Antenatal fetal magnetocardiography: a new method for fetal surveillance?

The magnetocardiogram

The magnetic field produced by the heart's electrical activity is called the magnetocardiogram (MCG). The first 20 years of MCG research established most of the concepts, instrumentation, and computational algorithms in the field. Additional insights into fundamental mechanisms of biomagnetism were gained by studying isolated hearts or even isolated pieces of cardiac tissue. Much effort has gone into calculating the MCG using computer models, including solving the inverse problem of deducing the bioelectric sources from biomagnetic measurements. Recently, most magnetocardiographic research has focused on clinical applications, driven in part by new technologies to measure weak biomagnetic fields.

Exploring the Influence of Fetal Sex on Heart Rate Dynamics Using Fetal Magnetocardiographic Recordings

Fetal sex has been associated with different development trajectories that cause structural and functional differences between the sexes throughout gestation. Fetal magnetocardiography (fMCG) recordings from 123 participants (64 females and 59 males; one recording/participant) from a database consisting of low-risk pregnant women were analyzed to explore and compare fetal development trajectories of both sexes. The gestational age of the recordings ranged from 28 to 38 weeks. Linear metrics in both the time and frequency domains were applied to study fetal heart rate variability (fHRV) measures that reveal the dynamics of short- and long-term variability. Rates of linear change with GA in these metrics were analyzed using general linear model regressions with assessments for significantly different variances and GA regression slopes between the sexes. The fetal sexes were well balanced for GA and sleep state. None of the fHRV measures analyzed exhibited significant variance heterogeneity between the sexes, and none of them exhibited a significant sex-by-GA interaction. The absence of a statistically significant sex-by-GA interaction on all parameters resulted in none of the regression slope estimates being significantly different between the sexes. With high-precision fMCG recordings, we were able to explore the variation in fHRV parameters as it relates to fetal sex. The fMCG-based fHRV parameters did not show any significant difference in rates of change with gestational age between sexes. This study provides a framework for understanding normal development of the fetal autonomic nervous system, especially in the context of fetal sex.

Recording and quantifying fetal magnetocardiography signals using a flexible array of optically-pumped magnetometers

OBJECTIVE

Fetal magnetocardiography (fMCG) is a non-invasive biomagnetic technique that provides detailed beat-to-beat fetal heart rate analysis, both in normal rhythm as well as in fetal arrhythmias. New cryogenic-free sensors called optically pumped magnetometers (OPMs) have emerged as a less expensive and more geometrically flexible alternative to traditional Superconducting Quantum Interference Device (SQUID) technology for performing fMCG. The objective of the study was to show the ability of OPMs to record fMCG using flexible geometry while seeking to preserve signal quality, and to quantify fetal heart rate variability (FHRV).

APPROACH

Biomagnetic measurements were performed with OPMs in 24 healthy pregnant women with uncomplicated singleton pregnancies between 28 and 38 weeks gestation (GA). A total of 96 recordings were analyzed from OPM data that was collected using sensors placed in two different maternal configurations over the abdomen. The fMCG signals were extracted and the quality of the recordings were quantified by peak amplitudes and signal-to-noise ratio (SNR). R peaks were used to perform both time and frequency domain FHRV analysis. FHRV measures obtained from OPMs were compared descriptively to the same measures obtained from GA-matched existing SQUID data.

MAIN RESULTS

The fMCG derived from OPMs were observed in 21 of the 24 participants. Higher detection rates (85%) of fMCG signals were observed in the data sets recorded at GA >32 weeks. Peak amplitudes and SNR values were similar between two maternal configurations, but peak amplitudes were significantly higher (p = 0.013) in late GA compared to early GA. FHRV indicators were successfully extracted and their values overlapped substantially with those obtained from SQUID recordings.

SIGNIFICANCE

Taking advantage of the geometric flexibility of the OPMs, we have demonstrated their ability to record and quantify fMCG in different maternal positions as opposed to rigid SQUID configurations.

Dynamics of the use of magnetocardiography in the study of the cardiac conduction system of the chick embryo

INTRODUCTION

Human fetal magnetocardiography (fMCG) has been done for several decades to evaluate fetal arrhythmias using a superconducting quantum interference device (SQUID) magnetometer, but there is little work in embryonic/fetal animal models. This study uses an optically-pumped magnetometer (OPM) to obtain an fMCG in the chick embryo.

METHODS

White Leghorn chick embryos were examined from incubation Day #10-19. Different examination chambers were tested to optimize embryonic thermal stability and magnetic signal acquisition. All examinations were done with magnetic shielding. The OPM sensors were placed next to the egg shell. The embryo's position was localized by transilluminating the intact egg or ultrasound imaging the egg with an open air cell to optimize sensor placement. The raw data for each embryo was postprocessed to obtain a fMCG composite waveform.

RESULTS

fMCG's were obtained in embryos from Day #12 to 19. The best success with intact eggs was obtained using five sensors; one at the bottom and four around the lower perimeter of the egg at 90° intervals with the egg oriented vertically and the air cell up. Using ultrasound imaging with the air cell open only two sensors were necessary, one at the bottom and one laterally next to the embryo. fMCGs were analyzed for heart rate and rhythm, each portion of the PQRST waveform, and the PR interval, QRS complex, RR interval, and QT interval.

