation of time dependency between two points of measurement (D) of the pulse wave to the enable the evaluation of time dependency between two points of measurement (D) of the pulse wave to the these signals (Salvi, 2012). timepoints difference of the pulse (D) wave distal to the proximal enable the evaluation of time dependency between two of measurement of distal the pulse wave to the these time difference of the the pulse pulse wave wave to the the proximal these signals signals (Salvi, (Salvi, 2012). 2012). time difference of distal to proximal (PTT). these signals (Salvi, 2012). time difference of the pulse wave distal to the proximal (PTT). (PTT). (PTT). Lukas Lukas Lukas Lukas
Peter ∗∗ ∗ Peter Peter Peter ∗
Ivo Ivo Ivo Ivo
Copyright © 2016, 2016 IFAC IFAC (International Federation of Automatic Control) 284Hosting by Elsevier Ltd. All rights reserved. 2405-8963 © Copyright 2016 IFAC 284 Copyright © 2016 IFAC 284 Peer review© of International Federation of Automatic Copyright ©under 2016 responsibility IFAC 284Control. 10.1016/j.ifacol.2016.12.048
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human body. For this kind of purpose we developed measurement system which include several channel for PPG measurement as well as one channel for ECG measurement for time dependency analysis. Our solution consists from two parts; hardware part for analogue preprocessing of measured signal and for AD conversion, and a software user interface for the display of measured signal and digital signal processing. Fig. 2. The Principle of transmission PPG. For reflective PPG measurement are receiver and transmitter placed side by side. PWV =
D . PTT
(1)
On the basis of the law on the transfer of shear waves, first published by (Posey and Geddes, 1973) and subsequently modified by (Bramwell and Hill, 1922) was created a mathematical model that combines the elasticity of the vascular wall inverse to square of the PWV. 3.57 2 Expandability = ( ) . (2) PWV Expandability is defined as the percentage difference vessel diameter for each increase in blood pressure of 1 mmHg. 1.1 Photoplethysmography Photoplethysmograph (PPG) is a non-invasive technique that measures relative blood volume changes in the blood vessels close to the skin. The PPG is evaluated as changes in light absorption of light which illuminates the skin. The change in volume caused by the pressure pulse is detected by illuminating the skin with the light from a lightemitting diode (LED) and then measuring the amount of light either transmitted or reflected to a photodiode (Fig. 2). The PPG monitors the perfusion of blood to the dermis and subcutaneous tissue of the skin. The blood flow to the skin can be modulated by multiple other physiological systems, the PPG can also be used to monitor breathing, hypovolemia, and other circulatory conditions. Additionally, the shape of the PPG waveform differs from subject to subject, and varies with the location and manner in which the sensor is attached.
• The hardware designed includes 6 channels for PPG measurement one channel for ECG measurement and an AD convertor. • The software part includes digital filtering of measured signals and signal processing which is needed for evaluation of state of cardiovascular system. The device had to fulfil several requirements for proper analysis of the signals: • all channels have to be measured simultaneously without time delay • a calibration system has to be implemented for accurate measurement • measurements should be comfortable for the patient • the device has to ensure patient’s safety during measurements 3. DESIGN OF THE MEASUREMENT SYSTEM The whole measurement system was developed as an independent device for the measurement of biological signals and it is composed of two parts. First, a hardware part for analogue signal preprocessing in order to ensure proper digital conversion has been designed; secondly a software part was developed for signal monitoring and digital processing of the signals measured. For our purpose, it was mandatory to ensure a proper synchronous measurement between the channels in order to avoid time delay between the samples, and also a higher sampling rate than usual was required. 3.1 Measuring hardware The device includes six individual channels for measuring pulse wave at different location on the human body and one channel for ECG measurement. The figure 3 shows the design of developed device.
2. IMPLEMENTATION OF NEW SOLUTION
For PPG measurement was used standard reflexive and transmission PPG sensors. Sub-DB9 connectivity was used for every PPG channel in order to ensure compatibility with most standard PPG sensors. The PPG signal was filtered by an analogue processing chain, mainly composed of: a Low-Pass filter (LPF) with a cutoff frequency of 6 Hz, a High-Pass filter (HPF) with a cut-off frequency of 0.8 Hz, and a Notch filter for 50 Hz power line removal. Additionally, an active LPF is used with a gain set to 31 and the output is finally filtered by a last LPF with a 4.8 Hz cut-off frequency. This processing chain is identical on the six channels used for pulse wave measurement.
