Fetal monitoring by physical methods I.Technical questions

EUROP. I. OBSTET. GYNEC. REPROD. BIOL., 1973, 3/2,57-66. EXCERPTA MEDICA

Review article

Fetal monitoring by physical methods

LTechnical quest ions J. DE HAAN”

Department of Obstetrics and Gynecology, Vrije Universiteit, Amsterdam, The Netherlands

DE HAAN, J. (1973): Fetal monitoring by physical methods. I. Technical questions. Europ. J. Obsfet. Gynec. reprod. Biol., 3/2, 57-66.

Several methods to record fetal heart signals and uterine activity simultaneously are available. Almost any of these methods presents certain technical questions based on the different nature and properties of the signals. The significance of these questions for the data obtained during fetal monitoring by physical methods is discussed.

fetal monitoring; fetal electrocardiography; fetal phonocardiography; ultrasonic Doppler signal; fetal heart rate recording

Introduction

Assessment of the fetal condition is based on information from the patient’s medical history, the

development during pregnancy and the conditions during delivery. Since the development of the fetus

and the delivery are both very dynamic processes, obtaining data concerning the fetus intermittently has

its disadvantages. Continuous recording of signals from the fetus in utero is possible by signals origi-

nating from the fetal heart; the fetal heart rate is one

parameter giving information on the fetal circulatory

condition; it is, however, an indirect approach. In as-

sessing the fetal condition during pregnancy and labor, the exact measurement and quantitative analysis of fetal heart rates and their disturbances are the

primary objectives, although changes in the fetal ECG

complex have been used, too. The effects in the heart

rate pattern caused by input-stimuli (for example uterine contractions or drugs) on the feto-maternal system have been especially studied.

* Present address: Department of Obstetrics and Gynecology,

University of Nijmegen, St. Radboud Ziekenhuis, Nijmegen,

The Netherlands.

Recording methods

Fetal heart activity can be recorded by using the electrical field potential: the fetal electrocardiogram,

or the fetal heart sounds: the fetal phonocardiogram. Moreover, fetal heart activity can be detected by

means of the ultrasonic Doppler signal. It applies for all three methods that the detectabil-

ity is a function of the growth of the fetal heart and

in order to perform signal transformation, the exist-

ing signal to noise ratio must be improved. The dis-

tance from and the tissue structures between the

transducer and the fetal heart, the movements of

mother and child, noise sources between transducer and fetal heart and externally generated disturbances

are also important factors in this respect. These prob-

lems are relevant for each of the three methods, al-

though they offer their specific problems and possi-

bilities, based on the different nature and properties

of the signals.

1. indirect fetal electrocardiography

This method can be used as of the 16th weak of pregnancy. The electrodes are usually placed on the midline of the maternal abdominal wall, although

58 J. de Haan, Fetal monitoring by physical methods

other locations have been described, e.g. the vagina (Cremer, 1906; Swartwout and Walter, 1959) and electrodes between the uterine wall and the membranes (Sureau, 1956). Most of the time, plaster or suction-cup electrodes are applied (Larks, 1961; Peeters, 1968) although intracutaneous electrodes have been also used (Friedman and Eckerling, 1969).

2. Direct fetal electrocardiography

Besides the fetal electrocardiogram, the abdominal recorded signal consists of several components; the maternal electrocardiogram, a noise component mainly consisting of the electromyogram, and a multiplica- tive component due to movements of mother (e.g. respiration) and fetus (Van Bemmel, 1968).

To obtain the signal to noise ratio as high as possible several conditions must be fulfilled: proper electrode placement, a low electrode-skin impedance and electrodes constructed out of material with a small polarization effect (silver-silverchloride), and shielding, grounding and electric isolation of the patient from the rest of the equipment (Van der Weide and Van Bemmel, 1968). Proper electrode placement means that the angle between the electrodes and the fetal heart must be as large as possible, and the distance between the electrodes and the fetal heart as small as possible (Van Bemmel, 1969). A low electrode-skin impedance is obtained by preparing the skin with alcohol and removing the horny layer of the epidermis by carefully scrubbing and impregnating the electrode sites with electrode paste. Optimum shielding is obtained by using a Faraday cage (De Haan, 1971) although such a supply is not necessary in routine monitoring.