CONCLUSIONS

This study validates the chick embryo as an animal model to study in a longitudinal and noninvasive fashion the fetal cardiac conduction system by using OPM magnetocardiography.

Fetal cardiac time intervals in healthy pregnancies - an observational study by fetal ECG (Monica Healthcare System)

BACKGROUND

Fetal electrocardiogram (fECG) can detect QRS signals in fetuses from as early as 17 weeks' gestation; however, the technique is limited by the minute size of the fetal signal relative to noise ratio. The aim of this study was to evaluate precise fetal cardiac time intervals (fCTIs) with the help of a newly developed fetal ECG device (Monica Healthcare System).

METHODS

In a prospective manner we included 15-18 healthy fetuses per gestational week from 32 weeks onwards. The small and wearable Monica AN24 monitoring system uses standard ECG electrodes placed on the maternal abdomen to monitor fECG, maternal ECG and uterine electromyogram (EMG). Fetal CTIs were estimated on 1000 averaged fetal heart beats. Detection was deemed successful if there was a global signal loss of less than 30% and an analysis loss of the Monica AN24 signal separation analysis of less than 50%. Fetal CTIs were determined visually by three independent measurements.

RESULTS

A total of 149 fECGs were performed. After applying the requirements 117 fECGs remained for CTI analysis. While the onset and termination of P-wave and QRS-complex could be easily identified in most ECG patterns (97% for P-wave, PQ and PR interval and 100% for QRS-complex), the T-wave was detectable in only 41% of the datasets. The CTI results were comparable to other available methods such as fetal magnetocardiography (fMCG).

CONCLUSIONS

Although limited and preclinical in its use, fECG (Monica Healthcare System) could be an additional useful tool to detect precise fCTIs from 32 weeks' gestational age onwards.

A Comparison of Five Algorithms for Fetal Magnetocardiography Signal Extraction

Fetal magnetocardiography (fMCG) provides accurate and reliable measurements of electrophysiological events in the fetal heart and is capable of studying fetuses with congenital heart diseases. A variety of techniques exist to extract the fMCG signal with the demand for non-invasively obtained fetal cardiac information. To the best of our knowledge, there is no comparative study published in the field as to how the various extraction algorithms perform. We perform a comparative study of the ability of five methods to extract the fMCG using real biomagnetic signals, two of those methods are applied to real fMCG data for the first time. Biomagnetic signals were recorded and processed with each of the five methods to obtain fMCG. The R peaks of the fMCG traces were obtained via a peak-detection algorithm. From whole recording for each method, the fetal heart rate (FHR) was calculated and used to perform FHR variability (FHRV) analysis. Additionally, we calculated durations from the PQRST complex from time-averaged data during sinus rhythm. The five methods recovered the fMCG signals, but two of them were able to extract cleaner fMCG and the morphology was observed from the continuous data. The time-averaged data showed very similar morphologies between methods, but two of them displayed a signal amplitude reduction on the R-waves and T-waves. Values of PQRST durations, FHR and FHRV were in the range of previous fetal cardiac studies. We have compared five methods for fMCG extraction and showed their ability to perform fMCG analysis.

Cardiac time intervals derived by magnetocardiography in fetuses exposed to pregnancy hypertension syndromes

OBJECTIVE

To test the hypothesis that fetuses exposed to maternal preeclampsia or chronic hypertension have deranged development of cardiac time intervals.

STUDY DESIGN

Pregnancies were divided into three groups: Intrauterine Growth Restricted (IUGR), Hypertensive, and Normal. Each group's mean fetal cardiac time intervals (P, PR, QRS and RR) derived by magnetocardiography were calculated using an analysis of covariance model's regression-adjusted estimates for a gestational age of 35 weeks.

RESULTS

We reviewed 141 recordings from 21 IUGR, 46 Hypertensive and 74 Normal patients. The IUGR, Hypertensive and Normal groups, respectively, had adjusted mean intervals in milliseconds of 66.4, 66.8 and 76.2 for P (P=0.001), 95.9, 101.6 and 109.6 for PR (P=0.002), 77.2, 78.7 and 78.7 for QRS (P=0.81) and 429.8, 429.2 and 428.5 for RR (P=0.97).

CONCLUSION

P and PR intervals are abbreviated in normotrophic fetuses exposed to maternal hypertension, suggesting shortened atrioventricular conduction times.

Fetal magnetocardiography measurements with an array of microfabricated optically pumped magnetometers

Following the rapid progress in the development of optically pumped magnetometer (OPM) technology for the measurement of magnetic fields in the femtotesla range, a successful assembly of individual sensors into an array of nearly identical sensors is within reach. Here, 25 microfabricated OPMs with footprints of 1 cm(3) were assembled into a conformal array. The individual sensors were inserted into three flexible belt-shaped holders and connected to their respective light sources and electronics, which reside outside a magnetically shielded room, through long optical and electrical cables. With this setup the fetal magnetocardiogram of a pregnant woman was measured by placing two sensor belts over her abdomen and one belt over her chest. The fetal magnetocardiogram recorded over the abdomen is usually dominated by contributions from the maternal magnetocardiogram, since the maternal heart generates a much stronger signal than the fetal heart. Therefore, signal processing methods have to be applied to obtain the pure fetal magnetocardiogram: orthogonal projection and independent component analysis. The resulting spatial distributions of fetal cardiac activity are in good agreement with each other. In a further exemplary step, the fetal heart rate was extracted from the fetal magnetocardiogram. Its variability suggests fetal activity. We conclude that microfabricated optically pumped magnetometers operating at room temperature are capable of complementing or in the future even replacing superconducting sensors for fetal magnetocardiography measurements.