The main objective was to develop a measurement device which would be able to measure pulse wave non-invasively, simultaneously, synchronously and accurately. Measurement of pulse wave should be done from several parts of
Pulse waves were measured simultaneously with one lead of ECG on a dedicated channel. ECG measurement is used for time synchronization and for the exact placement of each pulse wave to the right cardiac cycle. This will enable the evaluation of time dependency of pulse waves. ECG
The PPG measurement and PWV evaluation is non invasive and relatively comfortable for the patient (Salvi, 2012). The possibility to measure pulse waves from different locations on the human body simultaneously could provide more information about each segment of cardiovascular system. This is why the diagnostic methods based on processing of pulse wave could end up being a good candidate for the prevention of cardiovascular failure (Peter et al., 2015).
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Fig. 3. Multichannel photoplethysmography device with six channels for PPG sensors, potentiometers for adjusting of PPG wave and three connectors for ECG electrodes. ECG in this device is used for time synchronization of all of pulse wave during one cardiac cycle.
Fig. 4. The block scheme of developed hardware. measurement is obtained by using a one-lead ECG circuit. The analogue preprocessing consists of an instrumentation amplifier, active filters and amplification. The signal is the digitized and sent to the computer (Fig. 4). For an accurate measurement of the pulse wave, a calibration system has been implemented. Time delays between channels was measured using a pair of CD74HC4053F multiplexers. These multiplexors were controlled with the Arduino Nano board: when using calibration mode, all measurement channels are connected to a single transimpedance amplifier’s output. Having a strictly identical analogue signal at the filters’ input enable to evaluate and adjust time delays and gain drift between the PPG channels. 3.2 Digitalization of the signal For digitalization of the signal, control of multiplexers and communication with the computer, an Arduino Nano development board was used. This board is based on an ATMega328 microcontroller, which is an 8-bit AVR RISC-based microcontroller with an 8 channels, 10-bit ADC. The highest sampling rate of the converter is 1000 sps, thus 1400 sps for each channel. The samples are then transmitted to a host computer by the use of a Serial/USB CH340G converter. The microcontroller was also used to synchronously control the multiplexor using PWM. The device is connected and powered by a host computer using USB. After starting the Arduino board are initialized variables and setting the serial line to transmit data to a computer. The board starts to send signals of value 500 to a computer which is a kind of benchmark - isolines. The microcontroller is also working on command which entered by the user via the computer. 286
Fig. 5. Diagram of Arduino program sequence. It is control with developed LabVIEW application. There are two options (Fig. 5). The first option is to select the ”Normal”. The choice of this function sends symbol ”N” through the serial port the microcontroller starts to set the PWM output, which controls the multiplexers to LOW, thus multiplexers are inactive and transmits all the signals. Further activates the transfer of A/D converter and digital data are sequentially stored in an array of size seven, separated by tabs and gradually sent to the computer via a serial line. The choice of ”Calibration” sends letter ”K” and activates the calibration process. In this function the PWM output is set to HIGH which means multiplexers are active and to all inputs of A/D converter is bring the same signal which is transmitted over a serial line to the computer just as the case with the ”Normal”. The whole device is powered via the USB port providing a non-symmetrical +5 V power supply, and the current consumption remains below the 500 mA limit. 3.3 Measuring software Visualization and storage of data was done using applications developed using LabVIEW 2014. Serial line was opened for reading and writing data. Communication was made on port COM with 500000 baud rate. Data arriving via serial line are transferred to the individual values which correspond to each PPG and ECG channel. These values are then displayed using a block Waveform Chart. The saving of signal occurs with a period of 2 ms and output data is saved to a .txt file where the individual values are separated by a tab. When the application starts firstly is necessary to enter parameters of measured volunteer such as name, age and gender. After this procedure straight line with a value of 500 is displayed on each channel. To start measuring it is necessary to press ”Normal”. Calibration button is used
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to start the calibration mode and verify that there is no time delay between channels. When everything is set to measure, it is possible to press ”Record”. After pressing sequence of setting measuring is started. Firstly are displayed lines of value 500 on each channel. Secondly the calibration measurement is carried out for each channel. This procedure takes 10 seconds. The whole measurement takes 198 seconds and its progress is illustrated by the visual element. To end the application, click Stop.
Fig. 7. Detection of significant points on signals measured. R-peak of ECG is detected as point of heart’s systole, and subsequent valley and peak of pulse wave are detected on each channel. All of these point are used for statistical analysis and evaluation of relationship between parameters of pulse wave and state of cardiovascular system. values it is necessary to determine peak and valley of each pulse wave (Fig. 7). This detection is based on first derivation of the PPG signal.