In this method, the electrodes are placed directly on the fetus: transcervically (in case of ruptured membranes) on the presenting fetal part (Han, 1963), and transabdominally (with intact membranes) on the fetal buttock (Figuero-Longo, Poseiro, Alvarez and Caldeyro-Barcia, 1966). Various types of electrodes made of different materials have been used: clip electrodes covered with silver-silverchloride (Hon, 1963) stainless-steel spring-clip electrodes (Peeters, 1968; De Haan, Stolte, Eskes, Van der Weide, Van Bemmel en Braaksma, 197Oa), needle electrodes covered with gold (Saling, 1969), stainless-steel or platina wire electrodes (Baumgarten, 1969) stainless-steel spiral electrodes (Hon, Paul and Hon, 1972). Vasicka, Hutchin- son, Rijlander and Murray (1963) used transabdominally inserted intraamniotic platina wire electrodes. The results were sometimes very poor and our own experience with this technique has shown that the maternal ECG complex obtained often depends upon the position of the wire inside the uterus, which cannot be checked. At present, the most widely used electrode is the clip type, fixed on the presenting fetal part.

In a direct lead the maternal ECG is rarely visible and the electromyogram hardly present, so a rather noise-free fetal ECG is obtained (Fig. 2). However, the condition of ruptured membranes is a great disadvantage of this method. Complications produced by the electrode are rarely described (Hon and Hess, 1960). We ourselves did not observe any complication with the spring-clip electrode.

During pregnancy, almost all of these conditions can be fulfilled and multiple pregnancies, for example, can be detected (Fig. 1). However, during labor in non-sedated patients the electromyographic component in the signal most of time prevents continuous fetal monitoring with this technique.

3. Fetal phonocardiography As soon as the fetal heart sounds can be heard by

direct auscultation this method can be used. To obtain the signal to noise ratio as high as possible, the distance between the microphone and the fetal heart

Fig. 1 Indirect recording (abdominal leads) of the fetal and maternal electrocardiograms. Twin pregnancy, 33rd week.

J. de Haan, Fetal monitoring by physical methods 59

Fig. 2 Direct recording (scalp lead) of the fetal electrocardiogram.

must be as small as possible, as the detectability is mainly determined by this distance and the size of the fetal heart. Thus, the recording must be obtained from a position on the maternal abdomen where the fetal heart can be heard clearly.

The signal from the microphone contains several components besides the fetal heart sounds: the maternal pulse, movements of fetus and mother, and various outside noises, around the patient. By careful filtering in the frequency domain much of these dis- turbing components can be suppressed (Pommerenke, Bishop and Rochester, 1938; Smith and Hervert, 1940; Wood 1953; Dewhurst and Mainland, 1957; Smith, 19.57; Shelley, 1969). Disturbances occurring notwithstanding filtering can be eliminated by means of time-measuring and time-comparing circuits, which only accept signals with temporal relationships to the fetal heart sounds (Hammacher, 1962, 1967). Al- though this method has the advantage that it can be used with intact membranes, it also has some disadvantages: when the fetus is moving (presenting part not engaged) or when the woman is in heavy labor and does not, relatively speaking, lie quietly on her back, fetal signals are often insufficient for detection @lock and Jung, 1969).

4. The ultrasonic Doppler signal This method can be applied very early in preg-

nancy (Bishop, 1968; Kuah and Embrey, 1968). An ultrasound beam of constant frequency is reflected by a moving object at an altered frequency, depend- ing on the speed of the object. The detectability is mainly determined by the distance from the transmitting/receiving crystal to the fetal heart, and the size of the fetal heart. In the pelvis of a pregnant woman, however, many objects are present which can produce frequency alterations in a transmitted ultra-

sound beam: bowels, large maternal and fetal vessels, placenta and umbilical cord, and movements of mother and child. In order to obtain a high signal to noise ratio one must operate with an accurately directed beam of ultrasound reflecting only from objects in the beam. This is almost impossible without continuous manual control of the transducer (containing the transmitting and receiving crystal). Transducers with a large aperture or multiple receiving crystals located in a circle around a central transmitting crystal offer in this respect some advantage, but results in a smaller signal to noise ratio (Mosler, 1972). This method can also be applied with intact membranes, whereas an- other advantage is the possibility of detecting fetal heart activity very early in pregnancy.

The question of safety of this method has been raised several times. So far, no evidence of clinical damage has been reported when using ultrasonic energy as provided by the commercially available monitoring equipment. Different results and views have been reported about in vitro experiments with much higher energy levels (Bernstine, 1969; Macintosh and Davey, 1970; Loch, Fischer and Kuwert, 1971; Coakly, Hughes, Slade and Lawrence, 1971; Mannor, Serr, Tamari, Meshorer and Frei, 1972; Abramowski, Sturm, Jung and Austermann, 1972).

