FETUS, PLACENTA, AND NEWBORN
Instantaneous fetal heart rate monitoring by electromagnetic methods K.
M.S. M.D. PH.D. M.S.
R. LUKANDER, P. M;iKIPz%A, HeDinki,
Fetal magnetocardiography (FMCG) is a new complementary external method for accurate antepartul FHR recording. Because of the low magnetic noise level required it is not \let suitable for routine hospital use. Fifty-six siwlultaneous FMCG and external FECG measurements were made in order to compare these methods for instantaneous FHR recording. The error of the FHR value obtained with the instrumentation described is less than 1 per cent. Our results show that FMCG can be used for FHR processing from week 30 of gestation until term. Thirty-jive of the measurements were done during this period. From these, a readable FHR curue was obtained in 21 cases with FMCG and in 12 cases with external FECG. The maternal complexes were always present in the FECG, but in only 15 of the recorded FMCG’s.
during labor the QRS complexes of the fetal electrocardiogram (FECG) are picked up by an electrode attached to the presenting part of the fetus. Signal-tonoise ratio in the direct FECG is good enough for accurate continuous FHR monitoring. So far, external FHR monitoring is much more problematic than the direct method, yet most authors in this field find estimation of the beat-to-beat variation indispensable also during pregnancy before the actual labor begins.’ Phonocardiography (PCG) and sonocardiography (SCG, US, or ultrascan) are at present the most widely used methods for external FHR monitoring. Unfortunately, the signals obtained in these methods are heterogenetic and in FHR processing the trigger error too often exceeds one of the parameters to be measured, the FHR variability. External FECG is another recently used method for
fetal heart rate (FHR) monitoring during pregnancy and labor is routine procedure today in most well-established obstetric units. A recent report estimated that direct monitoring of labor very probably saves a vast sum in public funds by considerably reducing intrapartum fetal brain damage and subsequent mental retardation.’ In the direct method used CONTINUOUS
From the Department of Technical Physics, Helsinki University of Technology, and the First Department of Obstetrics and Gyrwcology, University Central Hospital. Supported by The Academy Jahnsson Foundation. Received for publication Accepted Janus?
and the Yj;
Reprint requests: Veikko Kariniemi, M.D., Department Obstetrics and Gynecology, University Central Hospital, Hanrtmanink. 2, HeOinki 29, Finland.
Hukkinen et al.
OISPLM AND PLOTTER
Fig. 1. The arrangement for electromagnetic FHR measurement. The magnetic signal and the electric signal can be obtained separately or simultaneously. Normally the signals are recorded on tape and processed separately later by the QRS detector and the FHR processor. The instantaneous FHR is displayed numerically and plotted on paper. FHR can also be obtained on-line without signal recording.
Fig. 2. Simultaneously The
recorded signals complexes are denoted
is possible is
if the the
estimation signal signals
is obtained. are
of the FMCG and abdominal F and the maternal M.
between gestation weeks 27 and 34 .3 Our group has previously described a new method for external fetal monitoring, fetal magnetocardiography (FMCG), in which the fetal QRS complexes are detected from above the mother’s abdomen with a superconducting magnetometer.4 Although this technique has its disadvantages, the need for magnetic shielding or an environment with low magnetic noise, we found it important to examine whether it is suitable for use in external fetal monitoring. We present here a FHR processor suitable for use with both the external FECG and the FMCG. We also compare the use of these two methods for instantaneous FHR processing. Method The block diagram for electromagnetic FHR recording is presented in Fig 1. The amplified signals can be analyzed on-line or recorded on magnetic tape and analyzed later. The QRS detector detects the fetal QRS complexes and generates a detection pulse when a complex is found. The output of the detector is further analyzed by the FHR processor which also calculates the instantaneous FHR for the plotter and display. FMCG instrumentation. A gradiometer type magnetometer at liquid helium temperature is used for FMCG measurements.5 The gradiometer is placed just above the mother’s abdomen, close to the fetal heart, without any physical contact with the mother. The magnetic measurement is done in an environment with low magnetic noise without any shielding. It is impor-
a fetus of 37 weeks.
