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Integrated Demonstration Module for Basic Biomedical Signal Processing

Daniel Santoso, F. Dalu Setiaji, Anantha Prasetya Dept. of Electronic and Computer Eng.

Satya Wacana Christian University Salatiga, Indonesia

daniel.santoso@staff.uksw.edu

Abstract—There are five vital signs normally used to monitor and document a patients’ health or deterioration during hospitalization. These vital signs consists of blood pressure (BP), oxygen saturation of the blood (SpO2), pulse rate, respiratory rate, and body temperature. Technological breakthroughs are revolutionizing the way healthcare service is being delivered including in vital signs measurement field. Most of vital signs now can be measured non – invasively and electronically. Most of vital signs monitoring devices are widely used in hospitals and health clinics. Healthcare providers require resident human resources capable of handling technical issues regarding the devices. The technician or engineer fulfilling such requirements usually graduated from study program that has specific courses related to medical devices within its curriculum. Lately, our department has been developing a course that includes biomedical variables measurement as one of the topics. Therefore a demonstration module is required as a means to provide better understanding to the students upon the concepts learned. The developed demonstration module has capability to measure and display heart signal, blood oxygen saturation, and blood pressure. The accuracy of the measurements has been verified relative to commercially available device. Experimental data set reveals that the bias and precision for heart rate measurement are 2 bpm and 6 bpm; for SpO2 measurement are 3% and 5%. The measurement bias for SP an DP is 1 mmHg and 5 mmHg respectively. The precision of developed blood pressure monitor is comparable with the one exhibited by reference device that is 6 mmHg vs. 5 mmHg for SP measurement and 5 mmHg vs. 4 mmHg for DP measurement.

Keywords—heart signal, blood oxygen saturation, blood pressure, demonstration, device

I. INTRODUCTION During hospitalisation or intensive care, traditionally, there

are five vital signs used for monitoring and documenting a patients’ health or deteoriation. These vital signs consists of blood pressure (BP), oxygen saturation of the blood (SpO2), pulse rate, respiratory rate, and body temperature [1]. Vital signs have obtained more awareness as means of identifying patients who probably at risk of harmful events and deterioration as complexity of patients in both inpatient and outpatient settings increased [2].

BP refers to the pressure exerted on the arterial walls when blood is pumped through vascular system. It’s value is determined by cardiac output, peripheral resistance, blood

volume and viscosity, and vessel wall elasticity [3]. BP measurement is considered important because it provides a reflection of how blood is flowing when the heart is contracting (systole) and relaxing (diastole). Cellular oxygen delivery can also be indicated by BP because it is the driving force of peripheral perfusion. Variations in BP may reflect body’s effort to maintain physiological equilibrium. For example, a drop in BP can be interpreted as a sign of imminent cardiac arrest [4].

SpO2 can be estimated non – invasively using clinical tool called pulse oximeter. SpO2 is a percentage of how much oxygen is being carried by oxyhemoglobin compared to its maximum capability. Normally, people need an oxygen saturation level of at least 89% to keep their cells function properly. Having a very low blood oxygen level may also cause damage to heart and brain. Therefore, should blood oxygen level is detected to be low on ambient air, external oxygen supply may be needed. The external oxygen administration, in term of quantity and time, can be determined with the help of oximeter. Some people need more oxygen supply when asleep than when awake, for instance. In other hand, other people may need more oxygen with activity than when at sedentary. The use of pulse oximeter have changed patient management and reduced the number of investigations undertaken [5].

Pulse can be defined as the rhythmic expansion of an artery produced by the increased volume of blood pushed into the vessel as a result of pumping action of the heart [6]. Several factors affecting pulse rate and intensity are age, current medical conditions, medications, and hydration status. Pulse represents cardiac impulse but does not always represent cardiac output; in case of aortic stenosis, in spite of strong cardiac contractions, pulse may lack of strength [7]. In order to obtain accurate reading, pulse rate should be assessed physically. In case of irregular pulse, even a pulse oximeter may provide inaccurate reading.