5. Uterine activity The uterine activity can be recorded by means of

intrauterine pressure recording or external tocodynamometry. Intrauterine pressure recording can be performed by an open-tip or by a sponge-tip fluid- filled catheter, transabdominally or transcervially inserted into the uterus and connected to a pressure transducer (Alvarez and Caldeyro-Barcia, 1950; Hen- dricks, Quilligan, Tyler and Tucker, 1959; Eskes, 1962; Baumgarten, Frohlich, Seidl and Hager, 1968;


Bengtsson, 1968). The transcervically introduced

sponge-tip or open-tip catheters can be introduced

into the uterus without rupturing the membranes

(Schatz, 1872; Bourne and Burn, 1927; Woodbury,

Hamilton and Torpin, 1938; Garrett, 1954; Embrey,

1958; Ingelman-Sundberg and Lindgren, 1953). In

our experience, however, the extraovularly placed

sponge-tip or small open-tip catheters are easily

blocked by blood and mucus. This disadvantage can

be circumvented by covering the end of the catheter

with a balloon or a rubber membrane; however, this

results in inaccurate pressure recordings (Braaksma,

Janssens, Eskes and Hein, 1971). With the commer-

cially available open-tip catheters which has a rather

large diameter and which are provided with a stiff introducer, membranes are easily ruptured when the

catheter is placed between the membranes and the uterine wall. This can be avoided by using a catheter

with a small diameter (outer diameter 0.9 mm) through a more flexible introducer with an outer di-

ameter of 3 mm (De Haan, Braaksma, Janssens and Van der Weide, 1971a). At present, the most widely

used method is the transcervically introduced catheter in case of ruptured membranes. The stiff intro- ducers, however, offer the risk of perforating the

uterine wall (Haverkamp and Bowes, 1971). This

technique yields quantitative results concerning all parameters of uterine activity: basal pressure, dura-

tion and maximum of the uterine contraction. With

this technique uterine activity can be expressed quan-

titatively in Montevideo Units (Caldeyro-Barcia and

Poseiro, 1959) or in Alexandria-Units (Sahwi, Gaafar

and Toppozada, 1967). External tocodynamometry is used especially to-

gether with fetal phonocardiography and ultrasonic

Doppler signal. It is assumed that the displacement of

the abdominal wall during uterine contractions gives

information about the uterine activity. The displace-

ment is dependent, however, upon the localization of the transducer on the abdominal wall (Reynolds, Heard, Bruns and Hellman, 1948) and moreover, the results of this method are strongly dependent upon the weight and the construction of the transducer (Smyth, 1957). Thus, the correlation between the

two methods is very poor (La Croix, 1968; Kolene, 1970) and the external tocodynamometry only provides information about the frequency of the contractions and no quantitative information about the basal

pressure, or exact information about the duration of

the contraction. This means that the uterine activity

cannot be expressed in quantitative units. This meth-

od can be used, however, in pregnancy and during

labor, in case of intact membranes; the fetal move-

ments can also be recorded (Hammacher, 1972).

Signal processing

Several parameters of the fetal heart action can be

obtained from the three different kinds of signals: the

shape of the fetal ECG complex, the fetal heart rate

(FHR), the detection of arrhythmia, the fetal heart sounds, the correlation between the electrical and

dynamical activity, and the analysis of the ultrasonic Doppler signal.

The maximum amplitude of the fetal R-wave re-

corded during pregnancy with abdominal leads is about lo-50 microvolts. Between the 25th and 35th

weeks of pregnancy, this amplitude is much smaller than before or after this period (Larks, Faust, Longo

and Anderson, 1960; Bolte, Bachmann and Kuhn,

1966). This is probably due to several factors: the development and the disappearing of the vernix layer, the growth of the fetal heart, and the fetal fat layer

formation (Caughey and Krohn, 1963). The maximum amplitude of the fetal R-wave in a direct lead

(e.g. with a scalp electrode) can come to several hun-

dreds of microvolts.

The shape of the fetal ECG-complex is not recog

nizable in the abdominal lead (Fig. 1). The waveform

can be estimated by averaging a certain number of

noisy complexes (Fig. 3) (Hon and Lee, 1964; Van Bemmel en Van der Weide, 1966; Favret and Mar-

chetti, 1966; Cox and Medgyesi-Mitschang, 1969;

Rhyne, 1968; Behrer, Glaeser, Cox and Woolf, 1968;

De Haan, Van Bemmel, Van der Weide, Versteeg and

Hellema, 1970b; Polvani, Brambati and Pardi, 1971). A correction must be made for the presence of the maternal ECG, as it gives an unneglectable contribu- tion in the resulting waveform (Favret and Marchetti, 1966; Van Bemmel, Peeters and Hengeveld, 1968).