Fig. 6. Synchronous signals from all of six PPG sensors which were measured simultaneously. Signals were measure also simultaneously with ECG which was used as time synchronization of each cardiac cycle. Blue curve shows differences between PPG which were measured on different places on human body.
It was evaluated PWV and thanks to equation 2 was evaluate expandability of cardiovascular segments. This value was compare to age.
4. SIGNAL ANALYSIS Preliminary tests were conducted on a group of 6 individuals. The group was composed of men and women from three categories of age. The youngest category was composed of two healthy sportsmen without any chronic disease. The second category was composed of middle-age man and woman with adequate body. The oldest category of patients were two people with cardiac problems. All measurements were made at rest. Each subject was sitting on a chair and at the same time wasn’t measured only pulse wave and ECG signal but also non invasive blood pressure was measured by standard automatic tonometer. Pulse waves were measured from: • • • • • •
index finger of right hand (PPG1) index finger of left hand (PPG2) index finger of left hand (PPG2) second finger of left foot (PPG4) right temporal bone (PPG5) left temporal bone (PPG6)
Fig. 8. Relationship between expandability of blood vessel and age of patient. With increasing of age; expandability of blood vessel decreases. Picture shows analysis which was made on signal from PPG1 (index finger of right hand. It can be seen that with increasing of age of patient, the expandability of blood vessel decreases (Fig. 8).
It is supposed that heart’s systole can be determine as R wave from ECG. It means that for processing of ECG signal, an algorithm for detection of R waves based on adaptive threshold was implemented to determine which peak should be taken. Digital processing of measured signals is focused on the computation of time delay between heart’s systole and arriving of pulse wave at periphery. This value is necessary for evaluation of PWV. For the determination of these 287
5. CONCLUSION It is very important to monitor state of cardiovascular system. Many times it is more useful to know information about each part of cardiovascular system. A possibility to evaluate parameters of each parts is to measure pulse wave which gives information about state of blood vessel reflecting state of cardiovascular system. Our main objective was to develop a system which could be used for this kind of purpose.
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Multichannel plethysmography extends the possibilities of standard plethysmography by enabling evaluation of the state of the cardiovascular system. Preliminary tests of the device and methods provided promising results. It was successfully tested in laboratory conditions and the next step will be to engage clinical trials. Indeed for the validation of this method it is necessary to confirm the preliminary results obtained on a bigger group of patients and compare them to standard methods for evaluation stiffness of blood vessels or blood pressure. Multichannel photoplethysmography is thus a promising method for long time monitoring of state of cardiovascular system which will bring better information about treatment of this system and it could prevent heart failure or; in the worst case, premature death. ACKNOWLEDGEMENTS The work and the contributions were supported by the project SV4506631/2101 ’Biomedic´ınsk´e inˇzen´ yrsk´e syst´emy XII’. REFERENCES Borik, S. and Cap, I. (2013). Measurement and analysis possibilities of pulse wave signals. Advances in Electrical and Electronic Engineering, 11(6), 514–521. Bramwell, J.C. and Hill, A.V. (1922). The velocity of the pulse wave in man. Proceedings of the Royal Society of London. Series B, Containing Papers of a Biological Character, 93(652), 298–306. Nichols, W., O’Rourke, M., and Vlachopoulos, C. (2011). McDonald’s blood flow in arteries: theoretical, experimental and clinical principles. CRC Press. Peter, L., Noury, N., and Cerny, M. (2014). A review of methods for non-invasive and continuous blood pressure monitoring: Pulse transit time method is promising? IRBM, 35(5), 271–282. Peter, L., Foltyn, J., and Cerny, M. (2015). Pulse wave velocity measurement; developing process of new measuring device. In Applied Machine Intelligence and Informatics (SAMI), 2015 IEEE 13th International Symposium on, 59–62. IEEE. Posey, J.A. and Geddes, L. (1973). Measurement of the modulus of elasticity of the arterial wall. Salvi, P. (2012). How vascular haemodynamics affects blood pressure. 2012, xii, 138, p125.
