Abstract--In noninvasive measurement of instantaneous blood pressure (BP) and cardiac output (CO) on a beat-by-beat basis, the key points in actual development are to reduce the inconvenience and discomfort due to cuff occlusion of the biological segment for BP measurement, to give a less troublesome measurement for the subject for CO measurement, and to improve the stability for the physiological information detection. In this system, we develop a new device and use a local pressurization method to overcome venous congestion at measuring site of the finger, use a spot-electrode array to replace the conventional band-electrode, and use information processing technology to overcome the problems such as motion artifact, detection accuracy and data storage at long term measurement. Experimental result shows that the monitoring system is reliable one capable of allowing noninvasive BP and CO measurement on a beat-by- beat basis, and its signal detecting and information processing are effective.

Index Terms—Noninvasive measurement, Cardiovascular Haemodynamic function, Biomedical, Information technology.

I. INTRODUCTION

lood pressure (BP) and cardiac output (CO) are physiologically fundamental variables for the assessment

of cardiovascular hemodynamic function. Noninvasive measurement of BP and CO are desirable for medical and bio-engineering fields such as clinic diagnosis and treatment, basic and sports medicine, psycho-physiological and bio-feedback research, health science, and so on. Up to now, the ideal technique for each measurement has to be developed and a lot of methods have been used. However, all of the methods have their own advantages and disadvantages [1,2]. It is therefore important to develop simple, convenient, and reliable instrumentation capable of allowing non-invasive BP and CO measurement.

BP and CO of human, as we know, are considering changes in their momentary levels, which are due to physiological

This work was supported in part by the Japan Science and Technology

Agency (pre-venture business project 2002). Y. Song is a Ph.D, professor at the School of Mechanical and Electronic

Engineering, Heilongjiang University, No. 74 Xuefu road, Harbin, 150080 China (e-mail: syl@ hlju.edu.cn).

S. Gao is a M.Sc., senior engineer at the School of Mechanical and Electronic Engineering, Heilongjiang University, No. 74 Xuefu road, Harbin, 150080 China.

A. Ikarashi is a M.Sc., researcher at the Faculty of Engineering, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192 Japan.

S. Tanaka is a Ph.D., associate professor at the Faculty of Engineering, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192 Japan.

K. Yamakoshi is a D.Med. & D.Eng., Professor at the Faculty of Engineering, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192 Japan.

conditions, physical activities, psychological and mental stresses, and environmental conditions [3-5]. Sometime, the changes are very big. For example, the change of BP may be as much as 70 mmHg for systolic and 40 mmHg for diastolic pressure. Therefore, a single measurement at a discrete point in time may be of little meaning [4,5]. For this reason, ambulatory and long-term monitoring of cardiovascular variables during daily life is one of the best ways for the assessment of cardiovascular hemodynamic function, also for early diagnosis and treatment of cardiovascular and other diseases.

This paper reports our currently developed multipurpose handy-type monitoring system for cardiovascular hemo- dynamic functions based on the volume oscillometric [6] and volume compensation method [7-8] for noninvasive BP measurement and electrical admittance method [9] for noninvasive CO measurement. The detection principle and signal processing method are also introduced in this paper.

II. METHOD

A. Description of the system The newly developed ambulatory system is based on the

simultaneous measurements of BP and CO on a beat by beat basis using a combination of the volume-compensation method and admittance cardiography with a µT-Engine digital technique. The system is also possible for intermittent measurement of BP using the volume-oscillometric method. Fig. 1 shows a block diagram of the system. The system consists of three parts. The first is the portable part consisting of a main portable unit, a cuff-pressure controller unit and a finger-cuff unit. The second is a cradle that connects the main portable unit and data processor unit and display device. And the third is data processor unit and display device for data retrieval and analysis and display.

As shown in Fig. 1, the main portable unit has seven functional roles: BP measurement, CO measurement, signal processing and control of each measurement using µT-Engine (32bit CPU), data storage using a memory media (SD card), data display using a LCD, and power supply using a lithium polymer rechargeable battery (7.4V, 2100mAh, Advanced Energy Industries, Inc., USA) which allows more than 300 min continues use. Also, the measured signals can be sent to the server in medical institution in real time using a PSH card on the µT-Engine board when the value of BP or CO of the subject is in the alarm condition set in advance. During monitoring, the subject carries the portable unit (72mm× 33.5mm×130mm) in his/her breast pocket of specially made holder.

The cuff-pressure (Pc) controller unit (77mm× 56mm × 30mm) consists of a newly developed nozzle-flapper type

A Multipurpose Handy-type Monitoring System for Cardiovascular Haemodynamic Functions Based on Biomedical and Information Technology

Y. Song , S. Gao, A. Ikarashi, S. Tanaka and K. Yamakoshi

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electro-pneumatic converter and an air pump (P36B01, Oken Seiko, Japan). A moving-coil type electro-magnetic actuator controls the distance between the flapper and the nozzle with

1.0mm inner diameter to adjust leakage of the airflow from an air pump. During monitoring, the unit is fixed to the wrist by a belt.