In case of a direct lead and a high signal to noise ratio, the fetal ECG complex can be recognized com- pletely (Fig. 2). Naturally, averaging is also possible.

The fetal ECG, phonocardiogram and the ultrasonic Doppler signal can be used for the computation

J. de Haan, Fetal monitoring by physical methods

t1

L 500m/sec ,

Fig. 3 Averaged fetal electrocardiograms. Abdominal leads

For each complex 120 samples have been used.

61

of the fetal heart rate (FHR). The fetal heart frequency is calculated by a rate meter which determines the

interval between two consecutive heart beats, the so-

called instantaneous FHR. This, in effect, means that

for each individual interval t there is one plotting

point. The reciprocal value 4, multiplied by 60 gives

the frequency per minute. The time between two

consecutive heart beats can only be determined accurately if the signal originating from the fetal heart

action is sharply defined and the signal to noise ratio

is high enough. The electric signal fulfils these two conditions the best. Although the maternal complex

is present in the abdominal lead, using the properties of the fetal ECG (frequency contents, amplitude and repetition rate) this technique can also be used for

calculating the FHR (Fig. 4) (Van Bemmel and Van der Weide, 1966).

In view of the nature of the heart sounds, which have a burst-like pattern, their occurrence is much less sharply defined than that of the electric signal,

which has a stochastic character. This results in a so-

called trigger inaccuracy, since the onset of the signal cannot be sharply defined, which, in turn, is reflected

in the recording, by wrong measurements presented

around the real frequency in case of a real beat-to-

beat recording method (Fig. 5) (Hammacher, 1971).

This disadvantage applies even more for the Doppler signal since the received signals have a strongly

stochastic character.

Fig. 4 Simultaneous recording of the fetal heart rate pattern (lower graph) and the fetal electrocardiogram (upper graph). Abdominal leads; 24th week of pregnancy.


Fig. 5 Simultaneous recording of intrauterine pressure, fetal heart rate pattern (silent pattern) and fetal electro*udiogram. Trig-

ger inaccuracy results in wrong measurements around the real frequency.

Fig. 6 Simultaneous recording of intrauterine pressure, fetal heart rate pattern and fetal electrocardiogram. Arrhythmia.


Fig. 7 Recording of maternal and fetal electrocardiograms by abdominal leads. Fetal arrhythmia.

The trigger-inaccuracy and the wrong measure-

ments can be avoided by averaging a certain number

of intervals. Such averaging, however, masks many of

the FHR changes which might indicate early fetal

compromise. With most of the commercially available monitor-

ing equipment, display of the unfiltered ECG com-

plex is not possible, so the origin of the arrhythmia

cannot be detected. For calculating the FHR, almost

any system makes use of the repetitive properties of

the heart action. A certain interval is computed once

and then stored into a memory for comparison with the consecutive interval. The latter is only accepted as

reliable if it does not differ more than a certain per- centage from the stored interval. Thus, arrhythmia is

never recorded in the fetal heart rate pattern, unless the equipment would be available with the possibility

to switch off the memory (Fig. 6). Fetal cardiac arrhythmia recorded by means of abdominal leads,

without the full ECG complex available (Fig. 7), can

be analysed by automatic processing methods using

the so-called second order interval distribution (De

Haan et al. 1970b; De Haan, Van Bemmel, Stolte,

Janssens, Eskes, Versteeg and Veth, 1971b). Murmers

in the fetal phonocardiogram can only be detected

when the response characteristics of the amplifying,

filtering and recording system are suitable for detect-

ing the different frequency components (Eskes, 1961). Correlation between the fetal phonocardio-

gram and the fetal electrocardiogram (Fig. 8) provides

data concerning the dynamics of fetal cardiac activity

(Persioninov, Ilyin, Karpman and Savelieva, 1966; Peeters, 1968).

Analysis of the received Doppler signal may, in future, provide more details about the mechanism of the fetal heart action itself (movements of the heart valves, heart muscle, blood velocity), but until now it

has hardly proved useful clinically (Wladimiroff, 1971; Abelson and Balin, 1972).

Since continuous fetal monitoring mostly concerns

the recording of the fetal ECG complex or the fetal

heart rate pattern using the electrical signal, the fetal

t 1 set

1

Fig. 8 Simultaneous recording of fetal phonocardiogram, maternal and fetal electrocardiograms. Abdominal leads.


heart sound or the ultrasonic Doppler signal, we will,

in a further paper, only discuss the significance of the fetal ECG complex and the fetal heart rate pattern in

assessing the fetal condition.

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Fetal monitoring by physical methods I.Technical questions