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IFAC-PapersOnLine 49-25 (2016) 284–288
Determination of Blood Vessels Determination of Blood Determination ofMultichannel Blood Vessels Vessels Expandability; Expandability; Multichannel Expandability; Multichannel Photoplethysmography Photoplethysmography Photoplethysmography
Vorek ∗∗ Bertrand Massot ∗∗ Iveta Bryjova ∗ ∗∗ ∗ Bertrand Massot ∗∗ Iveta Bryjova ∗ ∗ Vorek ∗ ∗ Bertrand ∗∗ Iveta Bryjova ∗ Vorek Massot Tomas Urbanczyk Vorek Bertrand Massot Iveta Bryjova ∗ ∗ Tomas Urbanczyk ∗ Tomas Urbanczyk Tomas Urbanczyk ∗ VSB-Technical University of Ostrava, Department of Cybernetics and ∗ ∗ VSB-Technical University of Ostrava, Department of Cybernetics and ∗ VSB-Technical of of BiomedicalUniversity Engineering, Ostrava,Department Czech Republic (e-mail: and VSB-Technical University of Ostrava, Ostrava, Department of Cybernetics Cybernetics and Biomedical Engineering, Ostrava, Czech Republic (e-mail: Biomedical Engineering, Ostrava, Czech Republic (e-mail: [email protected]) Biomedical Engineering, Ostrava, Czech Republic (e-mail: [email protected]) ∗∗ [email protected]) INL, UMR5270 CNRS-INSA Lyon, University of Lyon, 69621 [email protected]) ∗∗ ∗∗ INL, UMR5270 CNRS-INSA Lyon, University of Lyon, 69621 ∗∗ INL, UMR5270 CNRS-INSA Lyon, of Cedex, France INL, UMR5270 Villeurbanne CNRS-INSA Lyon, University University of Lyon, Lyon, 69621 69621 Villeurbanne Cedex, France Villeurbanne Cedex, France Villeurbanne Cedex, France Abstract: Clinicians would benefit from the evaluation of the state of the cardiovascular system, Abstract: from the evaluation the state of theusing cardiovascular system, Abstract: Clinicians would benefit from the of the of cardiovascular system, because it isClinicians one of thewould most benefit important system in humanof body, without invasive entrance. Abstract: Clinicians would benefit fromsystem the evaluation evaluation ofbody, the state state of the theusing cardiovascular system, because it is one of the most important in human without invasive entrance. because it is one of the most important system in human body, without using invasive entrance. Multichannel pulse wave measurement could be one possibility of how to noninvasively evaluate because it is one of the most important system in human body, without using invasive entrance. Multichannel pulse wave wave measurement could beproperties one possibility possibility of how how by to the noninvasively evaluate Multichannel pulse be one of to noninvasively the state of cardiovascular system. Pulsecould wave’s are affected properties evaluate of blood Multichannel pulse wave measurement measurement could beproperties one possibility of how by to the noninvasively evaluate the state of cardiovascular system. Pulse wave’s are affected properties of blood the state of cardiovascular system. Pulse wave’s properties are affected by the properties of blood vessels, like stiffness and diameter, and wave’s also byproperties heart function. An analysis of shape and time the state of cardiovascular system. Pulse are affected by the properties of blood vessels, like stiffness and diameter, and also by heart function. An analysis of shape and time vessels, like stiffness and diameter, and also by heart function. An analysis of shape and time properties of pulse wave propagation provides information about the state of the cardiovascular vessels, like stiffness and diameter, and also by heart function. An analysis of shape and time properties of pulse wave propagation about the of the cardiovascular properties of wave provides information about state of cardiovascular system. The measurement of pulse provides wave on information different locations of state human body could bring properties of pulse pulse wave propagation propagation provides information about the the state of the the cardiovascular system. The measurement of pulse wave on different locations of human body could bring system. The measurement of pulse wave on different locations of human body bring information about the different subparts of on thedifferent cardiovascular system. For this kindcould of purpose system. The measurement of pulse wave locations of human body could bring information about the different subparts of the cardiovascular system. For this kind of purpose information about the different subparts of the cardiovascular system. For this kind of purpose it is necessary to developed measurement system which can achieve pulse wave measurement on information about the different subparts of the cardiovascular system. For this kind of purpose it is necessary to developed measurement system which can achieve pulse wave measurement on it is necessary to developed measurement system which can achieve pulse wave measurement on several locations of the human body simultaneously and synchronously. The paper introduces it is necessary to developed measurement system which can achieve pulse wave measurement on several locations of the human body simultaneously and synchronously. The paper introduces several locations of the human body simultaneously and synchronously. The paper introduces developing of such system which is able to measure and displayed several PPG signals and several locations of the human body simultaneously and synchronously. The paper introduces developing ofone such system which istime ablereference to measure measure and displayed displayed several PPGand signals and developing such system which able to and PPG signals and also includeof ECG channel as ais for synchronization. Hardware software