DESIGN OF A PC-BASED PATIENT SIMULATOR FOR

TESTING AND CALIBRATION OF ELECTROMEDICAL

DEVICES USING LABVIEW

Pedro Pablo Escobar, Gerardo Acosta

INTELyMEC, Fac. of Engineering, UNCPBA University, Del Valle Av. 5737, Olavarría, Argentina

Marcos Formica

Fac. of Bioengineering, UNER University, Ruta 10, Km. 11, Oro Verde, Paraná, Argentina

Keywords: Patient simulation, testing, calibration, education, electromedicine.

Abstract: Modern digital biomedical devices need a testing and calibrating equipment to asses its functional state with

the appropiate technology, so patient simulators have become an essential tool for maintenance and

biomedical engineers. An interactive virtual instrument was developed in Labview for simulation of healthy

and pathological conditions to test biomedical devices which acquire, register and store temporal evolution

of human physiological variables. In this article we present some details of the design and implementation

of a simple pc-based patient simulator using Labview, in order to obtain a low cost solution for teaching and

practical purposes.

1 INTRODUCTION

Medical science relay for decades on biomedical

devices that technology developed continuously,

such as ECG, EEG and others which had very

limited features at the beginning and were relatively

expensive. As the years went by, we found that these

devices were reducing their size, enhancing

capabilities, adding new features and changing the

visualization and registration ways (Neer, 2003).

On the last two decades, biomedical devices have

had a vertiginous evolution which brought a lot of

new features including non-invasive signal

acquisition, data-processing, automation and others;

but also brought the necessity of calibrating them

and verify its functional state with an appropriate

technology (Bronzino, 1995). Then, patient

simulators for biomedical devices broke into the

market and nowadays they have become an essential

tool for biomedical and maintenance engineers on

every health institutions worldwide.

With the constant advance of technology, not

only biomedical devices but also simulators

incorporated new capabilities and connectivity

improvements. In spite of these evident advantages,

simulators have a high cost, especially for hospitals

and health centres in developing countries, situation

that sometimes turns them into prohibitive and

makes impossible the proper maintenance of the

devices, because of the lack of adequate evaluation

instrumental.

There is a bunch of interesting commercial

solutions available in the market, from many

manufactures, but most of them are designed for

specific medical applications so they can simulate a

set of parameters for these applications in particular,

for example, an ECG patient simulator for testing

ECG biomedical devices. The cost of any of these

analysis tools is high and this led us to design our

own tool using resources available at the university

labs and a moderated investment on technology,

with the main objective of develop a double purpose

device: a patient simulator and a teaching tool.

Teaching of Electromedicine is often based on

the integration of electronic and physiological

knowledge applied to the comprehension of how a

device works and why is it malfunctioning. It is

remarkable that patient simulators are extremely

useful for teaching and practicing electromedicine

anywhere, and for all the people who use and

maintain modern medical devices.

On our laboratories, the main obstacle for testing

and calibrating different kinds of electromedical

135

Pablo Escobar P., Acosta G., Acosta G. and Formica M. (2008).

DESIGN OF A PC-BASED PATIENT SIMULATOR FOR TESTING AND CALIBRATION OF ELECTROMEDICAL DEVICES USING LABVIEW.

In Proceedings of the First International Conference on Biomedical Electronics and Devices, pages 135-139

DOI: 10.5220/0001055201350139

 SciTePress

devices is the lack of a versatile and economic tool

to simulate many diverse physiological conditions,

normal and pathological, in order to measure the

output range values and verify that they meet

manufacturer’s specifications (Del Aguila, 1994).

Nowadays, it is a reality to acquire, digitalize,

process and visualize data by means of a personal

computer with the help of programming software for

instrumentation and measurement. Moreover, it is

also possible to integrate a wide variety of medical

devices into a wireless network (Chronaki, 2006).

In this brief article we present the design and

implementation of a simple PC-based patient

simulator using Labview® for the graphic user

interface, in order to obtain a low cost solution for

testing medical devices.

2 DESIGN

2.1 Requirements and Considerations

The main goal was to create a versatile tool for

testing and calibrating biomedical devices, suitable

for both practical and educational purposes. We

chose Labview

v8.0 for the programming

environment because it is available for several

computer platforms such as Linux, Windows, Palm

PC, etc, (Wang, 2003) and because its excellent

connectivity with the outside world through DAQ

modules, I/O and networks protocols which make it

a great tool to perform computer-based experiments

like acquisition, control, instrumentation and quick

engineering solutions development (Wells, 1997).

Our needs consisted on developing an interactive

virtual instrument (VI) for simulation of healthy or

pathological conditions in order to test different

biomedical devices. The VI also must have

capabilities for acquiring, registering and storing

temporal evolution of physiological variables like

biopotentials, respiratory frequency, pulse oximetry,

temperature and others (Kheir, 1995) sensed by

these devices in order to make a testing signals

repository available at any time in the computer hard

disk or any other non volatile storage system.

