Continuous monitoring of cervical dilatation during labour by ultrasonic transit-time measurement* Ivan Z a d o r t
M i c h a e l R. N e u m a n
R o b e r t N. W o l f s o n
Engineering Design Centre and Perinatal Clinical Research Centre, Case Western Reserve University, Cleveland, Ohio 44106, USA
Abstract---Instrumentation for continuously measuring cervical dilatation during labour based upon measuring the transit time of a short pulse of ultrasound across the intracervical space is described. I rnrn x I mm x 5 mm ultrasonic transducers are attached to diametrically opposite sides of the cervical rim by spring-loaded clips, and their separation increases as the cervix dilates. The velocity of ultrasound in the intracervical space has been determined to be 1.48 mrn/ Fs, making it possible to convert the transit-time measurements between the transducers to displacement, Samples are taken once a second, allowing a continuous recording to be made throughout labour. Examples of cervical dilatation recordings from primigravid and multiparous patients are presented. K e y w o r d s - - C e r v i c a l dilatation, Progress of labour, Ultrasonic transit time, Obstetrical monitoring
Introduction THE INCREASED interest in high-risk obstetrics has resulted in the development of electronic instrumentation and computing systems for continuously recording and evaluating the foetal electrocardiogram, the foetal heart rate, and the intrauterine pressure during labour (1--ION, 1974; NEUMAN et al., 1970). These intensive-care monitoring systems have aided clinicians in evaluating the foetus and uterus during active labour. The instrumentation systems, however, are currently unable to evaluate how well labour is progressing. Clinically, this is determined by periodic manual pelvic examination during which the degree of opening of the uterine cervix and the position of the foetus with respect to internal pelvic structure is determined by the examiner feeling the foetus and uterine cervix with two fingers inserted into the vagina or rectum. These infrequent examinations are not highly accurate, and the results can vary from one examiner to another. They also carry with them the increased risk of introducing infection into the uterine cavity as well as being uncomfortable for the patient. The infrequent sampling can also result in loss of important data. In this work, a new technique is described whereby a continuous recording of cervical dilatation during labour is obtained from ultrasonic transit-time measurements across a diameter of the cervical external os. Several investigators have described techniques for measuring cervical dilatation. However, these have not met widespread application. Mechanical linkages which transmit the cervical diameter to a scale located at the vaginal opening have been applied. Caliper-like devices of this sort were *First received 1st April and in final form 1lth July 1975 tCurrently with the Department of Obstetrics & Gynaeco/ogy, Cleveland Metropolitan Genera/Hospital, Cleveland, Ohio
Medical and Biological Engineering
May 1976
described by FRIEDMAN(1956) and later KREMENTSOV (1968), and were generally bulky, difficult to use, as well as to read. Friedman was able to demonstrate with his instrument that cervical dilatation measured by frequently reading the scale followed the wellknown curve which he had described from less accurate and less frequent digital internal examinations. Friedman later improved his device by replacing the scale with a variable-resistance angular position transducer and directly recording signals electrically (FRIEDMAN and VONMICSKY, 1963). Independent of this work, SIENER (1963) developed a similar electrical technique for continuously recording cervical dilatation. While this improvement made it easier to read and record data from the device, its mechanical nature still made it difficult to attach and maintain throughout labour. There also tended to be problems with the devices in that they often could not be used to record the changes in cervical dilatation throughout an entire labour. This was because the arms passing through the vagina from the cervix to the externally located position transducer were limited in motion by the vaginal walls. Furthermore, the bulk of the apparatus frequently resulted in its position changing during labour, which introduced errors into the determined cervical dilatation, and made continuous placement on a cervical diameter difficult. Another limitation of the device as described is that its output is nonlinear with respect to the cervical dilatation. Magnetic techniques which do not require the mechanical linkages of the systems described above have also been applied to this problem. SMYTH(1954) has constructed a mechanical device using a linear variable differential transformer; however, data from the use of this device has not been forthcoming and Friedman has questioned the overall applicability of the technique. RICE (1974) has measured cervical dilatation by placing coils on opposite sides of the 299
cervix, and determined the magnetic field induced in one from a current in the other. While this technique has eliminated the moving-parts problem of the other mechanical techniques, mutualinductance coils of this sort can only be used for determining displacement along a fixed axis. When angular displacement or a shift in axis coexists with this linear displacement, serious errors are introduced. The mutual inductance technique also produces a nonlinear output with respect to cervical dilatation. Recently, KRIEWALL(1974) has described a technique to measure cervical dilatation magnetically without encountering errors of the type described above. This is done by using a permanentmagnet dipole as the magnetic-field source and two Hall-effect magnetic-field transducers as the detector.
