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Microprocessor-based simulator of surface ECG signals

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2007 J. Phys.: Conf. Ser. 90 012030

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Microprocessor-based simulator of surface ECG signals

A E Martnez1, E Rossi

1and L Nicola Siri

1, 2,3

1Ctedra de Bioingeniera II, Facultad de Ingeniera, Universidad Nacional de EntreRos (FI-UNER), Ruta Provincial 11 Km.10 Oro Verde (Dpto. Paran) - Entre Ros,Argentina.

2Consejo Nacional de Investigaciones Cientficas y Tcnicas (CONICET), Av.Rivadavia 1917, (C1033AAJ) Ciudad Autnoma de Buenos Aires, Argentina

E-mail: nicolasirileo@yahoo.com

Abstract. In this work, a simulator of surface electrocardiogram recorded signals (ECG) ispresented. The device, based on a microcontroller and commanded by a personal computer,produces an analog signal resembling actual ECGs, not only in time course and voltage levels,but also in source impedance. The simulator is a useful tool for electrocardiograph calibrationand monitoring, to incorporate as well in educational tasks and in clinical environments forearly detection of faulty behaviour.

1. IntroductionSince 1985, Bioengineering is established as a degree-level career at the School of Engineering ofUniversidad Nacional de Entre Ros in Argentina. In its program, the undergraduate courseBioengineering II deals, among other things, with metrology applied to biomedical instrumentationand equipment, one of the competences that future Bioengineers must acquire. The syllabus contentsof Bioengineering II include the use of transducers and signal conditioning circuits for the design andcalibration of biomedical instruments. One of the fields concerning biomedical instruments -that wealso choose to train students in theoretical principles and practical aspects- is surfaceelectrocardiography (ECG).

From among the various didactic strategies opted for teaching purposes, one approach compelsstudents to develop several theoretical-practical tasks in the Instrumentation Laboratory. One suchtask consists on designing and developing a protoboard(MR) level of an ECG preamplifier, capableof recording, conditioning and exhibiting ECG in one of the standard bipolar leads. Recording andsignal conditioning specifications, similar to those encountered in current one-channel clinicalelectrocardiographs, such as preamplification, patient isolation, signal filtering, power amplification,must be satisfied by the design made by the student.

In order to tune-up the project, students perform standard electronic bench-tests, by setting eachstage (offset, gain, frequency response, etc.) using conventional waveforms (sinusoidal, triangular,square) as input signals, and by revisiting the complete design, if necessary [1], [2]. After the circuit

performance gives results within design specifications, the students make a field test on the entirecircuit, and record their own ECGs. This final stage of the calibration procedure is necessary because

3 To whom any correspondence should be addressed.

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waveforms used in bench-proofs do not completely reproduce actual ECG signal patterns, and becauserecording artifacts, similar to those produced in ECG cabinets under live recording conditions, need to

be recognized by Bioengineering students to learn how to prevent them.Abnormalities observed in the recording of ECG during the field test could be originated inimproper electrode location, a faulty circuit, or less probably in genuine cardiac anomalies in thesubject whose ECG is being recorded, borrowing the interpretation (and prevention) of anomalies inthe recording, which is one of the proposals of the exercise.

Because of this, we decided to feed the tested ECG amplifier with a pure ECG signal resemblingnormal, abnormal, and also artifact ECG recordings, prior to the recording of a live ECG with theamplifier under test. This strategy aims at reinforcing the students confidence on their owncapabilities to produce an efficient design. An antecedent of a device capable of generating anartificial ECG can be found in [3].

The recording of an artificial signal with known properties, similar to those expected in liverecordings, and the comparison in real time between input and output signals, will permit to perceive

distortions only related to the ECG amplifier. This is a useful complement to the classical bench-proofs referred to above.

2. Objectives

In the present work, we have developed a device (ECG simulator) to generate an analog signal withthe most salient attributes of a live ECG recording, which is capable of being connected to the inputstage of the one-channel ECG amplifiers designed by students.