tant that the mother does not have any ferromagnetic materials in her clothing (buttons, zippers etc.). The electronic equipment required for the magnetometer is placed nearby and the rest of the electronics further away to avoid magnetic disturbance. A typical noise level of the magnetic detector using a commercial electronic unit is 0.02 pT/%‘?% r.m.s. in the 0.05 to 100 Hz measurement band width. Power line disturbances are eliminated by using a 50 Hz band-reject filter. FECG instrumentation. The FECG is obtained with four disposable skin electrodes, of which one active electrode is placed just above the symphysis pubis and another on the fundus uteri. One common mode electrode is placed on each flank near the anterior ileal spine. Dead cells are rubbed from the skin and the skin is washed with acetone before atuching the electrodes. Twenty types of electrodes were tested for noise: 18 of them were almost equal and were found to be suitable for FECG measurements. The other two had a considerably higher noise level. The noise of the preamplifier is 6 PV peak-to-peak in the frequent) band 3 to 100 Hz. A 50 Hz band-reject filter is akcJ required for the FECG measurements. The QRS detector. The instantaneous FHR is evaluated from two successive fetal QRS complexes detected by the QRS detector.6 Because the shapes of the complexes both in the magnetic and the electric signal are almost identical both signals can be analyzed by the same instrument. Signals from a simultaneous magnetic and electric recording are shown in Fig. 2. The letters F and M denote the fetal and mateynal complexes, respectively. It can be seen that the disturbing effect of I he maternal complexes ir grratcl
FETAL -DETECTION PULSES *
Fig. 3. The QRS detector. The fetal and maternal QRS complexes are examined by comparators B and C. The reference voltages of these comparators are automatically controlled by the logic, which also generates the detection pulses and blocks out pulses due to maternal complexes detected by comparator M.
T COMPARISON OF INTERVALS
DIGITAL OUTPUT ,
Fig. 5. Correlation between the mean values of the magnetic QRS complexes and the gestation weeks. Bolte’9 diagram for the electric QRS complexes is presented for reference. Fig. 4. The FHR processor. The variations between the intervals of successive detection pulses are compared with the manually selected limits. The accepted intervals are converted to beats per minute for analogue and digital output. The blocks
are not shown
readable pottern by
readable FHR pattern by external FECG
in the figure.
in the electric signal. Because of this it is possible to detect all of the fetal complexes only from the magnetic signal. That is why the quality of the FHR curve from the magnetic signal is better than the one obtained from the electric signal. QRS detection is done by examining the polarity, the and negative duration. and both the positive amplitudes of the possible complexes. Only if all of the conditions set for the fetal QRS complex are fulfilled will a detection pulse be generated. The timing of this pulse is made accurate by separate peak detection. The maternal complexes are detected separately by a method that is based on their greater amplitude in order to block out detection pulses due to maternal complexes. This is usually not necessary when a magnetic signal is analyzed. The block diagram of the QRS detector is shown in Fig. 3. The signal is first filtered to pass only the main frequency contents of the fetal QRS complex. The
Fig. 6. The results from simultaneous FECG and FMCG measurements. An FHR pattern was obtained in 22 cases out of 35.
positive and negative amplitudes are examined by comparators B and C. The peak of the complex is detected from the derivative of the signal by comparator A. If maternai complexes are present in the signal they are detected by comparator M. The detection pulse is generated from the outputs of comparators A, B, C, and M by a logic circuit. The condition for the approval of a complex is that a pulse from comparator C must be followed by a pulse from
Fig. 7. FHR FMCG)
patterns obtained from the signals presented has more spots than the lower (from the FECG).
in Fig. 2. The
premature beats (upper trace). F,, 1O, 22, 2, are premature. and the following intervals are compensatory pauses. for comparison. Gestation week 33.