are attached to opposite sides of the cervix by means of small spring-loaded clips which hold them firmly against its rim. If carefully placed, the transducers can be attached to the cervix before extensive dilatation has begun, and with the foetal membranes intact. A clip with the attached transducer is illustrated in Fig. 1. The sharp points of the clip pierce the cervical tissue so as to hold the transducer firmly in place, thus minimising migration during labour. Note that the transducer is attached close to one of these teeth so that it approximates well against the cervical surface. The clip is applied by placing it in a specially designed pair of forceps which, when clamped, compress the spring so that the teeth of the clip are open. These are then placed against the portion of the
Fig. 1 An ultrasonic transducer and spring-loaded clip for attachment to the cervix
They are positioned in quadrature so that they pick up the radial and tangential components of the magnetic field from the permanent magnet. These two signals are then processed to determine the spacing. The main limitation of this elegant technique is that, over the normal range of cervical dilatation (approximately l0 to 100ram), the strength of the magnetic field varies by several orders of magnitude, which results in it being of the order of the Earth's magnetic field as the patient approaches the end stages of labour, making an accurate detection difficult. The ultrasonic transit-time technique described below was developed to eliminate or minimise the problems inherent in the other techniques to make it possible to record cervical dilatation continuously during labour on a routine basis.
Methodology and instrumentation Small ultrasonic transducers are placed on the lips of the external os of the cervix early in labour. These transducers measuring 1 mm x 1 m m x 5 mm 300
cervix to which they are to be attached either by direct observation through an endoscope or speculum or indirectly by palpating the position. Once the correct location has been established, the spring is released by opening the forceps, and the teeth puncture the cervical mucosa as the spring relaxes. As in the case of magnetic transducers, any deviation of the axis of one transducer with respect to the other for the ultrasonic transducers will result in a change in the amplitude of the ultrasound signal. Since time rather than amplitude is the important quantity, this is not a problem unless the amplitude diminishes to such an extent that the received signal cannot be detected out of the noise. To demonstrate that deviation of the axis was not a problem, field-strength patterns of the ultrasonic transducers were obtained in a water bath. Fig. 2 illustrates a typical pattern obtained from one of the transducers described above being rotated through 180~, as noted in the Figure. This sensor was located at various distances from a fixed similar tiansducer which was used as the source of ultrasonic energy. Medical and Biological Engineering
May 1976
It is noted from the Figure that, for amplitude variations within 3 dB, there is at least a 6 0 ~ angle over which the transducer is effective. Fig. 3 illustrates a block diagram of the electronic system used to obtain cervical dilatation. A 0.5/Ls pulse is generated once per second by the pulse generator and is amplified to a level of 30 V peak. It is then applied to the transmitting transducer through a coupling capacitor which isolates power
transferred from the counter to a register, which serves as a memory, and the counter is reset to await the next sample. A digital output can be obtained directly from the memory, or an analogue output is available from a digital/analogue convertor attached to the memory register. The instrument obtains the cervical dilatation by making the assumption that the velocity of ultrasound is constant in the medium located between
Fig. 2 Distribution of the amplitude of the ultrasonic field from a cervical transducer taken about the long axis of the ceramic ultrasonic element
supply voltages from the transducer on the patient. Isolation of the power supply from the mains by a low leakage-current transformer reduces 50 to 60 Hz current leakage at the transducers to less than 10pA. The pulse excites the transducer at its thickness mode resonance (nominally 1.5 MHz for the transducers used), producing a damped oscillation at that frequency. At the same time the excitation pulse opens a gate allowing a 2 MHz clock access to a binary counter. The ultrasonic energy produced by this technique is well within currently acceptable safe limits (ULRICh, 1974). The pulse voltage, transducer size and duty cycle are much less than those encountered with ultrasonic scanners. Only the exposure time is greater. When compared with ultrasonic Doppler foetal heart-rate monitors, the ultrasonic dosage is less owing to the low duty cycle, but the exposure time is similar. Thus, in terms of ultrasonic irradiation, this cervical dilatation monitor should share the same clinical acceptability as these other devices. When the ultrasonic signal generated by the self resonance of the transmitting transducer reaches the receiving transducer, it is amplified through a highgain low-Q factor amplifier and then detected by a pair of high-speed comparators. The signal from the comparators closes the gate between the clock and the binary counter; thus the number of counts in the counter is proportional to the time between the generation of the ultrasonic pulse at the transmitter and its reception at the receiver. This information is Medical and Biological Engineering
May 1976
the two transducers. This medium ranges from the foetal head itself to the amniotic fluid, mucous and vernix caseosa. The value of the velocity of ultrasound was taken as 1.48 mm/;us based on measurements made from external transducers across the maternal pelvis. The value was confirmed by transittime measurements from a pair of transducers fixed at a separation of 40 mm and placed against the foetal head during labour. The displacement between the two cervical transducers is obtained by multiplying the transit time as stored in the register by the velocity of ultra-
~ LJ~Tronsmzzrt~ I
ANALOG OUTPUt-OUTPUT Fig. 3 Block diagram of the electronic circuitry used in the ultrasonic c~rvimeter
301
sound in the intracervical space. Since the instrument described was to be used to make recordings on an oscillograph, this multiplication was carried out using a potentiometer to appropriately scale the analogue signal. In presenting the data, the recordings ot cervical dilatation were to be compared with those of the foetal heart rate and intrauterine pressure during labour, and so cervical dilatation was plotted on a third channel of an oscillograph adjacent to these other two channels of data at a chart speed of 0.5 mm/s. Cervical dilatation as a function of time is also important when presented as a labour graph (FRIEDMAN, 1954). In this case the recordings are made at a chart speed of 0.01 mm/s. The instrument is capable of measuring displacements ranging from 5 mm to 180 rnm; however, the practical range for cervical dilatation measurements is 10-100 ram.