Feasibility of noninvasive fetal electrocardiographic monitoring in a clinical setting

Cardiac rhythm is an essential component of fetal cardiac evaluation. The Monica AN24 is a fetal heart rate monitor that may provide a quick, inexpensive modality for obtaining a noninvasive fetal electrocardiogram (fECG) in a clinical setting. The fECG device has the ability to acquire fECG signals and allow calculation of fetal cardiac time intervals between 16- and 42-week gestational age (GA). We aimed to demonstrate the feasibility of fECG acquisition in a busy fetal cardiology clinic using the Monica fetal heart rate monitor. This is a prospective observational pilot study of fECG acquired from fetuses referred for fetal echocardiography. Recordings were performed for 5-15 min. Maternal signals were attenuated and fECG averaged. fECG and fetal cardiac time intervals (PR, QRS, RR, and QT) were evaluated by two cardiologists independently and inter-observer reliability was assessed using intraclass coefficient (ICC). Sixty fECGs were collected from 50 mothers (mean GA 28.1 ± 6.1). Adequate signal-averaged waveforms were obtained in 20 studies with 259 cardiac cycles. Waveforms could not be obtained between 26 and 30 weeks. Fetal cardiac time intervals were measured and were reproducible for PR (ICC = 0.89; CI 0.77-0.94), QRS (ICC = 0.79; CI 0.51-0.91), and RR (ICC = 0.77; CI 0.53-0.88). QT ICC was poor due to suboptimal T-wave tracings. Acquisition of fECG and measurement of fetal cardiac time intervals is feasible in a clinical setting between 19- and 42-week GA, though tracings are difficult to obtain, especially between 26 and 30 weeks. There was high reliability in fetal cardiac time intervals measurements, except for QT. The device may be useful for assessing atrioventricular/intraventricular conduction in fetuses from 20 to 26 and >30 weeks. Techniques to improve signal acquisition, namely T-wave amplification, are ongoing.

Diagnosis and treatment of fetal arrhythmia

AIMS

Detection and careful stratification of fetal heart rate (FHR) is extremely important in all pregnancies. The most lethal cardiac rhythm disturbances occur during apparently normal pregnancies where FHR and rhythm are regular and within normal or low-normal ranges. These hidden depolarization and repolarization abnormalities, associated with genetic ion channelopathies cannot be detected by echocardiography, and may be responsible for up to 10% of unexplained fetal demise, prompting a need for newer and better fetal diagnostic techniques. Other manifest fetal arrhythmias such as premature beats, tachycardia, and bradycardia are commonly recognized.

METHODS

Heart rhythm diagnosis in obstetrical practice is usually made by M-mode and pulsed Doppler fetal echocardiography, but not all fetal cardiac time intervals are captured by echocardiographic methods.

RESULTS AND CONCLUSIONS

This article reviews different types of fetal arrhythmias, their presentation and treatment strategies, and gives an overview of the present and future diagnostic techniques.

Fetal magnetocardiogram recordings and Fourier spectral analysis

Power spectral analysis of fetal magnetocardiogram (FMCG) data was evaluated in 64 pregnancies, using the non-invasive one channel superconducting quantum interference device (DC-SQUID), in order to investigate the power spectral amplitude distribution in the frequency range between 2 and 3 Hz. In all cases with normal and uncomplicated pregnancies, the data from the fetal heart and specifically the QRS complexes, were identifiable and unaffected by any maternal cardiac activity and furthermore the power spectral amplitudes, which varied between 120 and 350 fT/Hz, were directly related to gestational age.

Applicability of adaptive noise cancellation to fetal heart rate detection using photoplethysmography

In this paper, an approach based on adaptive noise cancellation (ANC) is evaluated for extraction of the fetal heart rate using photoplethysmographic signals from the maternal abdomen. A simple optical model is proposed in which the maternal and fetal blood pulsations result in emulated signals where the lower SNR limit (fetal to maternal) is -25dB. It is shown that a recursive least-squares algorithm is capable of extracting the peaks of the fetal PPG from these signals, for typical values of maternal and fetal tissues.

