Development of a Noninvasive Continuous Blood ...

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availability of this method for swift resetting of Vo during non- invasive beat-by-beat ... developed noninvasive blood pressure measurement system. Section III ...
IEEE/OSA/IAPR International Conference on Informatics, Electronics & Vision

Development of a Noninvasive Continuous Blood Pressure Measurement and Monitoring System Md. Manirul Islam1, Fida Hasan Md. Rafi1, Abu Farzan Mitul1 and Mohiuddin Ahmad1,2

M. A. Rashid3, Mohd Fareq bin Abd Malek 4 3,4

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Department of Electrical and Electronic Engineering 2 Department of Biomedical Engineering Khulna University of Engineering & Technology Khulna, Bangladesh Email: [email protected], [email protected] Abstract— Non invasive continuous blood pressure measurement system is more useful than conventional blood pressure measurement systems. The arterial system is extraordinarily well regulated blood delivery network in human body. It responds very quickly according to the body movements like altered body position, or in sudden excitation. So now a day’s continuous blood pressure monitoring devices are becoming more essential. Many types of blood pressure measurement devices are available. Those devices allow only few blood pressure readings in every 10 minutes. In contrast to them, we develop a very low cost noninvasive continuous blood pressure measurement and monitoring system. It measures blood pressure using volume oscillometric method and photoplethysmography technique during a long time period continuously. The rate of change of blood volume in an organ such as finger has a linear relationship with blood pressure. This rate of change of blood volume in finger is measured by an optical sensor network which estimates blood pressure. It displays the numerical value of systolic and diastolic blood pressure in a mini LCD. Our developed system is reliable, accurate and less expensive. Keywords-Noninvassive measurement; continuous pressure; monitoring system; oscillometric method;

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blood

INTRODUCTION

Arterial blood pressure is the force exerted by the blood on the wall of a blood vessel as the heart pumps (contracts) and relaxes. Blood pressure is comprised of two numbers: Systolic pressure (the force of blood in arteries as the heart contracts and pushes it out) and diastolic pressure (the force of blood in arteries as the heart relaxes). Understanding circulation will help about understanding and accurately measurement of blood pressure. Circulating blood provides transportation and communication system between the body's cells and serves to maintain a relatively stable internal environment for optimum cellular activity. Blood circulates because the heart pumps it through a closed circuit of blood vessels. Blood flow through the heart and the blood vessel is unidirectional, flowing into the heart from the pulmonary and systemic veins, and out of the heart into pulmonary and systemic arteries. Blood transports O2 and nutrients to tissues, and carries metabolic waste away from the cells. The transportation is made possible by a “pressurized vessel” system, the arteries,

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School of Electrical Systems Engineering University Malaysia Perlis (UNiMAP) Perlis, Malaysia E-mail: [email protected]

veins, arterioles, vacuoles and capillaries and out of the heart into pulmonary and systemic arterial. The standard arterial blood pressure curve is shown in Fig. 1. Blood pressure measurement system can be classified into two categories: (i) Invasive (direct) (ii) Noninvasive (indirect).

Figure 1. Standard Arterial blood pressure curve [10].

Invasive techniques of BP Measurement involve inserting a catheter into the vascular system which brings high risks of embolism, arrhythmia, heart attack and a certain percent of mortality. This method is not convenient for everyday application. It will only be used when absolutely necessary. The non invasive devices are safer, easier to use and can be utilized in most situations. Various noninvasive methods are available like Electronic Palpation method, Volume Oscillometric (VO) method, Volume Compensation (VC) method, Arterial Tonometry method etc. Among those ascultatory methods, Oscillometric methods are continuous. For conventional cuff-sphygmomanometer system the blood pressure readings are not continuous. Moreover it uses the invasive principle for blood pressure measurement which is bothersome for patients. So we have designed a continuous noninvasive blood pressure measurement and monitoring system. The overall cost for this system is also being lower than present devices. It is troublesome to monitor a patient’s blood pressure continuously during surgeries or in critical situations using conventional mechanical blood pressure measurement devices due to their invasive method and several faulty readings in

ICIEV 2012

IEEE/OSA/IAPR International Conference on Informatics, Electronics & Vision modern digital devices. But once our system is calibrated for a patient it will continuously show BP readings using non invasive measurement technique. The system is developed by considering volume oscillometric method. Volume-Oscillometric (VO) method is based on the vascular unloading principle and the characteristics of the pressure volume relationship in the artery. It employs photoelectric plethysmography [1] to detect the volume of blood changes in the artery. The VO method is similar to the oscillometric method except that it is based on arterial blood volume oscillations instead of cuff pressure oscillations. It can measure Systolic Pressure, Diastolic pressure, Mean Arterial Pressure and can be used for long term ambulatory monitoring. The concept for this volume oscillometric method embedded system is shown in Fig. 2. Blood

Finger

LED

LDR Light

Cover Figure 2. Photoplethysmography technique in finger.