Six of these patients were primigravidas (first pregnancy) while the remaining eight had had at least one uncomplicated vaginal delivery. Cervical transducers were prepared by sterilisation in ethylene oxide for 12 hours followed by at least 12 additional hours for degassing. A recording typical of the data obtained from the primigravid patients is illustrated in Fig. 4 starting in the latent phase of labour at a cervical dilatation of approximately 30 mm and continuously recording up to delivery. Note the slow progress of dilatation throughout the entire recorded labour. To demonstrate the dynamic behaviour of the cervix during a uterine contraction, a faster-speed recording was made at a cervical dilatation of approximately 60 ram. This recording shows that, even in the active phase, there is less than an 8 mm change in dilatation during uterine contractions. If these data are compared with those for a typical multiparous patient as shown in Fig. 5, it is seen that there is a much greater slope to the dilatation/ time curve for the latter. Furthermore, it is seen on the higher-speed recording that during the active phase of labour there is a greater change of cervical dilatation with each uterine contraction than was
Results
As a means of demonstrating the type of data that can be obtained from this instrument, a group of 14 patients were included in a preliminary study. GRAVlOI~1, PAPA 0
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Medical and Biological Engineering
May 1976
that of the stretched cervix as recorded on the instrument, once he removes his examining fingers, the cervical dilatation returns to its initial value, which was 15ram less than the stretched value during the examination.
observed for the primigravid patient, as well as the fact that the rise time of cervical dilatation during a contraction is greater than the fall time following the contraction. This, of course, leads to a more rapid progress of labour. None of the patients complained of pain due to the cervical clips following attachment. Furthermore. cervical trauma as a result of the clips was noted in only one patient. This was a small laceration attributed to an improper removal of the clip. One advantage of this technique of measuring cervical dilatation is that it does not interfere with any manual examinations which must be made during labour. This is illustrated by the data in Fig. 6 which shows a recording of the cervical dilatation taken simultaneously while the dilatation was being determined by the conventional digital technique. Note that once the physician inserts his fingers and starts to feel the cervix, he stretches it, and although his determined value corresponds to
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It was found from the preliminary sample of 14 patients that it was possible to obtain continuous recordings of cervical dilatation during labour. It was found to be unnecessary to have a sampling rate of any greater than 1 per second, since the observed data varied at a much lower rate. The low-sampling-rate, brief ultrasonic pulses were also desirable so as to minimise the ultrasonic irradiation of the foetus. The recordings show that it is possible to carry out manual internal manipulations on the patient while the instrument is attached and measurements are being made. In comparing results from the instrument with those made manually by
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Medical and Biological Engineering
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Fig. 5 Recording of cervical dilatation as a function of time from a typical multiparous patient
303
the physician managing the patient being studied, it was found that the agreement between the two was fairly good, with the discrepancy being generally no greater than 10-20 mm. During the early design work for this instrument the ultrasonic transmitters were attached to a Michelle clip which was held in place on the cervix. These clips were found to become easily detached, thereby giving erroneous data, and so a springloaded clip similar to the one described above but with shorter teeth was designed. These clips frequently came off or migrated during labour so that they were no longer diametrically opposed on the cervix and hence produced erroneous data. Patients studied using these preliminary designs gave uncertain results owing to the difficulty in maintaining the positions of the transducers. These problems were greatly minimised by the use of long sharp teeth on the spring-loaded clips as well as a strong spring so
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data from 14 patients have shown that recordings can be made throughout labour, and that these recordings correspond in general form to data described by Friedman using manual techniques on a large number of patients. Furthermore, dynamic recordings of changes in cervical dilatation during uterine contractions can be made throughout all stages of labour, and preliminary data show these changes to be different for primigravid and multiparous patients. Additional clinical trials of the technique are currently being undertaken to further demonstrate its clinical usefulness. Acknowledgment--This study was supported by the National Institute of Child Health and Human Development Grant No. HD-06735 and National Institute of General Medical Science Grant No. GM-14267-10, with patient support through the Perinatal Clinical Research Centre, Cleveland Metropolitan General Hospital,
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that the teeth must bite into the cervix. The series of 14 patients described above were studied using a clip of this design, and none of the transducers became dislodged. A major advantage of this instrument is that it provides the clinician with a continuous recording of cervical dilatation during labour so that he may watch the progress of labour without having to frequently examine the patient. This therefore can reduce the necessity for frequent digital examinations, but should not eliminate them altogether since foetal descent must still be determined in this way. Nonetheless, the reduction of the number of digital examinations can, in addition to reducing patient discomfort, be important for patients who are at risk of intrauterine infection. Conclusions It has been demonstrated that cervical dilatation can be measured continuously during labour by determining ultrasonic transit time. Preliminary
304
National Institutes of Health, Division of Research Resources Grant RR-00210. The authors also wish to express their gratitude to Messrs. W. Klinger, L. Greene, H. Durda and K, Dmytrus for technical assistance.