Fetal cardiac time intervals estimated on fetal magnetocardiograms: single cycle analysis versus average beat inspection

Fetal cardiac time intervals (fCTI) are dependent on fetal growth and development, and may reveal useful information for fetuses affected by growth retardation, structural cardiac defects or long QT syndrome. Fetal cardiac signals with a signal-to-noise ratio (SNR) of at least 15 dB were retrieved from fetal magnetocardiography (fMCG) datasets with a system based on independent component analysis (ICA). An automatic method was used to detect the onset and offset of the cardiac waves on single cardiac cycles of each signal, and the fCTI were quantified for each heartbeat; long rhythm strips were used to calculate average fCTI and their variability for single fetal cardiac signals. The aim of this work was to compare the outcomes of this system with the estimates of fCTI obtained with a classical method based on the visual inspection of averaged beats. No fCTI variability can be measured from averaged beats. A total of 25 fMCG datasets (fetal age from 22 to 37 weeks) were evaluated, and 1768 cardiac cycles were used to compute fCTI. The real differences between the values obtained with a single cycle analysis and visual inspection of averaged beats were very small for all fCTI. They were comparable with signal resolution (+/-1 ms) for QRS complex and QT interval, and always <5 ms for the PR interval, ST segment and T wave. The coefficients of determination between the fCTI estimated with the two methods ranged between 0.743 and 0.917. Conversely, inter-observer differences were larger, and the related coefficients of determination ranged between 0.463 and 0.807, assessing the high performance of the automated single cycle analysis, which is also rapid and unaffected by observer-dependent bias.

Gender-related changes in magnetocardiographically determined fetal cardiac time intervals in intrauterine growth retardation

Prenatal growth deficiencies as well as gender have been associated with cardiovascular disease in later life. It is also known that the duration of fetal cardiac time intervals (CTI) are dependent on fetal development. The aim of this work was to examine the relationship between fetal CTI in healthy and intrauterine growth retardation (IUGR) fetuses, taking gender into account. A total of 269 magnetocardiograms (MCG) were obtained in 47 healthy and 27 IUGR pregnancies. In each signal-averaged MCG, durations of CTI were determined. Age- and heart rate-corrected values were compared between normal and IUGR fetuses separately with respect to gender. Overall, there was an association between atrial and ventricular conduction times and estimated fetal body weight. In female fetuses, IUGR was associated with shorter P WAVE, PQ segment, PR interval, and QRS complex and longer STT and QT intervals. For males, this was so only for P wave, QRS complex, and STT interval. The shortening of conduction times in IUGR may be explained by reduced cardiac muscle mass associated with lower body weight. On the other hand, the gender-specific differences, particularly in the IUGR fetuses may be due to hormonal factors.

Differentiation of myomas by means of biomagnetic and doppler findings

Aim

To elucidate the hemodynamics of the uterine artery myomas by use of Doppler ultrasound and biomagnetic measurements.

Method

Twenty-four women were included in the study. Sixteen of them were characterised with large myomas whereas 8 of them with small ones. Biomagnetic signals of uterine arteries myomas were recorded and analyzed with Fourier analysis. The biomagnetic signals were distributed according to spectral amplitudes as high (140–300 ft/√Hz), low (50–110 ft/√Hz) and borderline (111–139 ft/√Hz). Uterine artery waveform measurements were evaluated by use of Pulsatility Index (PI) (normal value PI < 1.45).

Results

There was a statistically significant difference between large and small myomas concerning the waveform amplitudes (P < 0.0005) and the PI index (P < 0.0005). Specifically, we noticed high biomagnetic amplitudes in most large myomas (93.75 %) and low biomagnetic amplitudes in most small ones (87.5 %).

Conclusion

It is suggested that the biomagnetic recordings of uterine artery myomas could be a valuable modality in the estimation of the circulation of blood cells justifying the findings of Doppler velocimetry examination.

Cardiac time intervals of normal fetuses using noninvasive fetal electrocardiography

OBJECTIVES

To evaluate the fetal cardiac time intervals from the longitudinal analysis of noninvasive fetal electrocardiography (fECG) in normal pregnancies.

METHODS

One hundred singleton pregnancies were examined in this longitudinal study. Cardiac time intervals were derived from fetal electrocardiograms obtained noninvasively using three electrodes placed on the maternal abdomen. The variables measured included the durations of the P wave, PR interval, QRS complex, QT interval and T wave.

RESULTS

Success rates for detecting the P, QRS and T waves were 74.6, 91.0 and 79.3%, respectively. Cardiac time intervals were significantly influenced by fetal age. The mean P-wave duration increased from 43.9 (18--22 weeks) to 52.9 ms (>/=37 weeks) (p < 0.001). PR intervals were 102.1 and 110.1 ms, for fetuses at 18 to 22 and >/=37 weeks (p < 0.001), respectively. QRS intervals were 47.2 and 52.6 ms (p < 0.001), while QT intervals were 224.0 and 242.7 ms (p < 0.001), at 18 to 22 and >/=37 weeks respectively. From 18 to 22 weeks to >/=37 weeks, QT(c) values increased from 343.8 to 367.7 ms (p < 0.001), while T-wave durations increased from 123.8 to 152.4 ms (p < 0.001).

CONCLUSIONS

Serial noninvasive fECG of normal fetuses from 18 to 41 weeks of gestation show good success rates of fECG detection. Cardiac time intervals generally increased with increasing gestational age.

Correlation between biomagnetic and Doppler findings of the uterine artery in normal and preeclamptic pregnancies

OBJECTIVES

To elucidate the hemodynamics of the uterine artery in normal and abnormal pregnancies by use of Doppler ultrasound and SQUID biomagnetometry.