developing ofone such system which istime ablereference to measure and displayed several several PPGand signals and also include ECG channel as a for synchronization. Hardware software also include one ECG channel as a time reference for synchronization. Hardware and software materials developed forchannel the solution are described and preliminary results are presented. also include one ECG as a time reference for synchronization. Hardware and software materials developed for the solution are described and preliminary results are presented. materials developed for solution described and results are presented. materials developed for the the solutionofare are described and preliminary preliminary resultsLtd. areAll presented. © 2016, IFAC (International Federation Automatic Control) Hosting by Elsevier rights reserved. Keywords: ECG, PPG, Cardiovascular system, LabVIEW, PWV Keywords: ECG, PPG, Cardiovascular system, LabVIEW, PWV Keywords: ECG, ECG, PPG, PPG, Cardiovascular Cardiovascular system, system, LabVIEW, LabVIEW, PWV PWV Keywords: 1. INTRODUCTION 1. INTRODUCTION INTRODUCTION 1. 1. INTRODUCTION Pulse wave results from the work of the left part of heart. Pulse wave results from the work workduring of the the each left of Pulse wave results the of left part part of heart. heart. A single pulse wavefrom is generated cardio cycle, Pulse wave results from the workduring of the each left part of heart. A single pulse wave is generated cardio cycle, A single pulse wave is generated during each cardio cycle, defined by thewave interval between during two successive systoles. A single pulse is generated each cardio cycle, defined by the interval between two successive successive systoles. defined interval between two systoles. Most of by the the time, Pulse wave is measured from periphery defined by the interval between two successive systoles. Most of the time, Pulse wave is measured from periphery Most of the time, Pulse wave is measured measured from periphery like fingertip or from places showing good blood perfusion Most of the time, Pulse wave is from periphery like fingertip fingertip from showing like or from places showing good good blood blood perfusion perfusion ear lobe, or nose or places forehead. like from showing good blood perfusion like fingertip ear lobe, lobe, or nose or places forehead. like ear nose or forehead. like lobe, nose or forehead. The ear analysis of pulse wave provide relevant information The analysis of pulse wave provide provide relevant information The analysis of pulse wave relevant information aboutanalysis the state of cardiovascular system due to the influThe of pulse wave provide relevant information about the state of cardiovascular system due to the influabout the state of cardiovascular system due to the ence ofthe thestate different properties ofsystem blood due vessels andinflualso about of cardiovascular to the ence of the the different on properties of and blood vessels andinflualso ence of different properties of blood vessels and also of heart functioning the shape time properties of ence of the different properties of blood vessels and also of heart functioning onal., the2011). shape and and time time properties properties of of of heart functioning on the shape pulse wave (Nichols et of heart functioning onal., the2011). shape and time properties of pulse wave (Nichols et et pulse wave al., pulse wave (Nichols (Nichols al., 2011). 2011). Parameters of pulseetwave provide relevant information Parameters pulse relevant information Parameters ofrate pulse wave provide relevant about heart of andwave also provide about the good information functioning Parameters of pulse wave provide relevant information about heart rate and also about the good functioning about heart rate and also about the good of heart. Pulse wave velocity (PWV) which isfunctioning associated about heart rate and also about the good functioning of heart. Pulse wave waveof velocity velocity (PWV)elasticity which is is and associated of Pulse (PWV) which associated to heart. the calculation blood vessels blood of heart. Pulse wave velocity (PWV) which is associated to the calculation calculation of elasticity blood vessels elasticity and blood blood to the of blood vessels elasticity and pressure values. The of the blood vessels can to the calculation of blood vessels elasticity and blood pressure values. The elasticity elasticity of the the assessment blood vessels vessels can pressure values. The of blood can significantly contribute to the overall of the pressure values. The elasticity of the assessment blood vessels can significantly contribute to system the overall overall of the significantly contribute to the assessment the state of the cardiovascular (Borik and Cap, of 2013). significantly contribute to the overall assessment of the state of the theelasticity cardiovascular system (Borik (Borik and Cap, Cap, 2013). state of cardiovascular system and A reduced makes changes in pressure wave 2013). velocstate of theelasticity cardiovascular system (Borik and Cap, 2013). A reduced makes changes in pressure wave velocA elasticity makes changes wave velocityreduced and blood pressure value which in is pressure a complex indicator A reduced elasticity makes changes in pressure wave velocity andstate blood pressure value which aa complex indicator ity and blood value which is (Peter complex indicator about of pressure cardiovascular systemis et al., 2014). ity andstate blood value which a complex indicator Fig. 1. Different shape of pulse waves in each cardioabout of pressure cardiovascular systemis (Peter (Peter et al., al., 2014). about state of cardiovascular system et 2014). 