3. Design and developmentThe leading idea was to store a digitized live ECG, select an epoch from it (i.e. a complete cardiac

period), and send it to a digital-to-analog converter (DAC) at an adequate rate of refreshment to bringup a continuous analog ECG signal to the ECG amplifier through a driver amplifier. Our ECG

simulator (ECGS) was provided with digital storage capacity, serial communication and DAC, andwas designed based on a microcontroller and a solid state driver amplifier. The ECGS is to beconfigured and controlled from a personal computer (PC). Blocks diagram of the ECGS is shown infigure 1. It can be unfolded into five main stages: microcontroller (figure 2), serial communication(figure 3),EEPROM memory (figure 4), DAC (figure 5) and analog output (figure 6).

Figure 1. Block diagram of the ECGS. Personal computer (PC), serial communication

(RS232), ECGS memory (EEPROM), digital to analog converter (DAC).

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One cardiac period (ECG epoch) from a live human ECG in the DI lead, obtained from a normalindividual, was digitized (10 bits, 500 Hz) by means of a computerized polygraph constructed at our

Laboratory in an earlier stage [4], and the most significant 8 bits were stored in the hard disk (HD) of aPC. A dedicated PC-resident program sends this ECG epoch to ECGS through the PC serial port, to bestored in the ECGS memory by a dedicated program resident in the microcontroller.

Then this last program sends the ECG epoch from the memory of ECGS to the DAC of themicrocontroller at the same sampling rate (500 Hz) in order to present the ECG to the output amplifierof the ECGS. The transfer output cycle is repeated until an operator interruption stops the process. TheDAC output is smoothed via analogical low pass filtering (single pole, 100 Hz), and conditioned inimpedance and amplitude in order to simulate the ECG signal expected at the input of an ECGamplifier.

3.1. Hardware designThe microcontroller (PIC16C877-20/P, from Microchip, U.S.A.) commands several actions in the

ECGS: data management, serial communication with the PC, 8 KB EEPROM (24C64, fromMicrochip, U.S.A.) memory management, and 8 bit DAC (DAC0808, from National, U.S.A.)operation [5], [6]. Figure 2 shows the pin out of the microcontroller.

RA0/AN02RA1/AN13 RA2/AN24RA3/AN3/VREF5RA4/T0CKI6RA5/SS/AN47

RB0/INT33RB134RB235RB336RB437RB538RB639RB740

RC0/T1OSI/T1CKI 15RC1/T1OSI/CCP2 16RC2/CCP1 17

RC3/SCK/SCL 18RC4/SDI/SDA 23

RC5/SDO 24RC6/TX/CK 25RC7/RX/DT 26

RD0/PSP0 19RD1/PSP1 20RD2/PSP2 21RD3/PSP3 22RD4/PSP4 27RD5/PSP5 28RD6/PSP6 29RD7/PSP7 30

RE0/RD/AN5 8RE1/WR/AN6 9RE2/CS/AN7 10

VSS12VSS31

MCLR/VPP1OSC1/CLKI13

OSC2/CLKO 14

VDD 11VDD 32

PIC16C877-20/P

1 2Y118pFC3

18pFC5

+5V

TXRX

SDASCL

RB7RB6RB5RB4RB3RB2RB1RB0

Reset Circuit

Figure 2. Microcontroller module(PIC16C877-20/P) pin out. 20 MHz xtal.(Y1)

Figure 3shows the stage which adapts voltages between the RS232 serial port in the PC and thetransmission and reception pins of the microcontroller. It is based on a voltage adapter circuit(MAX232, from Maxim, U.S.A.).

Figure 4 shows the wiring of the EEPROM. The PC, through the RS232 serial port brings the ECGepoch previously stored in the HD to the microcontroller which stores it in the EEPROM memory.The microcontroller can, in turn, read the EEPROM to send the ECG epoch to the DAC in a repetitivecycle mode. The microcontroller and the EEPROM communicate under I2C protocol.

