INTERFACING FETAL MONITORS

IN COMPUTEIU.ZEDCARDIOTOCOGRAPHY SYSTEM
Janusz Jezewski, Janusz Wrbbel, Krzysztof Horoba, Shwomir Graczyk
Research & Development Centre of Medical Electronic Equipment OBREAM
ul. Wolnoki 345a, Zabrze, PL-41800, POLAND

Abstract SIC2 is applied to monitors with standard RS232 output

The paper presents an approach to interfacing various (HP's series 50, Team,Meridian). Circuits are built around
types of commercially available fetal monitors to the MAX232 transmitter-receiver. Supply +5V is derived from
KOMPOR System for computerized cardiotocography . a monitor. The isolation, based on a DC-DC converter and
Practical implementation of simple and universal interface optoisolators, has a very high noise immunity; the max.
circuits is described. Communicationsprotocols with popular length of a 3 a r e screened RS232 cable is about 70m.
fetal monitors are discussed. The proposed interfaces and the mA
L MONITOR CQMRmR
ISolATlON MONITOR COMWlER
method of automatic monitor identification have been veri-
fied in clinical practice. The circuits are also suitable for
interfacing other patient connected equipment to computer-
aided medical systems.
Introduction
In the KOMPOR System for computerized cardiotoco-
graphy a fetal monitor is an input device [l]. The fetal Fig. 1. Structure of Simple Interface Circuits
monitor provides fetal heart rate (FHR), uterine contractions
(UC) and fetal movement information to be accessed by Leading manufacturersusually provide their own commu-
computer. Communication with the computer is bidirectional nication protocols. A simple request-answer protocol with
via a simplified RS232 serial interface. Several monitors can software handshaking is applied in Oxford monitors. The
be connected to the system at the same time. Wall-mounted baud rate is 9600. Five single byte requests are issued by the
sockets for monitor connection are located anywhere within system to get different types of information from the monitor.
an obstetrical department. If fixedly attributed to a given Transmission starts from check request repeated till the
socket, the monitor, mostly mounted on mobile trolleys, is monitor acknowledgement is reached. The monitor gives the
most inconvienientin daily hospital practice. Additionally, for following types of two-byte answers: - FHR value, signal
better use of available resources, the system should handle quality indicator and recorder pen status, and a new heart
monitors equipped with any data interface types. beat flag (from channel 1 or 2); - UC value and a status of
Monitors with analog output only must be fitted with an transducers; - MARKER flag of fetal movements perceived
interface circuit with A/D conversion and transmission via by the mother. The system samples the FHR signal with 4Hz
RS232. A no less important issue is ensuring maximum safety and the UC with 2Hz. The sampling rate is high enough so
for a patient. According to IEC 601-1-1, safety level of the as not to miss a beat. Information from HP's series 50 is sent
entire system should not be lower than that of any of its within 32-byte data blocks surrounded by the block-start and
single components. To comply with the above and the IEC block-end. The speed is 12OObaud. The data block contains
601-1 general standard, the electrical isolation 24kV,, four values of FHR and UC, fetal movement profile, FHR
(50Hz) between the monitor and computer is applied. quality indicator, and a status of transducers. These blocks
Assuming that the patient has no contact with a computer, no are sent by monitor in automatic transmission mode in one-
additional adaptations are necessary. second intervals. The sampling frequency of both FHR and
UC is 4Hz. The checksum appended to the data block is used
Monitors with digital output to detect transmission errors. The system controls the monitor
The System accepts the following monitors with digital by issuing the commauds: start or stop of transmission. Fetal
output: M1350A, M1351A, M1353A (Hewlett-Packard)and movements are identified in two ways: by a patient-activated
FM7, Team, Meridian (Oxford-Sonicaid). Two Simple Inter- event marker or automatically by using the ultrasound channel
face Circuits have been designed (Fig.1). The SIC1 is pro- - fetal movement profile feature. Every time the event marker
posed for earlier models with CMOSITTL output (FM7). The button is activated, the message marker data block is
This work was supported by the State Committee for Scientific Research, transmitted in asynchronous mode.
Warsaw,Poland (KBN Grant No. 8 SS06 007 OS).