References FRIEDMAN, E. A. (1954) The graphic analysis of labor. Am. J. Obstet. Gynecol. 68, 1568. FRIEDMAN,E. A. (1956) Cervimetry; An objective method for study of cervical dilatation in labor. Am. J. Obstet. Gynecol. 71, 1189. FRIEDMAN, E. A. and VoNMICSKY, L. (1963) Electronic cervimeters: A research instrument for the study of cervical dilatation in labor. Am. J. Obstet. Gynecol. 87, 789. HON, E. H. (1974) Biophysical intrapartal fetal monitoring. Clinics in Perinatology 1, 149. KREMENTSOV,U. (1968) Improved technique for measurement of cervical dilatation. Biomed. Eng. (NY)2, 350 KRIEWALL, T. J. (1974) Measurement and analysis of cervical dilatation in human parturition. Doctoral
Medical and Biological Engineering
May 1976
Dissertation, University of Michigan, Ann Arbor, Michigan, USA. NEUMAN,M. R., PtCCONNOTrO,J. and Roux, J. F, (1970) A wireless radiotelemetry system for monitoring fetal heart rate and intrauterine pressure during labor and delivery. Gynecologic Investigation 1, 92. RICE, D. A. (1974) Mechanism and measurement of cervical dilatation. Doctoral Dissertation, Purdue University, Lafayette, Ind. USA.
StoNER, H. (1963) First stage of labour recorded by cervical tocometry. Am. J. Obstet. Gynecol. 86, 303. SMYTH, C. N. (1956) Measurement of the forces and strains of labour and the action of certain oxytocic drugs. Comptes-Rendus du Congres International de Gynecologie et d' Obstetrique S.A. George, Geneva. ULRICH, W. C. (1974) Ultrasound dosage for nontherapeutic use on human beings--Extrapolations from a literature survey. 1EEE Trans. BME-21, 48.
Surveillance continue de 1"61argissement cervical pendant le travail d'accouchement par mesures du delai de r6ponse ultra-sonore Sommaire--La description concerne des appareils destinrs h faire la surveillance continue de l'61argissement cervical pendant le travail d'accouchement, en mesurant le drlai de rrponse d'une courte impulsion ultra-sonore ~t travers l'espace intracervical. Des capteurs ultra-sonores 1 mm x I m m x 5 mm sont attachrs aux c6trs diamrtralement opposrs du rebord cervical avec des clips b. ressort, et l'rcartement entre les capteurs augmente au fur et ~t mesure que le col se dilate. La vitesse de l'ultra-son dans l'espace intracervical est calculre b_ 1.48 mm//zs; il est donc possible de transformer en d6placement les d61ais de rrponse mesurrs entre les capteurs. Les echantillons sont prrlevrs toutes les secondes, permettant ainsi de faire des enregistrements continus pendant le travail. Des enregistrements de l'61argissement cervical faits sur des malades primigestes et multipares sont prrsentrs.
Stiindige 0berwachung der Gebiirmutterhalsdilatation wiihrend der Wehen durch UltraschalI-Laufzeitmessung Zusammenfassung--Es werden Instrumente fiir die st~,ndige Messung der Gebiirmutterhalsdilatation w~ihrend der Wehen beschrieben, die die Laufzeit eines kurzen Ultraschallimpulses iiber die Gebiirmutterhals0ffnung messen. Am Gebiirmutterhalsrand werden durch gefederte Klemmen UltraschaUmel3wandler von 1 x I x 5 mm an gegeniiberliegenden Seiten befestigt. Ihr Abstand erh6ht sich, wenn sich der Geb~.rmutterhals erweitert. Die Geschwindigkeit des UltraschalIs in der Gebhrmutterhalsrffnung wurde mit 1.48 mm/tzs bestimmt, so dab es m~Sgiich ist, die Laufzeitmessungen zwischen den MeBwandlern im Verhiiltnis zur Verschiebung umzurechnen. Die Probenahme erfolgt einmal pro Sekunde, so dab wahrend der Wehen eine stiindige Aufzeichnung erfolgen kann. Es werden Beispiele ftir Gebiirmutterhals-Dilatationsaufzeichnungen yon erstgebhrenden und multiparen Patienten vorgestellt.