METHOD

Two hundred and three women (gestational age 28-42 weeks) were included in the study. Forty-three of them had preeclampsia and/or intrauterine growth restriction and 160 were normal. Uterine artery waveform measurements were evaluated by use of pulsatility index (PI) (normal value PI<1.45). Biomagnetic signals of uterine arteries were recorded and analyzed with Fourier analysis. The biomagnetic signals were distributed according to spectral amplitudes as high (140-300 fT/ radicalHz), low (50-110 fT/ radicalHz) and borderline (111-139 fT/ radicalHz).

RESULTS

There was a statistically significant difference between normal and abnormal pregnancies concerning the waveform amplitudes (p < 0.001) and the PI index (p < 0.001). Specifically, we noticed high biomagnetic amplitudes in most normal pregnancies (92.5%) and low biomagnetic amplitudes in most preeclamptic cases (90.7%).

CONCLUSIONS

Our study results indicated that biomagnetic measurements of uterine artery might prove to be useful in the evaluation of the fetal well being, especially in cases of preeclampsia and intrauterine growth restriction.

Fetal magnetoencephalogram recordings and Fourier spectral analysis

This study investigates the fetal brain activity in normal and pre-eclamptic pregnancies. Measurements were performed by means of a Superconducting Quantum Interference Device (SQUID) in an electrically shielded room of low magnetic noise. The study was prospective. Eleven pregnant women with pre-eclampsia and 21 normal pregnancies were included. All were at 37-40 weeks of pregnancy. Biomagnetic signals (waveforms), recorded from the fetal brains in the frequency range of 2-7 Hz, were expressed in terms of magnetic power spectral amplitudes. These were low (mean, 376.67; SD, 28.66) in almost all normal pregnancies, and high (mean, 554.91; SD, 149.56) in most pregnancies complicated with preeclampsia. These findings were of statistical significance (Student's t -test, P < 0.005). Biomagnetic measurements of fetal brain activity is a promising screening procedure for assessing the cerebral function, especially in high risk pregnancies.

Giant Fetal Magnetocardiogram P Waves in Congenital Atrioventricular Block

Background— Cardiogram signal amplitude is a key index of hypertrophy but has not been investigated extensively in utero. In this study, magnetocardiography was used to assess P and QRS amplitude in normal subjects and subjects with fetal arrhythmia.

Methods and Results— The study cohort consisted of 68 normal fetuses and 25 with various arrhythmias: 9 reentrant supraventricular tachycardia (SVT), 2 ventricular tachycardia (VT), 2 sinus tachycardia, 2 blocked atrial bigeminy, 2 congenital second-degree atrioventricular (AV) block, and 8 congenital complete AV block. Subjects with congenital AV block, all presenting with bradycardia, showed large QRS amplitude, exceedingly large P-wave amplitude, and long P-wave duration. The 2 subjects with VT, both with poor ventricular function, also exhibited large P waves. SVT was associated with only moderate signal amplitude elevation.

Conclusions— The data imply that AV block in utero is accompanied by hypertrophy, which is more pronounced for the atria than the ventricles. We hypothesize that the hypertrophy results from a compensatory response associated with regulation of cardiac output and is likely to be observable in other arrhythmias and disease states. Magnetocardiography may be more sensitive than fetal echocardiography for detection of atrial hypertrophy in utero.

Dependency of magnetocardiographically determined fetal cardiac time intervals on gestational age, gender and postnatal biometrics in healthy pregnancies

Background

Magnetocardiography enables the precise determination of fetal cardiac time intervals (CTI) as early as the second trimester of pregnancy. It has been shown that fetal CTI change in course of gestation. The aim of this work was to investigate the dependency of fetal CTI on gestational age, gender and postnatal biometric data in a substantial sample of subjects during normal pregnancy.

Methods

A total of 230 fetal magnetocardiograms were obtained in 47 healthy fetuses between the 15^th and 42^nd week of gestation. In each recording, after subtraction of the maternal cardiac artifact and the identification of fetal beats, fetal PQRST courses were signal averaged. On the basis of therein detected wave onsets and ends, the following CTI were determined: P wave, PR interval, PQ interval, QRS complex, ST segment, T wave, QT and QTc interval. Using regression analysis, the dependency of the CTI were examined with respect to gestational age, gender and postnatal biometric data.

Results

Atrioventricular conduction and ventricular depolarization times could be determined dependably whereas the T wave was often difficult to detect. Linear and nonlinear regression analysis established strong dependency on age for the P wave and QRS complex (r² = 0.67, p < 0.001 and r² = 0.66, p < 0.001) as well as an identifiable trend for the PR and PQ intervals (r² = 0.21, p < 0.001 and r² = 0.13, p < 0.001). Gender differences were found only for the QRS complex from the 31^st week onward (p < 0.05). The influence on the P wave or QRS complex of biometric data, collected in a subgroup in whom recordings were available within 1 week of birth, did not display statistical significance.