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RS232

R1 IN 13

R2 IN 8

T1 IN11

T2 IN10

GND

15

V+

2

V-

6

VCC

16

R1 OUT12

R2 OUT9

T1 OUT 14

T2 OUT 7

C1+1C1 -3

C2+4C2 -5

MAX232

100pF

C9

100pF

C8

100pFC6

100pF

C10

100pFC7

123

JP8TXRX

E01E12E23

GND4

SDA 5SCL6WC7

VCC 8

24C64

SCL SDA

+5V+5V

10KR21

+5V

10KR22

Figure 3. RS232 adapter.Voltage adapter module(MAX232). Connection with PC(JP8). Connection withmicrocontroller (TX, RX)

Figure 4. ECGS memory. 8 KB EEPROMmodule(24C64). Connection withmicrocontroller (SCL, SDA)

The circuit in figure 5 illustrates the connection between the microcontroller and the DAC in orderto bring the digitized ECG epoch stored in the EEPROM as an analog signal to the output stage. Theamplifier (LF353H from National, U.S.A.) converts DAC output current into a voltage signal.

A48A59A610A711A812

VDD 13

VREF+ 14VREF- 15

COMP 16

A37

NC 1GND2 VEE 3

IOUT- 4A15A26

DAC0808

RB7RB6RB5RB4RB3RB2RB1RB0

C

-12V

+5

+5V

123

4

8

A

ALF353H

-12V

+12

RR

R

V1

8

4

756

2

+12V

-12V50

10K

10K

15K 100pFV1

Electrode 1

Electrode 2

Electrode 3

Figure 5. DAC wiring. DAC module (DAC 0808).Connection with microcontroller (RB0..RB7).Current-to-voltage conversion (LF353H).Connection with output stage (V1)

Figure 6. Output stage. Connection withDAC (V1). Connections with ECG(electrodes 1, 2 and 3, see text)

Figure 6 shows the schemata of the low pass filter and the output amplifier (LF353H fromNational, U.S.A.) of the ECGS. The output signal is conditioned in voltage and impedance to simulatethe floating source which is expected to be presented by surface electrodes and patient cable to theECG input stage in a live ECG. The resistive voltage divider brings an ECG with 8 mV pp and 5 Koutput resistance [7]-[9]. Electrodes 1 and 2 must be connected to the corresponding input pins of the

electrocardiograph, having in mind the actual leads that generate the simulated ECG. Electrode 3must be connected to the right-leg driver terminal of the electrocardiograph.

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3.2. Software designThe program which transfers data from the PC to the ECGS was built in Delphi 6.0 (from Borland,

U.S.A.). This PC resident program permits the configuration of the RS232 serial port to comply withECGS requirements. It transfers the ECG epoch stored in the HD to the ECGS, exhibits the simulatedECG in the PC monitor and performs a transmission error-checking protocol. Figure 7 illustrates thedigitized ECG which is the source signal for ECGS, as seen in the visual environment of the program,on the PC monitor.

The program resident in the microcontroller was written in C-compiler Software DevelopmentTools (from CCS, U.S.A.). This program controls data transfer between EEPROM and MAX 232, and

between EEPROM and DAC.

Figure 7. Visual environment of ECGS.The upper trace shows the ECG epoch

previously stored in HD; the lower traceshows the ECG epoch after transmissionfrom PC, as it was recovered from

EEPROM by the transmission errorchecking protocol

4. ResultsIn order to verify the performance of the ECGS, two simultaneous recordings of the digitized ECGshown in figure 7 were performed. The first recording, showed in figure 8, was taken from the DACoutput with a precision digital oscilloscope (Scopemeter 190/C, from Fluke, U.S.A.); the second wasobtained by connecting the ECGS output to the patient cable inputs of an electrocardiograph(ECGView, from Eccosur, Argentina) and recording a DI lead ECG. A fragment of the ECGViewreport is reproduced in figure 9.