Medical and Biological Engineering
May 1976
305
M i c h a e l R. N e u m a n
R o b e r t N. W o l f s o n
Engineering Design Centre and Perinatal Clinical Research Centre, Case Western Reserve University, Cleveland, Ohio 44106, USA
Abstract---Instrumentation for continuously measuring cervical dilatation during labour based upon measuring the transit time of a short pulse of ultrasound across the intracervical space is described. I rnrn x I mm x 5 mm ultrasonic transducers are attached to diametrically opposite sides of the cervical rim by spring-loaded clips, and their separation increases as the cervix dilates. The velocity of ultrasound in the intracervical space has been determined to be 1.48 mrn/ Fs, making it possible to convert the transit-time measurements between the transducers to displacement, Samples are taken once a second, allowing a continuous recording to be made throughout labour. Examples of cervical dilatation recordings from primigravid and multiparous patients are presented. K e y w o r d s - - C e r v i c a l dilatation, Progress of labour, Ultrasonic transit time, Obstetrical monitoring
Introduction THE INCREASED interest in high-risk obstetrics has resulted in the development of electronic instrumentation and computing systems for continuously recording and evaluating the foetal electrocardiogram, the foetal heart rate, and the intrauterine pressure during labour (1--ION, 1974; NEUMAN et al., 1970). These intensive-care monitoring systems have aided clinicians in evaluating the foetus and uterus during active labour. The instrumentation systems, however, are currently unable to evaluate how well labour is progressing. Clinically, this is determined by periodic manual pelvic examination during which the degree of opening of the uterine cervix and the position of the foetus with respect to internal pelvic structure is determined by the examiner feeling the foetus and uterine cervix with two fingers inserted into the vagina or rectum. These infrequent examinations are not highly accurate, and the results can vary from one examiner to another. They also carry with them the increased risk of introducing infection into the uterine cavity as well as being uncomfortable for the patient. The infrequent sampling can also result in loss of important data. In this work, a new technique is described whereby a continuous recording of cervical dilatation during labour is obtained from ultrasonic transit-time measurements across a diameter of the cervical external os. Several investigators have described techniques for measuring cervical dilatation. However, these have not met widespread application. Mechanical linkages which transmit the cervical diameter to a scale located at the vaginal opening have been applied. Caliper-like devices of this sort were *First received 1st April and in final form 1lth July 1975 tCurrently with the Department of Obstetrics & Gynaeco/ogy, Cleveland Metropolitan Genera/Hospital, Cleveland, Ohio
Medical and Biological Engineering
May 1976
described by FRIEDMAN(1956) and later KREMENTSOV (1968), and were generally bulky, difficult to use, as well as to read. Friedman was able to demonstrate with his instrument that cervical dilatation measured by frequently reading the scale followed the wellknown curve which he had described from less accurate and less frequent digital internal examinations. Friedman later improved his device by replacing the scale with a variable-resistance angular position transducer and directly recording signals electrically (FRIEDMAN and VONMICSKY, 1963). Independent of this work, SIENER (1963) developed a similar electrical technique for continuously recording cervical dilatation. While this improvement made it easier to read and record data from the device, its mechanical nature still made it difficult to attach and maintain throughout labour. There also tended to be problems with the devices in that they often could not be used to record the changes in cervical dilatation throughout an entire labour. This was because the arms passing through the vagina from the cervix to the externally located position transducer were limited in motion by the vaginal walls. Furthermore, the bulk of the apparatus frequently resulted in its position changing during labour, which introduced errors into the determined cervical dilatation, and made continuous placement on a cervical diameter difficult. Another limitation of the device as described is that its output is nonlinear with respect to the cervical dilatation. Magnetic techniques which do not require the mechanical linkages of the systems described above have also been applied to this problem. SMYTH(1954) has constructed a mechanical device using a linear variable differential transformer; however, data from the use of this device has not been forthcoming and Friedman has questioned the overall applicability of the technique. RICE (1974) has measured cervical dilatation by placing coils on opposite sides of the 299
cervix, and determined the magnetic field induced in one from a current in the other. While this technique has eliminated the moving-parts problem of the other mechanical techniques, mutualinductance coils of this sort can only be used for determining displacement along a fixed axis. When angular displacement or a shift in axis coexists with this linear displacement, serious errors are introduced. The mutual inductance technique also produces a nonlinear output with respect to cervical dilatation. Recently, KRIEWALL(1974) has described a technique to measure cervical dilatation magnetically without encountering errors of the type described above. This is done by using a permanentmagnet dipole as the magnetic-field source and two Hall-effect magnetic-field transducers as the detector.