Another important variable worth to observe is respiratory rate. It serves as predictor of a number of critical illness such as acidosis [8] and cardiac arrest [9].

Body temperature indicates how thermal mechanism of the body works. The measured value is the balance between heat produced and heat lost. Clinically, there are two types of body temperature: the core body temperature and the surface body temperature. In clinical setting, body temperature refers to surface temperature. The reading may be affected by many

factors such as age, hot or cold fluids consumption, and skin exposure to ambient temperature. Wide variety of thermometer now can be used to measure body temperature. However, a study discovered notable deviations in term of accuracy and precision among commonly used thermometer types for measuring body temperature: tympanic, oral electronic, oral single – use, and temporal artery [10]. Therefore regular calibration and proper use should not be neglected, even for this simple measurement.

Developments and advancements in medical area in both method and technology have allowed better diagnosis and treatment of patients since the early time of the professional practice of medicine. Countless lives have been saved and overall quality of life keeps on improving over time because of the continous development in medical areas. In today’s modern life, technology plays an important role in every aspect as well as in our personal lives. Healthcare is definitely one of the most important factor responsible in improving and saving lives around the world.

There is no an industry that has not been impacted by modern technology revolution. Technological breakthroughs are revolutionizing the way health service is being delivered including in vital signs measurement procedure. Most of vital signs now can be measured non – invasively and electronically. Extensive research to refine human body parameter estimation, advancement in sensor material, together with intelligent processing unit underpin the technology implemented in current vital signs monitor available.

Most of vital signs monitoring devices are widely used in hospitals and health clinics. With constant change in vital signs monitor devices technology towards better performance, those healthcare providers require resident human resources capable of handling technical issues regarding the devices. The technician or engineer responsible to keep medical devices up and running first must understand the basic principles of common medical devices, create and monitor maintenance plan, and perform accurate troubleshooting actions should any problem occurs with the devices. In order to do the assigned tasks regarding medical devices maintenance, the technician or engineer should have adequate knowledge and skill in the related technical fields at least at electronics, physics, and biomedical engineering. Normally, the technician or engineer fulfilling such requirements graduated from study program that has specific courses related to medical devices – especially the electronic ones – within its curriculum.

Department of Electronic and Computer Engineering at Satya Wacana Christian University (SWCU) has been at front of creativity and new technology since it was founded. Lately, the faculty has been developing a course that includes biomedical variables measurement as one of the topics. This topic discusses about biopotentials and electrophysiology measurement, blood pressure measurement, and blood flow measurement. Compatible laboratory experiments also provided to accompany theoretical background presented in class. Furthermore, an integrated demonstration module is required as a means to help understand concepts learned. Therefore the purpose of the research is to develop a integrated demonstration module that can be used to demonstrate the

operation of sensors and circuits to acquire and process biomedical signals. The demonstration module has capability to pick up ECG signal at the chest and display the corresponding waveform on the LCD, to estimate blood saturation level using optical method and display the percentage on the LCD, and to perform sequence to measure blood pressure using oscillometric method the display the result on the LCD.

II. THEORETICAL BACKGROUND Biopotentials originate from organs are diverse. These

signals are categorized as very low amplitude (10 uV to 10 mV) and very low frequency signals (dc to few hundred hertz). Acquisitions of such signals are significantly affected by biological interference (skin impedance, electrode placement, patient motion, etc.) and disturbance from surrounding environment (power line, radio frequency, electromagnetic, etc.). Consequently, fundamental principles in acquiring biopotentials should be considered and complied. These principles involve.

Electrode design and its interface selected particularly to the application.

Amplifier circuit design to properly amplify the signal and to reject effectively noise and interference.

Good measurement techniques to reduce artifacts.

A. Heart Signal Acquisition Principles The heart continously emits biopotential signal with certain

signature called the electrocardiogram or ECG. The potential is created by sycnhronous contraction of heart muscle propagates electrical current throughout the body. The spreading electrical current generate potentials at particular points on the body. Leads are attached on the body surface in several predetermined locations to extract cardiac signal. The signal, typically 1 mVpp – 5 mVpp, is an ac signal with a bandwidth of 0.05 Hz to 100 Hz. The ECG is able to read heart electrical activity which is displayed as waveforms characterized by six peaks and valleys labeled with P, Q, R, S, T, and U. Fig. 1 illustrates the different waveforms for each of the specialized cells found in the heart and 3 – lead ECG system.