A high intensity LED and a LDR (Light Dependent Resistor) is placed at the edge of a finger as shown in Fig. 2. The resistance of the LDR changes according to the light intensity received by the LDR. The change in resistance is proportional to the change of blood volume and as well as blood pressure in the finger. This technique is used here. The main contribution in our work is the use of visible light instead of infra red (IR), ultrasonic sound or electromagnetic wave (EMW). These other sources have some adverse effect on human body if those are focused continuously on human body for a long period of time. Also light sources (LED) and sensors (LDR) are cheaper than previously mentioned sources. Moreover we have implemented the amplifier circuit with OpAmp 741 for high gain amplification and ATMEGA 8 microcontroller for compiling the blood pressure with preloaded program. The micro controller has a built-in ADC. Therefore no need for extra A/D converter and this results low cost compact BP measurement system. Our system kit can be used in clinics, hospitals and personal usages. It is very helpful in situations like surgery where continuous and accurate BP monitoring is needed. Author in [1, 5] designed a wearable blood pressure sensor that suits into the existing MEMSWEAR platform. The blood pressure measurement was semi continuous and using photoplethysmography technique. Author used a motor with a small sensor in the design to measure blood pressure. Authors in [3] developed a fully automated non-conscious monitoring system for home health care. In the paper, authors described the structural detail of a newly developed toilet-seatinstalled blood pressure measurement system and some results were obtained by the system. Authors also described the outline of a newly designed system for measuring hydrostatic

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pressure difference between the heart and the measuring site, i.e., thigh, during blood pressure measurement. Authors in [4] offered a novel alternative or companion to existing oscillometric BP measurements that uses natural arm motions to provide BP measurements. Their paper presented a new principle for noninvasive blood pressure measurements through a modified volume-oscillometric technique [2] that eliminates an inflatable pressure cuff, and instead takes advantage of natural hydrostatic pressure changes caused by raising and lowering the subject’s arm. This methodology provided the advantage of using an absolute gauge pressure reference for measurements, and does not necessarily require additional actuation. Authors in [6] estimated the blood pressure estimation method was based on a presumption that there was a singular relationship between the pulse wave propagation time in arterial system and blood pressure. The parameter used in this study is pulse wave transit time. In [7], authors demonstrated a new method for continuous real-time measurement of blood pressure during daily activities. This method was based on blood pressure estimation from pulse wave velocity calculation. Authors in [8] developed a system using Korean traditional medicine, the degree of the pulse depth to diagnosis & analysis with pulse wave. Using clinical data Authors selected APm (applied pressure which has a maximum value of pulse wave), elasticity of wrist tissue, depth of blood vessel, cardiac output and h1 as parameters to estimate blood pressure. They also showed the differences in sphygmomanometer data and their system’s data for SP, DP, MAP and PP, according to American Standard. Authors in [9] developed a new technique for determining servo reference value (Vo) for the volume-compensation method. In their method, the period of time for Vo determination was significantly reduced compared with the volume-oscillometric method. This result also indicates availability of this method for swift resetting of Vo during noninvasive beat-by-beat BP measurement. Compared to this method, we also get swift response of noninvasive measurement due to use of photoplethysmography technique BP using volume oscillometric method. The performance of our developed system is better compared to the performance of previous works. Our experiments result shows less mean difference (MD) and standard deviation (SD). This paper is organized as follows: Section II describes the developed noninvasive blood pressure measurement system. Section III explains the simulation results as well as calculation of real results. Finally, section IV concludes the entire paper. II.

PROPOSED DEVELOPED SYSTEM

Fig. 3 shows the block diagram of our developed system. Fig. 4 illustrates the pictorial view of the developed system. The detailed electronic circuit of the amplifier circuit with automatic reference selector is shown in Fig. 5. The working principle of the developed system for different blocks is discussed below.

ICIEV 2012

IEEE/OSA/IAPR International Conference on Informatics, Electronics & Vision In our designed system, a high intensity LED is placed at one side of a finger and a LDR (Light Dependent Resistor) is placed at another side. The light is absorbed by the blood, mussels, skin and bones of the finger. With the change of blood pressure the volume of blood vessels are varied. The volume of other parts of the finger remains constant. So the light absorption is varied only by the change of volume of blood. We know the resistance of the LDR is high in dark and becomes low when light falls on it. Its resistance is inversely proportional with light intensity.

low frequency signal is received by the LDR. The change in resistance of LDR is very low even in milliohm range. Therefore, the output voltage from the LDR has a large amount of dc component with a small amount of ac component. For example if the pulse rate is 75 BPM then frequency of ac signal is 75/60=1.25Hz. The electronic equivalent circuit of weak bio-signal is shown in Fig. 6.