1 10 nG
Fig. 8. A FMCG
signal with multiple intervals preceding them are short simultaneous maternal ECG is shown
when the derivative of the signal changes sign. The FHR processor. The FHR processor compares the intervals between detection pulses and accepts the intervals that are probably due to correctly detected fetal QRS complexes. The processor also converts these intervals to beats per minute (b.p.m.). The block diagram of the processor is shown in Fig. 4. An interval is accepted only if the corresponding heart rate differs from the previous FHR value by less than +- K b.p.m. K is chosen by the operator: 10, 20, or “no limits.” The correct position for this switch depends upon the type of signal (FMCG/FECG), the signal-to-noise ratio, and the amplitudes of the maternal complexes (if any). An accepted interval starts the digital counting of the heart rate, the result of which is converted to analogue form for the plotter. A point plotter is most suitable for the plotting of FHR patterns because it enables easy examination of the beat-to-beat variation. The interval that is to be converted must be between 0.25 and 2 seconds, which corresponds to FHR values from 240 to 30 b.p.m. There is also a pulse
digital display of the heart rate, which is turned off if a detection pulse is not received within two seconds. The error in the FHR value obtained is due partly to the error in the timing of the detection pulse and partly to the error in l/T-conversion. Together, these introduce a maximum error range of from - 1 to +0.6 pel cent at full scale. Patients Fifty-six simultaneous FMCG and external FECG recordings were made with the instrumentation described. The gestation weeks ranged from 20 to 43, The status of most of the patients was normal, but there were four cases of suspected placental insufficiency. No attempt is made here to draw clinical conclusions from the available data. Results QRS amplitude of the FMCG. The mean values of the magnetic QRS complexes are presented in Fig. 5 as a function of gestation. The magnetic complexes began to be detectable at about 30 weeks, the time at which the electric complexes were decreasing. No
decrease of the magnetic signal was seen until after 40 weeks. FHR obtained from the FMCG and FECG. As stated above, the amplitude of the magnetic QRS complexes is high enough for FHR processing from week 30 on. Thirty-five of the investigated pregnancies belonged to this period. FHR recording succeeded in 12 cases with external FECG and in 21 cases with FMCG, as can be seen in Fig. 6. There were 10 cases where FECG failed but FMCG gave a readable result. In one case a readable pattern was obtained with FECG but not with FMCG. The maternal complexes interfered with the FHR processing in 15 cases out of 35 using FMCG and in all the cases when FECG was used. Fig. 2 is a simultaneous recording of the abdominal FECG and FMCG. The signal-to-noise ratio in the upper (magnetic) trace is obviously better than in the lower (electric) signal. In the FECG the tall maternal peaks are disturbing. The FHR patterns plotted by the equipment are presented in Fig. 7. The quality of the FHR curve obtained from the FMCG is better than the one obtained from the FECG because there are more results or spots on the former. In the latter, about 20 per cent of the intervals have been lost because of the coincidental fetal and maternal complexes. Arrhythmias are known to be problematic for conventional cardiotocographs where sequential variations exceeding a preset value are neglected by logic. This instrument permits manual selection of a “no limits” position. where all intervals between 0.25 and 2 seconds are accepted. Fig. 8 is an FMCG in which there are four premature beats causing a short interval followed by a compensatory pause. The FHR processor is capable of plotting a FHR trace from this FMCG, as shown in Fig. 9. A premature beat causes two successive spots on the FHR curve, one above the normal values, the other below them. Some of these FHR values are about 210 b.p.m., a commonly used upper limit for the commercial cardiotocograph. In Fig. 10 there is another example of an FHR curve with multiple premature beats which disappear during accelerations.
9. FHR pattern obtained from the FMCG presented in Fig. 8. Each premature beat causes a spot above the normal value and a successive spot below the normal value. FHR values below 90 b.p.m. are caused by undetected QRS complexes. Fig.
Fig. 10. FHR pattern obtained from the FMCG of the same patient as in Fig. 9, but 2 weeks later (gestation week 35). The premature beats are seen, but they disappear during accelerations.
Comment Use of the abdominal FECG for antepartum FHR monitoring is seriously limited by the attenuation of the QRS complexes at the beginning of the last trimester. The results of our external FECG measurements confirm this observation. Our experience suggests that FMCG is a complementary method for external FHR recording, mainly because it is the only additional method of comparable
accuracy. With the present instrumentation the magnetic QRS complexes become detectable at the same time that the electric complexes disappear or diminish in size. Whether this phenomenon is a mere coincidence or dependent on a common cause is not yet clear. If attenuation of the electrical QRS complexes is caused by increasing vernix caseosa on the fetal skin, this insulating effect might cause a change in the current
distribution and an amplification of the magnetic fields. The amount of vernix decreases again after term, and in our diagram the size of the magnetic QRS complexes seems to diminish again after 40 weeks of gestation. When FMCG is compared with external FECG as a means of recording FHR with simultaneous measurements and both are successful, the FHR results agree with a difference no more than 1 b.p.m. Because a heart rate variation of less than 3 per cent is already considered pathologic, the total error of this equipment does not interfere seriously with the estimation of this variation.
Poor signal-to-noise ratio is still a problem in both FMCG and external FECG. Not much improvement i, expected in the electric method. However, the signalto-noise ratio of the magnetic equipment tan be considerably improved and such an instrument has alreadv been constructed.’ Thus FMCG 1~1’1 become a useful screening method in antepartum FHR monitoring, but the problem of tnagnetic noise must hc solved before it is usable in a hospital.
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