are attached to opposite sides of the cervix by means of small spring-loaded clips which hold them firmly against its rim. If carefully placed, the transducers can be attached to the cervix before extensive dilatation has begun, and with the foetal membranes intact. A clip with the attached transducer is illustrated in Fig. 1. The sharp points of the clip pierce the cervical tissue so as to hold the transducer firmly in place, thus minimising migration during labour. Note that the transducer is attached close to one of these teeth so that it approximates well against the cervical surface. The clip is applied by placing it in a specially designed pair of forceps which, when clamped, compress the spring so that the teeth of the clip are open. These are then placed against the portion of the
Fig. 1 An ultrasonic transducer and spring-loaded clip for attachment to the cervix
They are positioned in quadrature so that they pick up the radial and tangential components of the magnetic field from the permanent magnet. These two signals are then processed to determine the spacing. The main limitation of this elegant technique is that, over the normal range of cervical dilatation (approximately l0 to 100ram), the strength of the magnetic field varies by several orders of magnitude, which results in it being of the order of the Earth's magnetic field as the patient approaches the end stages of labour, making an accurate detection difficult. The ultrasonic transit-time technique described below was developed to eliminate or minimise the problems inherent in the other techniques to make it possible to record cervical dilatation continuously during labour on a routine basis.
Methodology and instrumentation Small ultrasonic transducers are placed on the lips of the external os of the cervix early in labour. These transducers measuring 1 mm x 1 m m x 5 mm 300
cervix to which they are to be attached either by direct observation through an endoscope or speculum or indirectly by palpating the position. Once the correct location has been established, the spring is released by opening the forceps, and the teeth puncture the cervical mucosa as the spring relaxes. As in the case of magnetic transducers, any deviation of the axis of one transducer with respect to the other for the ultrasonic transducers will result in a change in the amplitude of the ultrasound signal. Since time rather than amplitude is the important quantity, this is not a problem unless the amplitude diminishes to such an extent that the received signal cannot be detected out of the noise. To demonstrate that deviation of the axis was not a problem, field-strength patterns of the ultrasonic transducers were obtained in a water bath. Fig. 2 illustrates a typical pattern obtained from one of the transducers described above being rotated through 180~, as noted in the Figure. This sensor was located at various distances from a fixed similar tiansducer which was used as the source of ultrasonic energy. Medical and Biological Engineering
May 1976
It is noted from the Figure that, for amplitude variations within 3 dB, there is at least a 6 0 ~ angle over which the transducer is effective. Fig. 3 illustrates a block diagram of the electronic system used to obtain cervical dilatation. A 0.5/Ls pulse is generated once per second by the pulse generator and is amplified to a level of 30 V peak. It is then applied to the transmitting transducer through a coupling capacitor which isolates power
transferred from the counter to a register, which serves as a memory, and the counter is reset to await the next sample. A digital output can be obtained directly from the memory, or an analogue output is available from a digital/analogue convertor attached to the memory register. The instrument obtains the cervical dilatation by making the assumption that the velocity of ultrasound is constant in the medium located between
Fig. 2 Distribution of the amplitude of the ultrasonic field from a cervical transducer taken about the long axis of the ceramic ultrasonic element
supply voltages from the transducer on the patient. Isolation of the power supply from the mains by a low leakage-current transformer reduces 50 to 60 Hz current leakage at the transducers to less than 10pA. The pulse excites the transducer at its thickness mode resonance (nominally 1.5 MHz for the transducers used), producing a damped oscillation at that frequency. At the same time the excitation pulse opens a gate allowing a 2 MHz clock access to a binary counter. The ultrasonic energy produced by this technique is well within currently acceptable safe limits (ULRICh, 1974). The pulse voltage, transducer size and duty cycle are much less than those encountered with ultrasonic scanners. Only the exposure time is greater. When compared with ultrasonic Doppler foetal heart-rate monitors, the ultrasonic dosage is less owing to the low duty cycle, but the exposure time is similar. Thus, in terms of ultrasonic irradiation, this cervical dilatation monitor should share the same clinical acceptability as these other devices. When the ultrasonic signal generated by the self resonance of the transmitting transducer reaches the receiving transducer, it is amplified through a highgain low-Q factor amplifier and then detected by a pair of high-speed comparators. The signal from the comparators closes the gate between the clock and the binary counter; thus the number of counts in the counter is proportional to the time between the generation of the ultrasonic pulse at the transmitter and its reception at the receiver. This information is Medical and Biological Engineering
May 1976
the two transducers. This medium ranges from the foetal head itself to the amniotic fluid, mucous and vernix caseosa. The value of the velocity of ultrasound was taken as 1.48 mm/;us based on measurements made from external transducers across the maternal pelvis. The value was confirmed by transittime measurements from a pair of transducers fixed at a separation of 40 mm and placed against the foetal head during labour. The displacement between the two cervical transducers is obtained by multiplying the transit time as stored in the register by the velocity of ultra-
~ LJ~Tronsmzzrt~ I
ANALOG OUTPUt-OUTPUT Fig. 3 Block diagram of the electronic circuitry used in the ultrasonic c~rvimeter
301
sound in the intracervical space. Since the instrument described was to be used to make recordings on an oscillograph, this multiplication was carried out using a potentiometer to appropriately scale the analogue signal. In presenting the data, the recordings ot cervical dilatation were to be compared with those of the foetal heart rate and intrauterine pressure during labour, and so cervical dilatation was plotted on a third channel of an oscillograph adjacent to these other two channels of data at a chart speed of 0.5 mm/s. Cervical dilatation as a function of time is also important when presented as a labour graph (FRIEDMAN, 1954). In this case the recordings are made at a chart speed of 0.01 mm/s. The instrument is capable of measuring displacements ranging from 5 mm to 180 rnm; however, the practical range for cervical dilatation measurements is 10-100 ram.