1. shape pulse in about state of cardiovascular system (Peter et al., 2014). Fig. vascular 1. Different Different shape Aof of comparison pulse waves waveswith in each each cardiosegments. ECGcardiosignal The pulse wave velocity is equal to the distance between Fig. Fig. 1. Different shape of pulse waves in each cardiovascular segments. A comparison with ECG signal The pulse wave velocity is equal to the distance between vascular segments. A comparison with ECG signal The pulse wave velocity is equal the distance enable the evaluation of time dependency between two points of measurement (D) ofto the pulse wavebetween to the vascular segments. A comparison with ECG signal The pulse wave velocity is equal to the distance between enable the evaluation of time dependency between two points of measurement (D) of the pulse wave to the enable the evaluation of time dependency between two points of measurement (D) of the pulse wave to the these signals (Salvi, 2012). timepoints difference of the pulse (D) wave distal to the proximal enable the evaluation of time dependency between two of measurement of distal the pulse wave to the these time difference of the the pulse pulse wave wave to the the proximal these signals signals (Salvi, (Salvi, 2012). 2012). time difference of distal to proximal (PTT). these signals (Salvi, 2012). time difference of the pulse wave distal to the proximal (PTT). (PTT). (PTT). Lukas Lukas Lukas Lukas
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human body. For this kind of purpose we developed measurement system which include several channel for PPG measurement as well as one channel for ECG measurement for time dependency analysis. Our solution consists from two parts; hardware part for analogue preprocessing of measured signal and for AD conversion, and a software user interface for the display of measured signal and digital signal processing. Fig. 2. The Principle of transmission PPG. For reflective PPG measurement are receiver and transmitter placed side by side. PWV =
D . PTT
(1)
On the basis of the law on the transfer of shear waves, first published by (Posey and Geddes, 1973) and subsequently modified by (Bramwell and Hill, 1922) was created a mathematical model that combines the elasticity of the vascular wall inverse to square of the PWV. 3.57 2 Expandability = ( ) . (2) PWV Expandability is defined as the percentage difference vessel diameter for each increase in blood pressure of 1 mmHg. 1.1 Photoplethysmography Photoplethysmograph (PPG) is a non-invasive technique that measures relative blood volume changes in the blood vessels close to the skin. The PPG is evaluated as changes in light absorption of light which illuminates the skin. The change in volume caused by the pressure pulse is detected by illuminating the skin with the light from a lightemitting diode (LED) and then measuring the amount of light either transmitted or reflected to a photodiode (Fig. 2). The PPG monitors the perfusion of blood to the dermis and subcutaneous tissue of the skin. The blood flow to the skin can be modulated by multiple other physiological systems, the PPG can also be used to monitor breathing, hypovolemia, and other circulatory conditions. Additionally, the shape of the PPG waveform differs from subject to subject, and varies with the location and manner in which the sensor is attached.
• The hardware designed includes 6 channels for PPG measurement one channel for ECG measurement and an AD convertor. • The software part includes digital filtering of measured signals and signal processing which is needed for evaluation of state of cardiovascular system. The device had to fulfil several requirements for proper analysis of the signals: • all channels have to be measured simultaneously without time delay • a calibration system has to be implemented for accurate measurement • measurements should be comfortable for the patient • the device has to ensure patient’s safety during measurements 3. DESIGN OF THE MEASUREMENT SYSTEM The whole measurement system was developed as an independent device for the measurement of biological signals and it is composed of two parts. First, a hardware part for analogue signal preprocessing in order to ensure proper digital conversion has been designed; secondly a software part was developed for signal monitoring and digital processing of the signals measured. For our purpose, it was mandatory to ensure a proper synchronous measurement between the channels in order to avoid time delay between the samples, and also a higher sampling rate than usual was required. 3.1 Measuring hardware The device includes six individual channels for measuring pulse wave at different location on the human body and one channel for ECG measurement. The figure 3 shows the design of developed device.
2. IMPLEMENTATION OF NEW SOLUTION
For PPG measurement was used standard reflexive and transmission PPG sensors. Sub-DB9 connectivity was used for every PPG channel in order to ensure compatibility with most standard PPG sensors. The PPG signal was filtered by an analogue processing chain, mainly composed of: a Low-Pass filter (LPF) with a cutoff frequency of 6 Hz, a High-Pass filter (HPF) with a cut-off frequency of 0.8 Hz, and a Notch filter for 50 Hz power line removal. Additionally, an active LPF is used with a gain set to 31 and the output is finally filtered by a last LPF with a 4.8 Hz cut-off frequency. This processing chain is identical on the six channels used for pulse wave measurement.
