Digital communication with fetal monitors

Instruments & Methods DIGITAL

COMMUNICATION

WITH

FETAL MONITORS Zsolt Bozbki, MD

Background: Fetal heart rate (FHR) values in the averaged format that are provided by commercial computed cardiotocography analysis systems may be unsuitable for special analysis purposes. Method: I developed a communication software program to obtain any measured values of fetal monitors for individual analysis of computed cardiotocography. Experience: The software program was used to study the data continuity of beat-to-beat FHR values as an experiment for chaos theory and power spectrum analysis. The results indicated that the signal loss was recognized at a precision of 95%. Conclusion: The described method of digital communication with fetal monitors was found to be useful for individual purposes in the field of computed cardiotocography analysis. (Obstet Gynecol 1997;90:837-9. 0 1997 by The American College of Obstetricians and Gynecologists.)

Cardiotocography is one of the most efficient tools for the assessment of intrauterine fetal well-being. Sonicaid cardiotocographs (Oxford Instruments, Medical Systems Division, Abingdon, UK) are used worldwide. One of their advantages, not generally provided by the equipment of other manufacturers, is a digital interface through which all the characteristics measured by them can be read digitally. This can be useful if there is a need for an individual data analysis not included in commercially available cardiotocography analysis software, such as Oxford System 8000/8002 (Oxford Sonicaid Ltd., Chichester, UK). The data-averaging technique of the ?&-minute epoch used in the System 8000/8002’ (Oxford Sonicaid Ltd.) is sometimes too rough. For an

the Department of Obstetrics and Gynecology, Albert SzentGydrgyi Medical Unizlersity, World Health Organization Co/laborating Center gf Research in Human Reproduction, Szeged, Hungary. Supported by the National ScientiJic Research Fund (Hungary) and the Kaali Foundation (New York). From

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average heart rate of 144 beats per minute, instead of using the beat-to-beat fetal heart rate (FHR) values provided by the cardiotocograph, the commercial cardiotocography analysis software works with the averaged value of the last nine (9 X 16 = 144) heart beats. The present article illustrates the technique of digital communication with fetal monitors through a digital interface to obtain beat-to-beat FHR values and other characteristics measured by the cardiotocograph.

Method Sonicaid cardiotocographs (Oxford Instruments) provide additional information about all measured quantities and their actual operation mode in serial form transmitted through an RS232C connector (a widely used standard interface that covers the electrical connection between data communication equipment, such as a computer and a fetal monitor), allowing application of a computer system to collect data from the cardiotocograph for research, remote display, or further processing. The hardware involved is a simple three-wire connection. All the messages are small enough to be collected without data loss. The fetal monitor itself acts as a slave device and operates on request from the host computer with a delay of only a few milliseconds. One command byte must be sent from the com.puter to the fetal monitor; two bytes will be sent back as a reply, which is always the latest information available to the cardiotocograph (Table 1). Whenever a new fetal heart beat is recognized by the cardiotocograph, the “beat flag” (working as a signal) toggles. The status of this beat flag can be accessed with a computer. I wrote a software program using the Microsoft Visual Basic Programming System for Windows 3.0 (Ml.crosoft Corporation, Redmond, WA) which reads the beat flag of the fetal monitor continuously through the first serial port (COMl) of the computer. The beat flag toggles with every new beat recognized by the cardiotocograph. If the program finds that the beat flag has toggled, it reads and writes the FHR, tocograph fetal movement, signal quality, and beat flag values into a window displayed on the computer screen. Except for the latest one, values are written into a standard text file for safekeeping (Figure 1). This method provides beatto-beat FHR and other values in real time, and later processing also is possible by using text file data.

Experience When the fetus is moving, the fetal heart #often passes outside the range of the ultrasound beam. During this interval, which sometimes lasts for several seconds, the

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Table 1.

Description

of Command

Command byte (hexadecimal value) s9c

$91

$92

Bytes

16-bit re~lv

and

Reply

Words

word

Bits O-10: FHR beat interval in milliseconds. Range 0 -2000 Bit 11: beat flag Bits 13-15: confidence value (if bit 15 is set, there is no rate; if bit 14 is set, good confidence; if bit 13 is set, excellent confidence) Bits O-8: tocodynamometer value multiplied by 2. Range O-200 Bit 0: set if external event button pressed

FHR = fetal heart rate. Source: Sonicaid Fetal Monitor Serial Protocol Instruments, Medical Systems Division, Abingdon,

Booklet UK).

(Oxford

beat flag does not toggle. In response to the first recognized fetal heart beat, the flag toggles again. The program MISSBEAT.EXE (written by the author in Microsoft Visual Basic 3.0) measures the time that elapses between every two successive, recognized fetal heart beats and compares this time with the FHR value provided by the cardiotocograph. In this way, the existence and also number of missed beats can be recognized and stored in an output text file. At the end of the record, the program automatically gives the signal loss rate and writes it into an output file. Thus, data continuity can be verified for chaos theory2 and power spectrum analysis3 (two mathematical methods that recently appeared in connection with fetal surveillance), both of which require strictly continuous and highly precise FHR input values, as otherwise the results will be misleading. The data reading procedure can be verified readily by a crude simulation. If the ultrasound transducer is held in one hand with its abdominal surface toward the palm, and if this hand then is tapped with the other one, the fetal heart beating is simulated. At every tap the cardiotocograph provides the appropriate “FHR” value. When the hand is tapped at a frequency of 140-150 beats per minute (the average human rhythm sense is inaccurate enough to produce a variation in the intervals between tappings similar to that of a real FHR) but the chest sometimes is tapped instead of the hand, two, three, four, or more times in a random way, a check may be made to determine whether the program recognizes the existence and number of missed beats. The same signal processing is used for either the ultrasound or the scalp electrodes. This simulation technique was applied as validity measurement 100 times. In 95 cases the program recognized the correct number of simulated missed beats.