The local pressurizing finger-cuff unit is composed of a solid

shape cuff, a cuff-supporter, a reflectance photoplethysmo- graph, a pressure sensor and a magic tape belt, as shown in Fig. 2.

The reflectance photoplethysmograph is fixed inside the cuff,

which consists of a near-infrared LED with a wavelength 970nm and a red light LED with a wavelength 660nm as a light source, and a photodiode as a photo-detector. The cuff is attached on the inside, the pressure sensor on the outside of the cuff-supporter, on which there is a hole between the cuff and the pressure sensor so that the pressure sensor can detect the

pressure in the cuff. The reason for using the LEDs with two different wavelengths is to provide a function of arterial saturation monitoring based on the principle of the pulse oximetry. During monitoring, the unit is fixed to the index or middle finger by the magic tape belt.

Fig.3 Optimal spot-electrode array

An optimal spot-electrode array for electrical admittance

cardiography to estimate CO or stroke volume (SV) used in this system is shown in Fig. 3. A pair of spot-electrodes for current

Fig.1 Schematic block diagram of the multipurpose handy-type monitoring system

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injection is located on the positions behind ear and on the right lower abdomen. On the other hand, a pair of spot-electrodes for voltage pick-up places on the medial portion at the level of clavicle and on the portion above the xiphisternum. A current source is 50 KHz sinusoidal signal of 2 mArms. The current passes through a subject’s thorax by a pair of spot-electrodes, then the resultant voltage drop between a reference spot-electrode and each of voltage pick-up spot-electrodes is measured.

B. Volume-oscillometric method This method is based on the nonlinear nature of the

pressure-volume characteristics in the artery (arterial tube law) [6]. When applying a gradual change in counter-pressure (cuff pressure, Pc) externally to the biological segment, characteristic changes in the amplitude of arterial volume pulsation, ∆V, produced by the pulse pressure are observed due to the arterial tube law. When a finger is used as a measuring site, ∆V can be easily detected indirectly by a transmittance (or reflectance) photoplethysmograph (PGp; ∆V PGp) using a near-infrared LED as a light source and a sensitive photodiode as a photodetector. The determination of indirect systolic blood

pressure (SBP) and mean pressure (MBP) by this method is given as the Pc values corresponding respectively to the systolic end point (SEP) and the point of maximum amplitude of volume pulsation (MAP), see the left part in Fig. 4. Diastolic pressure (DBP) cannot be obtained with this method directly, but it can be calculated from SBP and MBP determined above based on the waveform of the volume pulsation.

C. Volume-compensation method The measuring method is briefly described as follows. When

Pc is gradually increased, the unloaded vascular volume (Vo) is determined from the mean level of the DC component of the PG signal (PGdc) at point of maximum amplitude of the pulsation signal of PG (PGac), based on the principle of the volume-Oscillometric method just mentioned above. A servo control error, produced by subtracting the instantaneously measured arterial volume from the reference value Vo, is then fed to the compensator to clamp the vascular volume at the reference value. In this way, the intra-arterial pressure can be indirectly obtained by measuring Pc, see the middle part in Fig. 4.

D. Cardiac output measuring method The most commonly used cardiac output measuring method

is the electrical impedance cardiography (ICG) [10], or its reciprocal electrical admittance cardiography (ACG). As shown in the right part of Fig. 4, two outer band-electrodes are placed on the neck and the abdomen impose sinusoidal current (usually, 50KHz, 2mArms), and two inner band-electrodes are placed more than 3 cm apart from each current electrode detect the potential difference between a length L. If the thorax is

roughly assumed to be a two-compartment coaxial cylindrical model composed of the aorta and its surrounding tissues, as modified by an original proposal given by Nyboer [11], the total thoracic admittance Y after blood inflow into the aorta can be represented by an electrically parallel connection with the total admittance Y0 before inflow and the admittance component due to the increase in blood volume (∆V) in the aorta (Yb=∆V/ρbL2). Therefore, Y can be expressed as Y=Y0+Yb. Accordingly, the admittance change before and after the blood

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Fig. 4 Principle of non-invasive BP·CO measuring technique

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inflow is derived as ∆Y= Y -Y0= Yb. Assuming that the volume of blood inflows into the aorta without outflow during the ventricle ejection time (Ts), the admittance change can be expressed as ∆Y*= dY /dt|max·Ts. Replacing this value into ∆Y in the equations yields the formulas for estimating SV and CO, shown in below right part of Fig. 4. Where, ρb is the blood resistivity and is a function of hematocrit. Usually, ρbis inΩcm, L in cm, Y in S, dY/dt in S/s and Ts in s, and SV is calculated in mL.

E. BP measurement Automatic BP measurement based on the volume

servo-control is already described in the volume-compen- sation method. The cuff pressure (Pc) detected by the air pressure sensor as the indirect BP waveform is led to the µT-Engine board (32 bit CPU) via a 12-bit A/D converter (sampling frequency of 300Hz), and then processed to determine the value of SBP, MBP, and DBP on a beat by beat basis.