The main objective was to develop a measurement device which would be able to measure pulse wave non-invasively, simultaneously, synchronously and accurately. Measurement of pulse wave should be done from several parts of
Pulse waves were measured simultaneously with one lead of ECG on a dedicated channel. ECG measurement is used for time synchronization and for the exact placement of each pulse wave to the right cardiac cycle. This will enable the evaluation of time dependency of pulse waves. ECG
The PPG measurement and PWV evaluation is non invasive and relatively comfortable for the patient (Salvi, 2012). The possibility to measure pulse waves from different locations on the human body simultaneously could provide more information about each segment of cardiovascular system. This is why the diagnostic methods based on processing of pulse wave could end up being a good candidate for the prevention of cardiovascular failure (Peter et al., 2015).
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Fig. 3. Multichannel photoplethysmography device with six channels for PPG sensors, potentiometers for adjusting of PPG wave and three connectors for ECG electrodes. ECG in this device is used for time synchronization of all of pulse wave during one cardiac cycle.
Fig. 4. The block scheme of developed hardware. measurement is obtained by using a one-lead ECG circuit. The analogue preprocessing consists of an instrumentation amplifier, active filters and amplification. The signal is the digitized and sent to the computer (Fig. 4). For an accurate measurement of the pulse wave, a calibration system has been implemented. Time delays between channels was measured using a pair of CD74HC4053F multiplexers. These multiplexors were controlled with the Arduino Nano board: when using calibration mode, all measurement channels are connected to a single transimpedance amplifier’s output. Having a strictly identical analogue signal at the filters’ input enable to evaluate and adjust time delays and gain drift between the PPG channels. 3.2 Digitalization of the signal For digitalization of the signal, control of multiplexers and communication with the computer, an Arduino Nano development board was used. This board is based on an ATMega328 microcontroller, which is an 8-bit AVR RISC-based microcontroller with an 8 channels, 10-bit ADC. The highest sampling rate of the converter is 1000 sps, thus 1400 sps for each channel. The samples are then transmitted to a host computer by the use of a Serial/USB CH340G converter. The microcontroller was also used to synchronously control the multiplexor using PWM. The device is connected and powered by a host computer using USB. After starting the Arduino board are initialized variables and setting the serial line to transmit data to a computer. The board starts to send signals of value 500 to a computer which is a kind of benchmark - isolines. The microcontroller is also working on command which entered by the user via the computer. 286
Fig. 5. Diagram of Arduino program sequence. It is control with developed LabVIEW application. There are two options (Fig. 5). The first option is to select the ”Normal”. The choice of this function sends symbol ”N” through the serial port the microcontroller starts to set the PWM output, which controls the multiplexers to LOW, thus multiplexers are inactive and transmits all the signals. Further activates the transfer of A/D converter and digital data are sequentially stored in an array of size seven, separated by tabs and gradually sent to the computer via a serial line. The choice of ”Calibration” sends letter ”K” and activates the calibration process. In this function the PWM output is set to HIGH which means multiplexers are active and to all inputs of A/D converter is bring the same signal which is transmitted over a serial line to the computer just as the case with the ”Normal”. The whole device is powered via the USB port providing a non-symmetrical +5 V power supply, and the current consumption remains below the 500 mA limit. 3.3 Measuring software Visualization and storage of data was done using applications developed using LabVIEW 2014. Serial line was opened for reading and writing data. Communication was made on port COM with 500000 baud rate. Data arriving via serial line are transferred to the individual values which correspond to each PPG and ECG channel. These values are then displayed using a block Waveform Chart. The saving of signal occurs with a period of 2 ms and output data is saved to a .txt file where the individual values are separated by a tab. When the application starts firstly is necessary to enter parameters of measured volunteer such as name, age and gender. After this procedure straight line with a value of 500 is displayed on each channel. To start measuring it is necessary to press ”Normal”. Calibration button is used
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to start the calibration mode and verify that there is no time delay between channels. When everything is set to measure, it is possible to press ”Record”. After pressing sequence of setting measuring is started. Firstly are displayed lines of value 500 on each channel. Secondly the calibration measurement is carried out for each channel. This procedure takes 10 seconds. The whole measurement takes 198 seconds and its progress is illustrated by the visual element. To end the application, click Stop.