We consider the design of a VI with a simple

user interface but with full capability to interact with

six different medical devices available at the lab as

educational modules: a bi-directional vascular

Doppler (frequency and beat noises); an incubator

(room and skin temperatures); an external

peacemaker (frequency and amplitude); an

electrocardiograph; a neonatal cardiac monitor and

a cardiac monitor (signal shape and frequency).

2.2 Design of the Patient Simulator

The first version was designed choosing a physical

model for each device and selecting the basic

parameters we wanted to control from the user

interface. Then, the mathematical models were built

to simulate different signal conditions varying some

parameters of the differential equations while others

remained constant or stable (Zeigler et al., 2000).

Each parameter was controlled by a graphical

component of the VI’s front panel (e.g. a slider or a

knob). The signals generated by the mathematical

model could be stored in an excel sheet or a text file,

creating this way a repository of signals that allowed

us to test the devices mentioned before, and then re-

define the model if necessary (Dorf, 1998).

A second version was developed to overcome

some limitations of the first one, and to simplify the

design. In this case, mathematical models were

replaced by a repository of signals acquired by

different methods. This version provides two main

modalities: simulation and testing. Simulation

allows for sending signals from the computer to a

device, and testing allows for acquiring signals from

a device.

Figure 1 shows the general system overview for

this second version. It can be seen that the Virtual

Instrument we developed can interact with all the

devices through three different interfaces: a

multipurpose DAQ Board that we designed and

built, the PC sound card, and the SCXI-1000 DAQ

device form National Instruments. Details of each

will be provided ahead.

2.3 Simulation Modality

This modality is not applicable for all the devices,

for example the incubator or the external pacemaker.

As it was introduced on the previous section, the

signals for the simulations of the cardiac monitors

(adult an neonatal) and the ECG device were

obtained in an “offline” fashion and stored locally

on the hard drive. For example, a wide variety of

Figure 1: General system connection.

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136

normal an pathological cardiac signals were scanned

from ECG traces and processed using the open

source software Engauge Digitizer

v4.12 which

converts an image file into a bidimensional array of

numbers (X,Y), that can be read on the screen and

stored in a text file or a spreadsheet (Salatino et al,

2007). Figure 2 shows a capture of the software

during a digitalisation process of an ECG signal and

the numeric output file format.

Figure 2: ECG signal digitalisation process using Engauge

Digitizer v4.12.

In our case, sample signals were selected in order to

cover a wide variety of cases like regular rythms,

tachycardia, arrhythmias, idioventricular rythms,

wandering pacemakers, conduction defects, infarcts,

anomalous waves, fibrillation, coronary diseases and

others (Lopez et al, 2005).

Sound signals from the bi-directional vascular

Doppler were acquired connecting its audio output

to the Line-in input of the PC sound card.

All kind of signals were located in a signal

repository on the hard disk and used for simulation

without the need of a patient, and also for testing and

calibrating the devices.

2.4 Testing Modality

In this modality, we wanted to test the functional

state of a particular device by sending a stored signal

to it and checking the values it is sensing. The signal

is represented on a chart and compared in shape and

values with the signal on the monitor.

For achieving this, the virtual instrument has

some features for interfacing with a medical device,

as we mentioned before. Because of the nature of the

signals the devices can handle, we needed to

implement different interfacing ways. One of the

ways we proposed was using a SCXI-1000 DAQ

chassis from National Instruments with its respective

modules: 1200, 1161, 1122; and the 1302 accessory.

The chassis can connect to the computer through its

serial or paralell port. The second alternative was a

multipurpose DAQ board that was designed and

built here in our lab. The third, the PC sound card.

For each of them, the digital simulation signal is

converted to analogical output by these interfaces

and used to stimulate the biomedical devices.

In the case of the external pacemaker, it wasn’t

able to visualize the stimulating spikes on the device

because it is old and it has no display. So, testing

was accomplished using a virtual oscilloscope

library for Labview 5.1 or superior available on

Internet. The test of the incubator was performed by

means of thermistor with a conditioning circuit and

also a traditional bulb thermometer, in order to

compare the evolution of temperature with the one

that the device’s probe sensed.

3 RESULTS

3.1 The Virtual Instrument

Once we determined all the requirements for each

device, the virtual instrument was programmed and

named SIMPAC, version 1.0. Figure 3 shows the VI

graphical user interface (GUI).

Figure 3: The SIMPAC v1.0 Virtual Instrument GUI.

The GUI was designed as simple as possible; it

contains a tab for each device and the controls and

indicators for the required parameters. There were

included two extra tabs; one for the help and the

other for a configuration utility which allows for

configuring the serial communication, the buffer

size, sampling frequency and other settings.

In figure 4 we can see the Labview design

window, showing the graphical code of the

incubator sub VI.

DESIGN OF A PC-BASED PATIENT SIMULATOR FOR TESTING AND CALIBRATION OF ELECTROMEDICAL

DEVICES USING LABVIEW

137

Figure 4: Design view of the incubator Sub VI.