Fig. 1. Electrophysiology of the heart, typical ECG and 3 – lead ECG system.

A dipole or vector between two charged points can be used to depict heart’s electrical activity. Electrodes arrangement on the body establish the view of the vector as a function of time. In its most basic form, electrode can be arranged based on Einthoven’s triangle, known as 3-lead ECG system. .

Each apex of imaginary triangle drawn surrounding the heart represents the location of the fluids around the heart link electrically with the limbs. Lead I provides the potential difference of right and left arms, Lead II measures difference between the right arm and left leg, and Lead III between the left arm and left leg.

B. Pulse Oxymetry Principles Blood red cells contain essential protein named

hemoglobin. This protein later will react with oxygen to combine and form oxyhemoglobin (HbO2). Oxygenated hemoglobin inside blood red cells circulate through whole body, supplying tissues with oxygen. When blood gets in contact with a cell, oxyhemoglobin releases oxygen and becomes deoxyhemoglobin (Hb). Then, blood without oxygen returns to heart’s right atrium to repeat the gas exchange process.

Pulse oximetry or photopletyhsmography (PPG) is the non – invasive method to measure of the oxygen saturation (SpO2). The reading represents the amount of oxygen dissolved in blood based on the detection of HbO2 and Hb. The bloodstream color is affected by the concentration of HbO2 and Hb, and their absorption coefficient are measured using light source with two wavelengths; 660 nm (red light spectrum) and 940 nm (infrared light spectrum). HbO2 and Hb have different light absorption characteristic. HbO2 has higher absorption rate at 940 nm while Hb has higher absorption rate at 660 nm. A photo – transistor is used to receive non-absorbed light spectrum emitted from infrared / red LED. The signal received by photo – transistor represents the light that has been absorbed by body part (usually finger) and consists of dc component and ac component. The dc components represents the light absorption by the tissue, venous blood, and non – pulsatile arterial blood. Pulsatile arterial blood represented by ac component of the signal. Fig. 2 illustrates HbO2 / Hb light absorption characteristic and processed signal received by photo – transistor.

Fig. 2. HbO2 / Hb light absorption chart and signal components received by photo – detector.

The pulse oximeter calculates analyzes the light absorption of two wavelengths from the pulsatile – added volume of oxygenated arterial blood (ac / dc) and calculates the absorption ratio, as in:

(1)

SpO2 is then calculated with empirical formula:

(2)

C. BP Monitor Operating Principles The origin of blood pressure is the pumping action of the heart. Its value mainly depends on the cardiac output and peripheral resistance. The term blood pressure most commonly refers to arterial blood pressure which is the pressure exerted on the arterial walls when blood flows through the arteries. Systolic pressure (SP) is the highest blood pressure which occurs when the heart contracts and eject blood to the arteries and diastolic pressure (DP) is the lowest blood pressure occuring between the ejections of blood from the heart. The values of blood pressure vary significantly during the course of 24 hours according to an individual’s activity. In a healthy adult at rest, SP and DP is approximately 110 mmHg and 70 mmHg respectively.

Most electronic blood pressure monitor nowadays works on oscillometric principle because of its simplicity and repeatibility. Blood pressure measurement using the oscillometric principle depends on the intra – arterial pulsation propagated to the occluding cuff encircling the upper arm. An occluding cuff usually secured on the left upper arm and is connected to a miniature air pump and a pressure sensor. Cuff is rapidly pumped to about 30 mmHg above typical SP. The pressure is then slowly released. As cuff losses pressure, when SP value approaches, pulsations start to be noticeable. These pulsations is caused by contraction of heart ventricle and can be used as a means to calculate heart rate. Pulsations grow in amplitude until mean arterial pressure (MAP) is reached then decrease until subside completely. Fig. 3 shows the cuff pressure versus pulsations.