Fig. 7. Detection of significant points on signals measured. R-peak of ECG is detected as point of heart’s systole, and subsequent valley and peak of pulse wave are detected on each channel. All of these point are used for statistical analysis and evaluation of relationship between parameters of pulse wave and state of cardiovascular system. values it is necessary to determine peak and valley of each pulse wave (Fig. 7). This detection is based on first derivation of the PPG signal.
Fig. 6. Synchronous signals from all of six PPG sensors which were measured simultaneously. Signals were measure also simultaneously with ECG which was used as time synchronization of each cardiac cycle. Blue curve shows differences between PPG which were measured on different places on human body.
It was evaluated PWV and thanks to equation 2 was evaluate expandability of cardiovascular segments. This value was compare to age.
4. SIGNAL ANALYSIS Preliminary tests were conducted on a group of 6 individuals. The group was composed of men and women from three categories of age. The youngest category was composed of two healthy sportsmen without any chronic disease. The second category was composed of middle-age man and woman with adequate body. The oldest category of patients were two people with cardiac problems. All measurements were made at rest. Each subject was sitting on a chair and at the same time wasn’t measured only pulse wave and ECG signal but also non invasive blood pressure was measured by standard automatic tonometer. Pulse waves were measured from: • • • • • •
index finger of right hand (PPG1) index finger of left hand (PPG2) index finger of left hand (PPG2) second finger of left foot (PPG4) right temporal bone (PPG5) left temporal bone (PPG6)
Fig. 8. Relationship between expandability of blood vessel and age of patient. With increasing of age; expandability of blood vessel decreases. Picture shows analysis which was made on signal from PPG1 (index finger of right hand. It can be seen that with increasing of age of patient, the expandability of blood vessel decreases (Fig. 8).
It is supposed that heart’s systole can be determine as R wave from ECG. It means that for processing of ECG signal, an algorithm for detection of R waves based on adaptive threshold was implemented to determine which peak should be taken. Digital processing of measured signals is focused on the computation of time delay between heart’s systole and arriving of pulse wave at periphery. This value is necessary for evaluation of PWV. For the determination of these 287
5. CONCLUSION It is very important to monitor state of cardiovascular system. Many times it is more useful to know information about each part of cardiovascular system. A possibility to evaluate parameters of each parts is to measure pulse wave which gives information about state of blood vessel reflecting state of cardiovascular system. Our main objective was to develop a system which could be used for this kind of purpose.
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Multichannel plethysmography extends the possibilities of standard plethysmography by enabling evaluation of the state of the cardiovascular system. Preliminary tests of the device and methods provided promising results. It was successfully tested in laboratory conditions and the next step will be to engage clinical trials. Indeed for the validation of this method it is necessary to confirm the preliminary results obtained on a bigger group of patients and compare them to standard methods for evaluation stiffness of blood vessels or blood pressure. Multichannel photoplethysmography is thus a promising method for long time monitoring of state of cardiovascular system which will bring better information about treatment of this system and it could prevent heart failure or; in the worst case, premature death. ACKNOWLEDGEMENTS The work and the contributions were supported by the project SV4506631/2101 ’Biomedic´ınsk´e inˇzen´ yrsk´e syst´emy XII’. REFERENCES Borik, S. and Cap, I. (2013). Measurement and analysis possibilities of pulse wave signals. Advances in Electrical and Electronic Engineering, 11(6), 514–521. Bramwell, J.C. and Hill, A.V. (1922). The velocity of the pulse wave in man. Proceedings of the Royal Society of London. Series B, Containing Papers of a Biological Character, 93(652), 298–306. Nichols, W., O’Rourke, M., and Vlachopoulos, C. (2011). McDonald’s blood flow in arteries: theoretical, experimental and clinical principles. CRC Press. Peter, L., Noury, N., and Cerny, M. (2014). A review of methods for non-invasive and continuous blood pressure monitoring: Pulse transit time method is promising? IRBM, 35(5), 271–282. Peter, L., Foltyn, J., and Cerny, M. (2015). Pulse wave velocity measurement; developing process of new measuring device. In Applied Machine Intelligence and Informatics (SAMI), 2015 IEEE 13th International Symposium on, 59–62. IEEE. Posey, J.A. and Geddes, L. (1973). Measurement of the modulus of elasticity of the arterial wall. Salvi, P. (2012). How vascular haemodynamics affects blood pressure. 2012, xii, 138, p125.
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