Conclusion

We conclude that 1) from approximately the 18^th week to term, fetal CTI which quantify depolarization times can be reliably determined using magnetocardiography, 2) the P wave and QRS complex duration show a high dependency on age which to a large part reflects fetal growth and 3) fetal gender plays a role in QRS complex duration in the third trimester. Fetal development is thus in part reflected in the CTI and may be useful in the identification of intrauterine growth retardation.

Reproducibility and reliability of fetal cardiac time intervals using magnetocardiography

We investigated several factors which may affect the accuracy of fetal cardiac time intervals (CTI) determined in magnetocardiographic (MCG) recordings: observer differences, the number of available recording sites and the type of sensor used in acquisition. In 253 fetal MCG recordings, acquired using different biomagnetometer devices between the 15th and 42nd weeks of gestation, P-wave, QRS complex and T-wave onsets and ends were identified in signal averaged data sets independently by different observers. Using a defined procedure for setting signal events, interobserver reliability was high. Increasing the number of registration sites led to more accurate identification of the events. The differences in wave morphology between magnetometer and gradiometer configurations led to deviations in timing whereas the differences between low and high temperature devices seemed to be primarily due to noise. Signal-to-noise ratio played an important overall role in the accurate determination of CTI and changes in signal amplitude associated with fetal maturation may largely explain the effects of gestational age on reproducibility. As fetal CTI may be of value in the identification of pathologies such as intrauterine growth retardation or fetal cardiac hypertrophy, their reliable estimation will be enhanced by strategies which take these factors into account.

Review of electromagnetic source investigations of the fetal heart

There is at present no reliable clinical technique for the assessment of cardiac electrophysiological activity in the fetus. There are two primary requirements of this type of monitoring: (i) sequential assessment of morphological and temporal parameters of cardiac electrical activity during advancing gestation, and (ii) description of the cardiac electrical activity in terms of an electrophysiologically realistic model. Fetal electrocardiography may be performed using maternal abdominal electrodes but this is only reliable prior to the 27th week of gestation. This is primarily because of the electrically insulating effects of the vernix caseosa and the existence of preferred conduction pathways between the fetal heart and maternal abdomen after this time. Fetal magnetocardiography is largely unaffected by these factors and so enables a reliable assessment of fetal electrocardiological activity throughout the second and third trimesters of pregnancy. This method can also be used to model fetal electrophysiological activity in terms of a current dipole or magnetic dipole. The vectorcardiogram is a plot of the dynamic change in dipole parameters during the cardiac cycle, allowing the study of growth-related or pathology-related electromagnetic changes in the heart. Fetal magnetocardiography and the fetal vectorcardiogram may thus provide important additions to current methods of antenatal monitoring.

Influence of intrauterine growth restriction on cardiac time intervals evaluated by fetal magnetocardiography

OBJECTIVE

Differences in the cardiac excitation cycle between normotrophic and intrauterine growth-restricted fetuses were to be investigated by fetal magnetocardiography (fMCG).

STUDY DESIGN

In this study, the time intervals of the fMCG signal in dependence on gestational age were compared between a group of 30 growth-restricted fetuses and 60 normotrophic fetuses by using Spearman's correlation coefficient and two-way analyses of variance.

RESULTS

A significantly increasing duration of the P wave and the QRS complex could be observed with advancing gestational age in the normotrophic collective. This prolongation was not evident in the group of growth-restricted fetuses. The QRS complex showed a significant difference between both groups. In regard to the duration of the PR and the QT intervals, neither a distinct increase nor a clear difference between both groups was observable.

CONCLUSION

In contrast to the observations in the normally grown fetuses, none of the cardiac time intervals in the group of the growth-restricted fetuses were significantly correlated with gestational age. More especially, the results of the QRS complex could be an indicator of the altered conditions when intrauterine life is complicated by intrauterine growth restriction (IUGR).

Noninvasive antepartum recording of fetal S-T segment with a newly developed 151-channel magnetic sensor system

OBJECTIVE

The objective of this study was to evaluate the efficiency of the detection of the S-T segment in the fetal PQRST complex that is recorded in the antepartum period with the use of a newly developed noninvasive 151-channel magnetic sensor array.

STUDY DESIGN

One hundred two fetal magnetocardiographic recordings were performed on normal fetuses with gestational ages that ranged from 27.5 to 39.5 weeks. After the removal of the interfering maternal heart signals, the fetal heart data were inspected to detect the presence of P, QRS, and T segments.

RESULTS

The QRS complex was detectable in 100%, the P wave was detectable in 95.1%, and the T wave was detectable in 87.3% of the recordings.

CONCLUSION

Fetal magnetocardiography was recorded successfully, the QRS complex was observed in all subjects, and the T detection rate increased, with the gestational age reaching a peak of 97%. Further study of the S-T segment through the antepartum period could lead to advances in the detection of fetal jeopardy before labor.

Chaotic and periodic analysis of fetal magnetocardiogram recordings in growth restriction

OBJECTIVE

We studied how chaotic and periodic heart rate dynamics differ between normal fetuses (n = 19) and intrauterine growth restricted fetuses (n = 11) at 34 to 37 weeks of gestation. We quantified the chaotic dynamics of each heart rate time series obtained by fetal magnetocardiography (FMCG) using correlation dimension.