The value of SP is usually taken from cuff pressure when the pulsation amplitude is 55% of the maximum amplitude. DP value is also taken from cuff pressure, that is when the pulsation amplitude is 85% of the maximum amplitude.

Fig. 3. Indirect blood pressure measurement: oscillometric method.

III. DESIGN AND IMPLEMENTATION The main goal of the research is to design and realize a

prototype of integrated training module that has ability to perform ECG, SpO2, and BP measurements and display the results on 2.8” touchscreen graphical LCD. The touchscreen also serves as user interface to select which measurement action to be performed. Arduino Mega 2560 is employed as the controller. Fig. 4 shows overall block diagram of the system.

Fig. 4. Design overview of the integrated training module.

A. ECG Module ECG module is built based on AD8232 chip. It is an

integrated signal conditioning block designed to extract, amplify, and filter ECG signal in the presence of noisy conditions. The noise may be caused by body motion or remote electrode placement. The AD8232 is basically an instrumentation amplifier combined with configurable low – pass filter and high – pass filter for eliminating motion artifacts and additional noise. An amplifier for driven leg circuit is also included to improve common – mode rejection of the line frequencies in the system.

In this application, the chip is configured to monitor ECG waveform with assumption that the patient remains still and therefore motion artifacts are not significant. The cut – off frequency of the high – pass amplifier is set to 0.5 Hz followed by 40 Hz low pass – filter. The third electrode is driven to improve common – mode rejection ratio. The low – pass filter stage also includes a gain of 11 to bring the total system gain approximately 1100. The fully conditioned ECG signal is present at chip’s output and interfaced directly to the controller’s analog input. Biopotential signal from the heart is obtained using three electrodes placed on the chest, as illustrated in Fig. 5.

Fig. 5. Block diagram of ECG module.

B. PPG Module The pulse oximeter module requires two different

wavelength to estimate oxygen saturation in blood. Two LEDs are used to emit these wavelengths, a LED emitting visible red light at 660 nm and the other LED emitting infrared light at 940 nm. The light passes through the finger and the remaining intensity received by photo – transistor. Since there is only one photo – transistor, signal to drive the LEDs must be multiplexed. Fig. 6 shows block diagram of PPG module.

Fig. 6. Module to perform pulse oximetry function.

A LED driver based on transistor is used to provide each LED with sufficient power to work properly. Red LED and infraredred LED are illuminated alternately to obtain light absorption by Hb and HbO2 respectively. The output generated by the photodetector is a current that represents the light absorption. The current is then converted to voltage to be filtered and fed to the controller. The filter is a band – pass filter that has lower cut – off frequency at 0.5 Hz and higher cut – off frequency at 5.3 Hz. This filter also provides a gain of 101. Dc component of the signal appears on the output of photo – transistor (after converted to voltage) while ac component appears on the output of the filter. Both signals are sent to controller’s analog input.

Samples are stored on a software buffer, averaged, and plotted on the LCD. Absorption ratio and SpO2 value is calculated using (1) and (2) respectively. A peak detection algorithm is used to perform beats per minute (bpm) calculation.

C. BP Module BP module incorporates oscillometric pricnciple for blood

pressure measurements. This non – invasive method requires external arm cuff to occlude blood flow in patient’s arm. The arm cuff is inflated using 12 V miniature air pump controlled with an controller’s digital output and deflated by activating an 12 V exhaust valve controlled with another digital output. A driver is needed to bridge controller’s control signals with the components to activate. The pump is able to deliver pressure up to 450 mmHg while the valve can hold up well up to 350 mmHg.

The pressure variations in the arm cuff is measured by using MPX5050GP sensor which has sensitivity of 12 mV / mmHg. This sensor is directly connected to the filtering and amplification stage. The band – pass filter allows signals with frequency between 0.5 Hz – 5.3 Hz to pass while providing gain of 100. The circuit equipped with virtual ground to enable negative signal reading by the controller. Fig. 7 shows block diagram of BP module.