Six of these patients were primigravidas (first pregnancy) while the remaining eight had had at least one uncomplicated vaginal delivery. Cervical transducers were prepared by sterilisation in ethylene oxide for 12 hours followed by at least 12 additional hours for degassing. A recording typical of the data obtained from the primigravid patients is illustrated in Fig. 4 starting in the latent phase of labour at a cervical dilatation of approximately 30 mm and continuously recording up to delivery. Note the slow progress of dilatation throughout the entire recorded labour. To demonstrate the dynamic behaviour of the cervix during a uterine contraction, a faster-speed recording was made at a cervical dilatation of approximately 60 ram. This recording shows that, even in the active phase, there is less than an 8 mm change in dilatation during uterine contractions. If these data are compared with those for a typical multiparous patient as shown in Fig. 5, it is seen that there is a much greater slope to the dilatation/ time curve for the latter. Furthermore, it is seen on the higher-speed recording that during the active phase of labour there is a greater change of cervical dilatation with each uterine contraction than was
Results
As a means of demonstrating the type of data that can be obtained from this instrument, a group of 14 patients were included in a preliminary study. GRAVlOI~1, PAPA 0
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Medical and Biological Engineering
May 1976
that of the stretched cervix as recorded on the instrument, once he removes his examining fingers, the cervical dilatation returns to its initial value, which was 15ram less than the stretched value during the examination.
observed for the primigravid patient, as well as the fact that the rise time of cervical dilatation during a contraction is greater than the fall time following the contraction. This, of course, leads to a more rapid progress of labour. None of the patients complained of pain due to the cervical clips following attachment. Furthermore. cervical trauma as a result of the clips was noted in only one patient. This was a small laceration attributed to an improper removal of the clip. One advantage of this technique of measuring cervical dilatation is that it does not interfere with any manual examinations which must be made during labour. This is illustrated by the data in Fig. 6 which shows a recording of the cervical dilatation taken simultaneously while the dilatation was being determined by the conventional digital technique. Note that once the physician inserts his fingers and starts to feel the cervix, he stretches it, and although his determined value corresponds to
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It was found from the preliminary sample of 14 patients that it was possible to obtain continuous recordings of cervical dilatation during labour. It was found to be unnecessary to have a sampling rate of any greater than 1 per second, since the observed data varied at a much lower rate. The low-sampling-rate, brief ultrasonic pulses were also desirable so as to minimise the ultrasonic irradiation of the foetus. The recordings show that it is possible to carry out manual internal manipulations on the patient while the instrument is attached and measurements are being made. In comparing results from the instrument with those made manually by
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Medical and Biological Engineering
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Fig. 5 Recording of cervical dilatation as a function of time from a typical multiparous patient
303
the physician managing the patient being studied, it was found that the agreement between the two was fairly good, with the discrepancy being generally no greater than 10-20 mm. During the early design work for this instrument the ultrasonic transmitters were attached to a Michelle clip which was held in place on the cervix. These clips were found to become easily detached, thereby giving erroneous data, and so a springloaded clip similar to the one described above but with shorter teeth was designed. These clips frequently came off or migrated during labour so that they were no longer diametrically opposed on the cervix and hence produced erroneous data. Patients studied using these preliminary designs gave uncertain results owing to the difficulty in maintaining the positions of the transducers. These problems were greatly minimised by the use of long sharp teeth on the spring-loaded clips as well as a strong spring so
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data from 14 patients have shown that recordings can be made throughout labour, and that these recordings correspond in general form to data described by Friedman using manual techniques on a large number of patients. Furthermore, dynamic recordings of changes in cervical dilatation during uterine contractions can be made throughout all stages of labour, and preliminary data show these changes to be different for primigravid and multiparous patients. Additional clinical trials of the technique are currently being undertaken to further demonstrate its clinical usefulness. Acknowledgment--This study was supported by the National Institute of Child Health and Human Development Grant No. HD-06735 and National Institute of General Medical Science Grant No. GM-14267-10, with patient support through the Perinatal Clinical Research Centre, Cleveland Metropolitan General Hospital,
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that the teeth must bite into the cervix. The series of 14 patients described above were studied using a clip of this design, and none of the transducers became dislodged. A major advantage of this instrument is that it provides the clinician with a continuous recording of cervical dilatation during labour so that he may watch the progress of labour without having to frequently examine the patient. This therefore can reduce the necessity for frequent digital examinations, but should not eliminate them altogether since foetal descent must still be determined in this way. Nonetheless, the reduction of the number of digital examinations can, in addition to reducing patient discomfort, be important for patients who are at risk of intrauterine infection. Conclusions It has been demonstrated that cervical dilatation can be measured continuously during labour by determining ultrasonic transit time. Preliminary
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National Institutes of Health, Division of Research Resources Grant RR-00210. The authors also wish to express their gratitude to Messrs. W. Klinger, L. Greene, H. Durda and K, Dmytrus for technical assistance.