METHODS

The FMCG was recorded digitally by a single-channel biomagnetometer in an electrically shielded room of low magnetic noise. The position of the fetal heart was determined using ultrasonography.

RESULTS

The correlation dimension was significantly lower in IUGR than in normal fetuses (p < 0.001, t-test). The periodic dynamics were also obtained by FMCG and measured by power spectrum. The low-frequency components and therefore the periodicity of the low-frequency range were significantly higher in IUGR than in normal fetuses (p < 0.001, t-test).

CONCLUSIONS

The analysis of FMCG recordings may offer important perspectives to understand significant features of the heart function of the fetuses. This technique improves the recognition of IUGR fetuses over healthy ones and may help improve perinatal morbidity and mortality.

Correlation between biomagnetic and Doppler findings of umbilical artery in fetal growth restriction

OBJECTIVE

To assess the value of biomagnetic recordings of the umbilical artery over Doppler ultrasound screening in order to predict complications of impaired uteroplacental blood flow in fetuses with intrauterine growth restriction (IUGR).

METHODS

Our study population included 11 IUGR preeclamptic (34-37-weeks gestation; birth weight 1969 +/- 129 g) and 19 normal pregnancies (34-37-weeks gestation; birth weight 3195 +/- 229 g). Umbilical artery Doppler ultrasound waveform measurements were expressed in terms of pulsatility index (PI). Biomagnetic signals (waveforms) recorded from the IUGR umbilical artery were expressed in terms of magnetic power spectral amplitudes.

RESULTS

In all cases, the frequency band considered was 2 to 7 Hz. The spectral amplitudes were low (mean: 117+/-24 fT/ radical Hz) in most (90.9%) IUGR pregnancies and high (mean: 224+/-37 fT/ radical Hz) in most normal pregnancies (89.5%). There was a statistically significant difference between normal and IUGR pregnancies with respect to spectral amplitudes (p < 0.0001), Doppler PI (p < 0.0005), pH (p < 0.0005) and Apgar score (p < 0.0005). Multiple linear regression analysis revealed an influence of PI, pH and Apgar scores on the biomagnetic values (ANOVA: p < 0.0005).

CONCLUSION

Our data imply that biomagnetic values of the umbilical artery flow could prove to be a helpful method for the evaluation of fetal well being especially in pregnancies complicated with preeclampsia and growth restriction.

Differential diagnosis of prostate lesions with the use of biomagnetic measurements and non-linear analysis

The purpose of this study was to investigate the biomagnetic activity emitted from the prostate gland and to differentiate cancerous from benign prostate lesions with the use of biomagnetic measurement and non-linear analysis. Magnetic recordings were obtained from 47 patients with palpable prostate lesions. Histology revealed 24 prostate cancer patients and 23 benign prostate hyperplasia (BPH) patients. The superconducting quantum interference device (SQUID) biomagnetometer was used to measure the prostate's magnetic field by placing the SQUID detector 3 mm above the symphysis pubis. The magnetic field recorded in the 2-7 Hz frequency range was of high amplitude in most malignant lesions whereas all benign cases were of low amplitude. According to our results, the sensitivity, specificity, positive predictive value and negative predictive value were 83.33%, 100%, 100% and 85.18%, respectively. By applying the Grassberger-Procaccia algorithm to the magnetoprostatogram time series in malignant and benign prostate lesions we found clear saturation in malignant prostate lesions with m>7 whereas in the benign lesions there was not clear saturation. Prostate cancer emits higher biomagnetic activity than the BPH. This confirms a higher angiogenic activity in prostate cancer than the BPH lesions. Furthermore, the saturation value in the estimation of the correlation dimension of the attractor for the cancer lesions confirms the lower complexity of the system in comparison to the BPH, which is characterized by higher complexity.

Multicentre study of fetal cardiac time intervals using magnetocardiography

OBJECTIVE

A database with reference values of the durations of the various waveforms in a magnetocardiogram of fetuses in uncomplicated pregnancies is assessed. This database will be of help to discriminate between pathologic and healthy fetuses. A fetal magnetocardiogram is a recording of the magnetic field in a location near the maternal abdomen and reflects the electric activity within the fetal heart. It is a non-invasive method, which can be used with nearly 100% reliability from the 20th week of gestation onward.

DESIGN

Durations of the waveforms were assembled from averaged magnetocardiograms and statistically processed.

SETTING

Fetal magnetocardiograms were measured with different magnetocardiographs. All measurements were carried out in magnetically shielded rooms.

SAMPLE

Fetal magnetocardiograms were obtained for 582 healthy patients.

METHOD

The durations of the waveforms were extracted from fetal magnetocardiograms measured at the cooperating centres. The variables collected included the duration of the P-wave, the PR interval, the PQ interval, the QRS complex, the QT interval and the T-wave and QTc value. The results were compared with values extracted from electrocardiograms of fetuses measured via electrodes attached to the maternal abdomen, from electrocardiograms measured during labour using a scalp electrode, and from electrocardiograms recorded in newborns, that were found in the literature.

MAIN OUTCOME MEASURES

Values of the durations are given as a function of gestational age including the regression line as well as the bounds marking the 90%, 95% and 98% prediction interval.