F. CO measurement Signal processing and the determination of SV and CO on a

beat-by-beat basis have been described in the cardiac output measuring method. In the bio-admittance meter in the main portable unit (Fig. 1), an ECG signal can be detected from the voltage pick-up electrodes to provide as a timing signal for the signal processing. Also, a total admittance signal (Y) is taken from the meter to obtain respiration rate (Resp) by detecting a peak-to-peak time interval (Tresp) of the slow change in the Y signal due to inspiration and expiration. The outputs from the meter, Y, dY/dt, and ECG signals, are sent to the µT- Engine via the 12-bit A/D converter to perform the necessary processing. In order to eliminate the baseline fluctuation of the dY/dt signal caused by normal respiration and body movements, a digital filtering function is provided.

G. Signal processing Fig. 5 shows a chart of the signal processing from the

measured signals. Following an ECG-R wave, detections of a bottom peak followed by a maximal first peak value from the BP curve are made to give a DBP and an SBP value, respectively. An MBP value is determined from the integration of the BP curve during the same cardiac cycle. At this cycle, ECG-R to –R interval (RR), pulse transit time (PTT), pre-ejection period (PEP), and ventricle ejection time (Ts) are processed from a time interval between two peculiar points of ECG, BP and dY/dt waves. A maximum dY/dt is measured by the height from the baseline to a peak point of the dY/dt wave.

Respiration interval is also determined from the Y signal that varies as a function of inspiration and expiration. Signal processing is executed by µT- Engine. Processed signals are stored in the memory media (SD card) when ambulatory use, or are displayed on the display device via cradle for making the real-time observation when stationary use.

III. RESULTS OF MONITORING EXAMPLE Fig. 6 shows an example of about 3h trends for the

ambulatory monitoring of beat-by-beat cardiovascular variables obtained by the prototype of this system (the present system is been modified and tested). The subject is a healthy adult (43yrs, male) and does his daily activities such as sitting (ST), desk work (DW), standing work (SW), driving (DR), walking (WK), going up the stair (GU) and riding a bicycle (BC) during the measurement. The monitoring lasting for 200 min is made on the subject. The results are satisfactory except for a small amount of bad data due mainly to motion artifacts. The bad data is about 3% of the beat-by-beat datasets.

The venous congestion caused by the local pressurizing finger-cuff unit using in the present system and a convention band cuff is assessed by the blood volume measurement at the fingertip distal to the BP measuring site. The blood volume is directly evaluated by another photoplethysmograph (FPG). FPG is composed of a reflectance photosensor, which is attached on the palm side of the fingertip. Result shows the venous congestion to be decreased greatly by using the finger-cuff unit.

The system appears ready to provide the essential parameter of RR, BP, CO, Rp, and Resp on a beat- by-beat basis for detailed assessment of the cadiovascular hemodynamic functions. The system involving monitoring PEP, PTT, Ts, and RPP may also useful for further assessment of impaired circulatory functions. It creates an exciting opportunity to investigate the behaviors of these additional variables in clinical applications.

Fig. 5 Signal processing of measured signals

Mean value mean value

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IV. CONCLUSION Using the prototype of this system, the noninvasive

measurement of instantaneous (BP) and (CO) on a beat-by-beat basis has been successfully and satisfactiorily executed with only a little subject’s troublesomeness and discomfort: in particular, the system allows the measurement with a little venous congestion due to the cuff occlusion and a little discomfort due to the attachment of the spot electrodes, resulting in capable of monitoring the cardiovascular hemodynamic function in a longer period of time. However, several minor but practical problems are occurred during the measurement, such as signal processing for the motion artifact, acoustic noise produced from the air pump. Now, the present system is been modified and tested for improving its performance such as stability for signal detecting and processing, lower acoustic noise, and so on. Overall, this system offers a good way to monitor the cardiovascular hemodynamic function ambulatory.

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[6] K. Yamakoshi, H. Simazu, M. Shibata and A. Kamiya "New oscillometric method for indirect measurement of systolic and mean arterial pressure in the human finger Part 1 and Part 2 " Med & Biol. Eng. & Comput, 1982, 20, 307-313 and 314-318.

[7] K. Yamakoshi, H. Simazu, M. Shibata and T. Togawa, "Indirect measurement of instantaneous arterial blood pressure in the rat" Am. J. Physiol. 1979, 237, H632-H637.

[8] K. Yamakoshi, H. Simazu, M. Shibata and T. Togawa "Indirect measurement of instantaneous arterial blood pressure in the human finger by the vascular unloading technique" IEEE Trans. Biomed. Eng. 1980, BME-27, 150-155.

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[11] J. Nyboer, "Electrical impedance plethysmography; A physical and physiological approach to peripheral vascular study” Circulation, 1950, 2, 811-821

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Fig.6 Example of ambulatory monitoring in normal subject doing daily activities

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