References FRIEDMAN, E. A. (1954) The graphic analysis of labor. Am. J. Obstet. Gynecol. 68, 1568. FRIEDMAN,E. A. (1956) Cervimetry; An objective method for study of cervical dilatation in labor. Am. J. Obstet. Gynecol. 71, 1189. FRIEDMAN, E. A. and VoNMICSKY, L. (1963) Electronic cervimeters: A research instrument for the study of cervical dilatation in labor. Am. J. Obstet. Gynecol. 87, 789. HON, E. H. (1974) Biophysical intrapartal fetal monitoring. Clinics in Perinatology 1, 149. KREMENTSOV,U. (1968) Improved technique for measurement of cervical dilatation. Biomed. Eng. (NY)2, 350 KRIEWALL, T. J. (1974) Measurement and analysis of cervical dilatation in human parturition. Doctoral
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May 1976
Dissertation, University of Michigan, Ann Arbor, Michigan, USA. NEUMAN,M. R., PtCCONNOTrO,J. and Roux, J. F, (1970) A wireless radiotelemetry system for monitoring fetal heart rate and intrauterine pressure during labor and delivery. Gynecologic Investigation 1, 92. RICE, D. A. (1974) Mechanism and measurement of cervical dilatation. Doctoral Dissertation, Purdue University, Lafayette, Ind. USA.
StoNER, H. (1963) First stage of labour recorded by cervical tocometry. Am. J. Obstet. Gynecol. 86, 303. SMYTH, C. N. (1956) Measurement of the forces and strains of labour and the action of certain oxytocic drugs. Comptes-Rendus du Congres International de Gynecologie et d' Obstetrique S.A. George, Geneva. ULRICH, W. C. (1974) Ultrasound dosage for nontherapeutic use on human beings--Extrapolations from a literature survey. 1EEE Trans. BME-21, 48.
Surveillance continue de 1"61argissement cervical pendant le travail d'accouchement par mesures du delai de r6ponse ultra-sonore Sommaire--La description concerne des appareils destinrs h faire la surveillance continue de l'61argissement cervical pendant le travail d'accouchement, en mesurant le drlai de rrponse d'une courte impulsion ultra-sonore ~t travers l'espace intracervical. Des capteurs ultra-sonores 1 mm x I m m x 5 mm sont attachrs aux c6trs diamrtralement opposrs du rebord cervical avec des clips b. ressort, et l'rcartement entre les capteurs augmente au fur et ~t mesure que le col se dilate. La vitesse de l'ultra-son dans l'espace intracervical est calculre b_ 1.48 mm//zs; il est donc possible de transformer en d6placement les d61ais de rrponse mesurrs entre les capteurs. Les echantillons sont prrlevrs toutes les secondes, permettant ainsi de faire des enregistrements continus pendant le travail. Des enregistrements de l'61argissement cervical faits sur des malades primigestes et multipares sont prrsentrs.
Stiindige 0berwachung der Gebiirmutterhalsdilatation wiihrend der Wehen durch UltraschalI-Laufzeitmessung Zusammenfassung--Es werden Instrumente fiir die st~,ndige Messung der Gebiirmutterhalsdilatation w~ihrend der Wehen beschrieben, die die Laufzeit eines kurzen Ultraschallimpulses iiber die Gebiirmutterhals0ffnung messen. Am Gebiirmutterhalsrand werden durch gefederte Klemmen UltraschaUmel3wandler von 1 x I x 5 mm an gegeniiberliegenden Seiten befestigt. Ihr Abstand erh6ht sich, wenn sich der Geb~.rmutterhals erweitert. Die Geschwindigkeit des UltraschalIs in der Gebhrmutterhalsrffnung wurde mit 1.48 mm/tzs bestimmt, so dab es m~Sgiich ist, die Laufzeitmessungen zwischen den MeBwandlern im Verhiiltnis zur Verschiebung umzurechnen. Die Probenahme erfolgt einmal pro Sekunde, so dab wahrend der Wehen eine stiindige Aufzeichnung erfolgen kann. Es werden Beispiele ftir Gebiirmutterhals-Dilatationsaufzeichnungen yon erstgebhrenden und multiparen Patienten vorgestellt.
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May 1976
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