3.2 The Multipurpose DAQ Board

Some modern devices come equipped with a PC

interface, generally a RS232 protocol or a GPIB488

connector. Other older devices do not come

equipped for interfacing a PC but it is possible to

build a low cost specific board in order to replace a

paper registration system or similar, with a PC

which can acquire, show, store and process

information sensed by these devices.

A multipurpose board was designed in our lab

for a previous work and used here as another

alternative for interfacing biomedical devices with

the virtual instrument on the PC. In this case, the VI

(master) commands the start and stop of the

acquisition, and also handles the occurrence of

possible errors.

The PC establishes communication via RS232

with an 8-bit microcontroller (MCU, slave) 68HC08,

that read the sensors and send data to the PC. The

MCU provides the A/D converting module (ADC)

with 8 multiplexed channels and a full-duplex serial

asynchronous communication module (SCI). The

signals of the sensors are applied to the ADC inputs

of the MCU through a voltage follower with high

impedance and unitary gain.

Signal conditioning is made using the MAX232

IC, which adapts voltage levels between the MCU

and the PC. Communication is made at 9600 bps

baud rate with a format of 1 Start bit, 8 data bits, 1

Stop bit and no parity bit. Other baud rates and

transmission formats were tested but we chose the

mentioned above because of its simplicity and

because it is a MCU common configuration.

3.3 Simulation and Testing

Six devices were connected to the virtual instrument

for testing or simulation. In the case of the cardiac

monitors, all were tested using the signals

repository. This modality show great usefulness

because it allows for comparing signals displayed on

the charts of the VI with the others on the monitors.

Taking advantage of the fact that any common

PC has integrated a 16-bit sound card, we connected

the audio output of the vascular Doppler device

(Fetal monitor) to the “Line-in” input of the sound

card. Labview provides all necessary drivers to

handle the incoming data from the sound card, to

plot the signal shape and frequency spectrum in real

time and also record the signals into a file. This

allows for recovering signals at any time and to

perform cuantitative and qualitative assesments of

the signal like beat noises, frequency, amplitude, etc.

The incubator was tested by comparing the

temperature sensed by the room electrode and skin

electrode against a thermistor (with a conditioning

circuit) and also a mercury bulb thermometer. We

could see that the room electrode wasn’t working

properly and its values were almost 5° centigrades

far from the real temperature. Figure 5 shows the

incubator VI during a test.

Figure 5: Working with the incubator Sub VI.

The electrocardiograph’s capabilities were enhanced

when it was connected to the virtual instrument. It

delivers the amplified signals of the electrical

activity of the heart, with the respective derivations,

to the A/D converter of the MCU and then taken to

the PC for processing. The PC allows for displaying

all derivations simultaneously and also for making a

vectorial sum of each derivation in order to build a

3D graph of the cardiac vector, performing a

vectocardiography, a feature that is not available on

modern ECG devices. The Labview ECG VI can

also work as a Holter device registering ECG signals

and storing them.

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4 CONCLUSIONS

Basic simulators used for medical training are

useless for testing and calibrating electromedical

devices. The need of a simple and powerful tool and

the avalability of new programming technologies

motivated us to develope our own tool with the

students of the last year.

A PC-based patient simulator was designed and

built with minimum investment in comparison with

the commercial solutions available in the market. It

took only a standard PC with Windows 98 (for

Labview v5.1 or v6.1) or Windows XP (for Labview

v8.0) as the operating systems to run the

programming environment. Three alternatives were

used to interface Labview and biomedical devices: a

16-bits sound card, a SCXI-1000 DAQ device (with

1200, 1161 and 1122 modules and the 1302

accessory) and a multipurpose DAQ board designed

and built at our laboratories.

Six different biomedical devices were connected

to the virtual instrument, which can work in two

modalities: simulation and testing. Both modalities

were acomplished succesfully.

One of the problems found was the limited

numbers of digital outputs of the DAQ device. This

turns into a limitation when we want to test an ECG

device using more than 6 derivations. Other

limitations found were dued to the few parameters

selected for each device. Further versions of the

patient simulator will overcome these limitations

and add more devices in a closer future, such as new

VI’s for mechanical ventilation devices, EEG,

electronic sthetoscopes, tensiometers, biostimulators

and others (Murita et al, 2004).

The patient simulator is not fully developed yet,

but this partial success has encouraged us to

continue this way in order to improve the

capabilities of SIMPAC v1.0.

The secondary goal or this work, education and

training, was fully achieved. Teaching has been

traditionally based on concept transmission;

however, in practical careers like electromedicine or

bioengineering most of the time it is necessary to

integrate basic and clinical science concepts with

electronics to improve critical analysis of real life

working situations. Simulations are used to visualize

real world dynamic processes in classes and to

enhance students’ knowledge.

Labview shown to be a powerful tool and an

excelent resource for building human-machine

interfaces, taking advantage of the great connectivity

it provides by different means. Testing capabilities

were of great practical relevance.

The designed patient simulator is a low cost,

very useful tool, developed entirely as an original

work.

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DESIGN OF A PC-BASED PATIENT SIMULATOR FOR TESTING AND CALIBRATION OF ELECTROMEDICAL

DEVICES USING LABVIEW

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