Gain control

Automatic reference selector Mini LCD

Amplifier

Sampler

ADC

Microcontroller

Calibration control

Figure 3. Block diagram of our developed system. Figure 5. PSpice simulation of double stage amplifier.

Figure 4. Pictorial view of our developed system.

Figure 6. Electronic equivalent of Weak bio signal.

In our designed system, a high intensity LED is placed at one side of a finger and a LDR (Light Dependent Resistor) is placed at another side. The light is absorbed by the blood, mussels, skin and bones of the finger. With the change of blood pressure the volume of blood vessels are varied. The volume of other parts of the finger remains constant. So the light absorption is varied only by the change of volume of blood. We know the resistance of the LDR is high in dark and becomes low when light falls on it. Its resistance is inversely proportional with light intensity.

This weak bio signal is then amplified by a double stage very high gain amplifier using Op Amp as shown in Fig. 5. But the output voltage of the LDR has a large amount of dc component as mentioned earlier and amplifying the signal directly results in saturation of the amplified signal. So to avoid this phenomenon, a subtractor circuit is used which can automatically null the dc component. But filter can’t be used here as frequency of ac signal is 0.8 Hz to 1.4 Hz. Therefore the subtractor circuit is made by an Op-amp which acts as automatic reference selector to suppress the dc component.

When systolic blood pressure occurred in the human body, the blood volume in the finger becomes maximum and light absorption is also maximum. Therefore light falls on the LDR is minimum and its resistance is maximum. During systolic pressure resistance of LDR is high (maximum). Similarly during diastolic pressure resistance of LDR is low (minimum). So, it can be concluded that blood pressure is directly proportional to the resistance of LDR. A low magnitude and

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For first stage (From Fig. 5), V01 = −

RF1 (Vi − Vref ) = A1 (Vi − Vref ) R1

When R1=R2 and RF1=R3. Here, RF1= 820KΩ and R1=10 KΩ. So A1=82

ICIEV 2012

(1)

IEEE/OSA/IAPR International Conference on Informatics, Electronics & Vision Pulse Rate (BPM) = 60/ Pulse to Pulse Interval (seconds).

For 2nd stage (From Fig. 5), V02 = −

RF 2 (V01 ) = A2 (V01 ) R9

The mini LCD display is interfaced with microcontroller as shown in Fig.9.

(2)

Where RF2 =820 KΩ and R9=1KΩ.So the gain A2=820

Sensor

Finally,

Amplifier

RF 1× RF 2 V02 = (Vi − Vref ) = At (Vi − Vref ) R1 × R9 820 × 820 = (Vi − Vref ) = 67240 (Vi − Vref ) 10 × 1

(3)

Where At is the total gain of the amplifier and At = 67240. A variable resistor may be used instead of RF2 and the total gain of the amplifier can be varied from 82 to 67240. So we can easily analyze the low amplitude (may be in micro volt range) bio-signal from the LDR using this amplifier circuit. The amplifier’s output is then fed to a microcontroller where it is sampled and quantized. To find out the largest (represents SP) and the smallest (represents DP) value form the output sampled and quantized voltage a program is written using BASCOM-AVR software into the micro controller. The microcontroller displays SP, DP and Pulse rate in mini LCD using the following algorithm of Fig. 7. Moreover the flow chart of the developed system is shown in Fig. 8. In Fig. 8 x1, x2, x3 are continuous sampled data. As systolic pressure is the largest value among the sampled data for a particular period, so x2 must greater than x1 and x3. For diastolic pressure this condition is vice versa. More measurements can be loaded to the micro controller using these formulas.

main () { start: get_adc_input(); store_input(); check_systolic_pressure(); check_diastolic_pressure(); check_pulse_rate(); if (systolic){display_systolic_pressure();} if (diastolic){display_diastolic_pressure();} if (pulse_rate){display_pulse_rate();} goto start; } Figure 7. Algorithm for systolic and diastolic pressure and pulse rate measurement in the microcontroller.

Pulse Pressure (mm Hg) = Systolic Pressure - Diastolic pressure. Mean Arterial Pressure (MAP) (mm Hg) = 1/3 (Pulse pressure) + Diastolic pressure.

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ADC

Sampling(x1, x2, x3)

if x1>x2

if x1x2

if x3