838 Boz6ki

Fetal

Monitor

Communication

Comment Although commercial cardiotocography analysis programs, such as the System 8000/8002 (Oxford Sonicaid Ltd.), offer complex analysis possibilities, the commonly used data-averaging technique (eg, %6-IrdnUk epoch) may be a disadvantage in some cases. The described method of digital communication with Sonicaid fetal monitors (Oxford Instruments) can be used for many individual purposes in the field of computed cardiotocography analysis. I have used it to check whether the FHR values provided by a high-tech cardiotocograph (using an external abdominal transducer or intrapartum scalp electrodes) are strictly continuous and highly precise data, as required by chaos theory and power spectrum analysis. I have used it to make truly simultaneous objective cardiotocography recordings in twin pregnancies and to detect objectively synchronous accelerations with or without fetal movements in twin pregnancies. With its help, I have succeeded in developing a quicker, more stable, and more reliable method of baseline fitting to FHR data than that of the System

Set COMl port parameters

e

1Start software loop for watching beat flag 1 1 Send l-byte-long datum request code 1t

I

YES 1 1 Get beat flag 1 i

-1

Data DrLessing

Figure 1. Algorithm of digital monitors (Oxford Instruments).

I

communication

with

Sonicaid

fetal

Obsfef,+s 6 Gynecology

8000/8002 (Oxford Sonicaid Ltd.). I also have devised a method of internal trend analysis of System 8000/8002 (Oxford Sonicaid Ltd.) cardiotocography records. An exchange of information can prove constructive, and is encouraged.

R$erences 1. Dawes GS, Moulden M, Redman CW. Criteria for the design of fetal heart rate analysis system. Int J Biomed Comput 1990;25:287-94. 2. Chaffin D, Goldberg C, Reed K. The dimension of the chaos in the fetal heart rate. Am J Obstet Gynecol 1991;165:1425-9. 3. Akselrod S, Gordon D, Ubel FA, Shannon DC, Berger AC, Cohen RJ. Power spectrum analysis of heart rate fluctuation: A quantitative probe of beat-to-beat cardiovascular control. Science 1981;213: 220-2.

CROSS-SECTIONAL

IMAGING

ANATOMY

ANAL

OFTHE

SPHINCTERS Ursula M. Peschers, MD, John 0. L. DeLancey, MD, Helga Fritsch, MD, Leslie E. Quint, MD, and Martin R. Prince, MD, PhD Background:

To

describe

cross-sectional

anatomy of the anal sphincter mechanism relevant to magnetic resonance imaging (MRI) and ultrasound cross-sectional images. Method: Axial, sagittal, and coronal 5-mm sections of female pelves were reviewed from six cadaver specimens (ages 24-72 years). Fetal anatomy was studied in plastinated histologic sections from 19 and 26 weeks’ gestation. Images of the anal sphincter were obtained by MRI in six and by ultrasound using an exoanal technique in 12 nulliparous volunteers.

Experience: The internal

the

anal sphincter

is clearly visible in

anatomic

sections central to the external sphincter and is visible in MRI and ultrasound images. The external anal sphincter can be subdivided into a subcutaneous and a deep portion. On anatomic sections and on MRI, the subcutaneous part shows as two parallel muscle strips in the axial plane; the deep portion presents with a characteristic teardrop form in the section perpendicular to the axis of the anal canal. The puborectalis muscle and the external anal sphincFrom the Departments @Obstetrics and Gynecology and Radiology, The University ofMichigan Medical Center, Ann Arbor, Michigan; and the Anatomical Institute, Medical University, Luebeck, Germany. Dr. Peschers was supported by agrant (Pe 610/l-lifiom the Deutsche Forschungsgemeinschq?.

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Address reprint requests to: Zsolt Bozdki, MD Department of Obstetrics and Gynecology Albert Szent-Gyiirgyi Medical University Semmelweis u. 1 H-6725 Szeged Hungary E-mail: [email protected]

Received May 7, 1997. Received in revised form Accepted July 18, 1997.

]uly

8, 1997.

Copyright 0 1997 by The American College of Obstetricians Gynecologists. Published by Elsevier Science Inc.

and

bump” ter form a “double in the sagittal section. The longitudinal muscle can be identified by its fiber orientation in anatomic sections but is not clearly visible in imaging studies. Conclusion: This information should make it possible to identify accurately anal sphincter anatomy in twodimensional sectional Gynecol 1997;90:839-44. of Obstetricians

and

images of the anal 0 1997 by The

sphincter. American

(Obstet College

Gynecologists.)

Anal sphincter damage at childbirth has been recognized to be associated with the development of fecal incontinence.‘,2 New imaging techniques have begun to yield important information about the structural defects responsible for this increasingly recognized dysfunction following birth.’ Ultrasound and magnetic resonance imaging (MRI) have emerged as important techniques that allow detailed sectional images of the pelvic floor structures involved in continence.1*3-” In reviewing ultrasound and magnetic resonance images of the sphincter complex, it is apparent that detailed structural information is visible in these scans that has not been defined completely. 13ecause the anatomy of the anal sphincter is both complex and controversial,6 it is important to establish the correct identity of sphincter components in each of the various planes to avoid errors in diagnosis and interpretation. Previous studies have been limited to extrapolating information found on dissection to cross-sections3,4 or have been limited to a single plane.7 This study compared sectional anatomy and sectional images to provide an accurate basis for interpreting sphincter images acquired in sphincter studies.

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