Fig. 7. Functional block diagram for BP module.

IV. RESULT AND ANALYSIS A prototype of laboratory training module has been

successfully designed, implemented, and tested. The module integrates three circuits to acquire, process, and display biomedical signals including heart signal, blood oxygen saturation, and arterial pressure. Fig. 8 shows the front view of the prototype when being used to SpO2.

Fig. 8. The LCD shows PPG, heart rate of 96 bpm, and SpO2 of 98%.

The accuracy of the measurements has been verified relative to commercially available device. The outcome of ECG monitor and SpO2 monitor of the prototype was compared to the reading from Riester ECG dan SpO2 monitor. The experimental data set reveals that the bias between two devices for heart rate measurement is 2 bpm and the precision is 6 bpm. The precision of the reference device is 4 bpm. The ECG was clearly displayed on LCD and P, Q, R, S, T marks also noticeable. Fig. 9 shows the LCD displaying ECG while being compared to the printout from the reference device.

Fig. 9. The ECG printout compared to the waveform on LCD.

Performance evaluation on developed SpO2 monitor indicates that the measurement bias is 3% and the precision is 5%. The precision of reference device is 1%, which is significantly better. This may be due to non – standard sensor used in prototype. The blood pressure monitor was compared relatively to the digital blood pressure monitor from Omron. The experimental data set reveals the measurement bias for SP an DP are 1 mmHg and 5 mmHg respectively. The precision of developed blood pressure monitor is comparable with the one exhibited by reference device that is 6 mmHg vs. 5 mmHg for SP measurement and 5 mmHg vs. 4 mmHg for DP measurement.

V. CONCLUSION AND FUTURE WORK In this paper, we present a prototype of a integrated

laboratory training module for measuring biomedical signals, including heart signal, blood oxygen saturation, and arterial blood pressure. The prototype exhibits comparable performance to the reference instruments in term of bias and precision for most of the measurement parameters. Note should be taken for SpO2 measurement performance as it has significantly lower precision compared to the reference instrument.

Next generation of the prototype will employ circuit enhancements for reducing electric interference, filtering noise, and reduction of artifacts. In attempt to improve precision for SpO2 reading, the sensor will be replaced with the one which complies medical standard. Hopefully those improvements can yield a signal acquisition of acceptable quality in most laboratory senttings.

REFERENCES

[1] T. Ahrens, “The most important vital signs are not being measured,” Aust. Crit. Care, vol. 21, no. 1, Feb., pp.3 – 5, 2008.

[2] M. Elliot and A. Coventry, “Critical care: the eight vital signs of patient monitoring,” British Journal of Nursing, vol. 21, no. 10, pp. 621 – 625, 2012.

[3] S. Fetzer, Vital sign, 6th ed., St. Louis, Mosby / Elsevier, 2006.

[4] K. Rich, “In – hospital cardiac arrest: pre – event variables and nursing response,” Clin. Nurse. Spec., vol.13, no. 3,pp. 147 – 53, 1999.

[5] C. Lockwood, T. Conroy – Hiller, T. Page, “Vital signs,”JBI Reports vol. 2, no. 6, pp. 207 – 30, 2004.

[6] T. Piper, Stedman’s medical dictionary for the health professions and nursing, 6th edn., Philadelphia, Lippincot, 2008

[7] S. Smith, D. Duell, B. Martin, Clinical nursing skills: basic to advanced skills, 7th edn., Upper Saddle River, Pearson, 2008.

[8] N. Cooper, K. Forrest, P. Cramp, Essential guide to acute care, 2nd edn. Oxford, BMJ Books, 2006.

[9] M. Cretikos, J. Chen, K. Hillman, R. Bellomo, S. Finfer, A. Flabouris, “The objective medical emergency team activation criteria: a case control study,” Resucitation, vol. 73, no.1,pp. 62 – 72, 2007.

[10] T. Frommelt, C. Ott, V. Hays, “Accuracy of different devices to measure temperature”. Med. Surg. Nurs., vol. 17, no. 3, pp. 171 – 4.