0-7803-2050-6/94 $4.00 01994 E E E 922

 Monitors with analog output
SERIAL PORT INlERRUPT
The HP8041A is the most popular monitor where analog
output is the only one available. For interfacing this model
the Intelligent Interface Circuit (IIC) has been developed.
From two possible solutions of the circuit: as a plug-in
computer board or as an external module, the latter was
chosen. Placing the IIC close to the monitor shortens the
cables transmitting analog signals [2]. It is easier to apply
electrical isolation of required dielectric strength in the ex-
ternal module. The circuit is powered from the monitor side.
The IIC (Fig.2) is centered around the NEC 78310 8-bit
microcontroller. It supports an internal 4-channel 8-bit A/D
converter (18Onsec), interrupt controller, serial and parallel
port. The circuit is housed in a small box (90~40xllOmm).
The HP8041A transfers to the IIC the following signals:
* FHR: lOmVhpm, limits: U,=O.60+2.10V, LtS5mA, Fig.3. Flow chart of IIC control program
* UC: 100mV/unit, limits: U,=Ot lOV, IWtG m A ,
* FHR-V: validity of FHR, TTL, tion, conditioningshould provide lLSB136.8mV in FHR, and
* MARKER: fetal movement flag, TTL. 1LSB=5OmV in UC. Any program upgrades or customized
enhancements can be realized by simply repracing the
M O m R , COuIwl€R
EPROM.
Monitor handling procedure
In the on-line monitoring mode, data received from fetal
monitor are dynamically presented on a screen as moving
graphic, and on-line analysed. Received data are streamed to
hard disk for archiving; one-hour trace requires 4OKbyte. The
informationobtained from monitors depends on its type (fetal
movement profile from M1351A or new heart beat flag from
Team). For effective storage management and to prevent any
data loss, the automatic monitor identification and then data
acquisition according to relevant protocol are performed.
Fig.2. Block diagram of Intelligent Interface Circuit
To identify the monitor type, the system sends alternately
Reference voltage of A/D converter, FHR and UC signal start command (HP) and check request (Oxford). If the
conditioning are selected to achieve 1 bpm resolution in FHR system receives block-start then it continues data exchange
(lLSB=lOmV), and 1 unit in UC (lLSBAl00mV). Data according to HP's protocol. If the system receives the check
exchange between the system and monitor is via interface answer, in the next step additional bits of MARKER answer
RS232 according to a user defined protocol (Team's protocol are analysed to differentiate among Oxford's family and HP
is default). 8041A along with IIC.
The flow chart of the control program is shown in Fig.3.
Conclusions
Four hardware interrupts are used : TEST - non-maskable
interrupt of TEST key (the highest priority), MARKER - an The presented approach to monitors interfacing applied in
event marker interrupt, FRS - transmission error interrupt the KOMPOR System has been verified in clinical practice.
and RS - interrupt generated by serial port of the micro- The economic advantage of interface circuits enabling the use
controller (the lowest priority). After receiving the request to of various fetal monitors is particularly attractive in Poland.
send FHR or UC signal values, AID-conversion starts and The circuits are versatile enough for interfacing other patient
then digital data are transferred to the system. If FHR is not connected equipment to computer-aided medical systems.
valid it is replaced by zero value. In TEST mode a series of References
gradual increases and decreases of FHR and UC is sent. The
J.Jezewski, J.Wr6be1, "Fetal Monitoring with Automated Analysis of
control program is written in the 78310 assembly language. Cardiotocograms: The KOMPOR System", Proc. 15th Ann. Conf.
A simple adaptation of the IIC to other monitors with IEEE/EMBS, pp.638-639, 1993.
analog output: HP8040, HP8030 or FM6 is possible. The J.Francis, T.Martin, "Principles of Interfacing Computers to Medical
new signal conditioning is only needed. For instance, to Equipment",Bailliere's Clb. Obstet. Gynaecol., pp.787-795, 4, 1990.
obtain in FM6 monitor: FHR 1 bpm and UC 1 unit resolu-

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