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Figure 8. ECG input signal. SimulatedDI lead ECG. Same digitized ECGthan in figure 7, recorded at the output(pin 7) of of LF353 7 in figure 6 bymean of an oscilloscope, gain (1V/div), timebase (500 msec/div), probeattenuation (10:1)

Figure 9. ECG recorded signal. Transcription of ECG report from ECGView.DI lead. Gain (5 mm./mV), timebase (50 mm/sec).

5. DiscussionAs a gross view of the ECG stored in HD (figure 7), the ECG signal driven to the electrocardiograph(figure 8) and the ECG finally recorded by the electrocardiograph (figure 9), permits recognize a veryhigh concordance among them.

We thus conclude that the ECGS developed in this work has the capability of reproducing a storeddigitized ECG trace and transforming it into an analog signal which is, in turn, suitable for beingreproduced without undesired artifacts by a standard commercial electrocardiograph. Nevertheless, aquantitative comparison among the three traces could be useful in order to better characterize the

performance of this ECGS.

As a result, we now have a device which can be easily installed in the teaching laboratory, and thestudents will have another tool for completing the calibration procedure of their ECG amplifiers.The ability of the device to feed an electrocardiograph with normal or abnormal ECGs, and to add

programmed artifacts to the recording by digitizing actual ECG records, or by loading the ECGS withsynthetic ECG-like waveforms, will not only expand the possibilities of the educational tasks, but also

provide a method for early detection of electrocardiograph faults.This easy-to-operate ECGS, together with its low-cost characteristic, make it suitable for use in an

Electrocardiology Department in hospitals where technicians could consequently obtain recordings ofsimulated ECGs with their in-use electrocardiographs and, by comparing actual recordings with theexpected ECG as visualized in the PC monitor, will be able to decide on the recalibration of theelectrocardiograph by the Bioengineering section of the hospital, instead of recording unsuspectedwrong ECGs.

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AcknowledgementsThis research was suported by project PFIP 2006-1: Laboratorio de Ensayos y Calibracin de

equipamiento mdico Instalacin y puesta en funcionamiento para la prestacin de servicios aterceros, granted to Ms., Bioingineer M C Mntaras by Universidad Nacional de Entre Ros (UNER)and by Secretara de Ciencia, Tecnologa e Innovacin Productiva (SECyT) from Argentina.We express our thanks to MSc. Engineer Norberto Lerendegui for theoretical suggestions, toBioengineer Carlos Pais for ECG records, and to Prof. D Waigandt for the English revision.

References[1] Palls Areny R 2001 Sensores y Acondicionadores de Seal 3ra Edicin corregida (Barcelona:

Alfaomega/Marcombo)[2] Vasilevich M Y 2004, A Generator for Testing Signals of Electroencephalographs and

Electrocardiographs, Biomedical Engineering, 38(3), 162-4[3] Bio-Tek Instruments Inc. 1989 ECG Performance Analyzer Model ECGPlus: Users guide.

(Winooski: Bio-Tek Instruments Inc.)[4] Marchand C and Rossi E 2004 Polgrafo computarizado de bajo costo; Tesis para obtener elgrado de Bioingeniero (Oro Verde - Paran: Centro de Medios / Facultad de Ingeniera /Universidad Nacional de Entre Ros-ARGENTINA)

[5] Kontodimopoulos N et al. 1998 In-house development of test equipment for quality control andtraining. Case study: a prototype ECG simulator-tester, Medical Engineering & Physics, 20(10),71721

[6] Caicedo H et al. 2005 Equipo transportable basado en dsp- para el almacenamiento y anlisis deseales biomdicas, Rev. Colomb. Fs., 37(2): 418-25

[7] Castellanos brego P et al. 1997 Electrofisiologa Humana: Un enfoque para ingenieros 1raEdicin (Mxico DF: Universidad Autnoma Metropolitana)

[8] Ferrero Corral JM 1993 Bioelectrnica: Seales bioelctricas (Valencia: Servicio dePublicaciones-Universidad Politcnica de Valencia)

[9] Geddes L A and Baker L E 1989 Principles of Applied Biomedical Instrumentation3rd Edition(New York: John Wiley)

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