RESULTS

The durations of the P-wave, the PR interval, the QRS complex, the QT interval and QTc value increase linearly with gestational age. The durations of the PQ interval and the T-wave are independent of fetal age.

CONCLUSION

The values found agree with those found in the literature. The scatter of the data is wide due to the variation in normal physiology, the measuring system and signal processing and the subjectivity of the researcher. However, the system can define normal ranges and may be used in diagnosis.

Fetal magnetocardiography: development of the fetal cardiac time intervals

OBJECTIVES

To analyse the physiologic development of fetal cardiac time intervals throughout gestation using fetal magnetocardiography (FMCG).

METHODS

FMCG data of 163 uncomplicated pregnancies (19th and 42nd gestational week) were analysed. Mean value, standard deviation, minimum and maximum of the duration of the P-wave, the QRS-complex, the PR and the QT-interval were plotted against gestational age.

RESULTS

QRS-complex, P wave and QT-interval showed a significant lengthening between the 20th and 42nd gestational week. The mean of the QRS complex raised from 36+/-4.7 ms (week 21-24) up to 48+/-5.2 ms (> or =37th week), (p=0.0001). The mean of the P-wave was between 47+/-5.9 ms (week 21-24) and 53+/-9.5 ms (> or =37th week), (p=0.05) and the mean of the QT-interval was 198+/-18 ms (week 21-24) and increased up to 244+/-23.9 ms (> or =37th week), (p=0.009). The PR-interval did not show a correlation with gestational age.

CONCLUSION

FMCG provides sufficient information about all parts of the fetal cardiac conduction system from the 19th gestational week on. It offers the possibility to analyse the shape and the duration of the PQRST-complex.

SQUID biomagnetometry of the uterine arteries in normal and pre-eclamptic pregnancies

AIM

This study was designed to investigate the hemodynamics of the uteroplacental circulation in normal and pre-eclamptic pregnancies using the biomagnetometer SQUID.

METHOD

Twenty-two pregnancies complicated by pre-eclampsia and 49 normal pregnancies were included in this study. All were near term. Biomagnetic signals were recorded from the uterine arteries. After statistical Fourier analysis, the findings were designated in terms of spectral amplitudes as high (140-300 fT/square root of Hz), low (50-110 fT/square root of Hz) and borderline (111-139 fT/square root of Hz).

RESULTS

The uterine artery waveforms and the corresponding spectral densities were of high amplitudes in most (89.7%) normal pregnancies and of low amplitudes in most (81.8%) pregnancies complicated by pre-eclampsia (p < 0.005). These findings were of statistical significance and were correlated with fetal heart rate (FHR) monitoring, pH, Apgar score at 1 and 5 minutes and birth weight percentiles: high amplitude cases were related with normal FHR patterns, pH > 7.25, Apgar score > 7 and birth weight > 75th percentile, while low amplitude recordings were connected with abnormal FHR patterns, pH < 7.25, Apgar score < 7, and birth weight < 10th percentile (8 cases) and < 50th percentile (10 cases).

CONCLUSION

Biomagnetic measurement of the uterine artery flow, is a promising procedure in assessing fetal health, especially in high-risk pregnancies.

Detection of cardiac hypertrophy in the fetus by approximation of the current dipole using magnetocardiography

To determine the developmental changes in the myocardial current during fetal life, and to evaluate the clinical usefulness of magnetocardiography for prenatal diagnosis of cardiac hypertrophy or enlargement, we approximated the magnitude of the one-current dipole of the fetal heart using fetal magnetocardiography (fMCG). A total of 95 fetuses with gestational age of 20-40 wk were included in this study. fMCG was recorded with a nine-channel superconducting quantum interference device system in a magnetically shielded room. The magnitude of the dipole (Q) was calculated using an equation based on the fMCG amplitude obtained on the maternal abdomen and the distance between the maternal surface and fetal heart measured ultrasonographically. In uncomplicated pregnancies, the Q value correlated significantly with gestational age, reflecting an increase in the amount of myocardial current, i.e. myocardial mass. Moreover, the Q values in fetuses with cardiomegaly caused by various cardiovascular abnormalities tended to be higher than the normal values. Although there are some limitations of the methodology based on the half-space model, and fetal orientation may influence the magnitude of the dipole, making it smaller, fMCG recorded with a multichannel superconducting quantum interference device system is a clinically useful tool for noninvasive, prenatal, and electrical evaluation of fetal cardiac hypertrophy.

The application of fetal magnetocardiography (FMCG) to investigate fetal arrhythmias and congenital heart defects (CHD)

OBJECTIVES

Fetal magnetocardiography (FMCG), a new non-invasive diagnostic tool in the analysis of the electrophysiological changes of the heart, was selectively applied in cases of fetal arrhythmias and congenital heart defect (CHD) to demonstrate its value for diagnosis and prenatal management.

METHODS

The FMCG was analysed and compared to the postnatal ECG in four cases of fetal arrhythmia [supraventricular tachycardia (two cases), complex tachy-/bradycardia (one case), ventricular extrasystoles (one case)] and a case of right heart hypoplasia diagnosed by established methods prior to investigation.

RESULTS

A Wolf-Parkinson-White (WPW) syndrome was diagnosed by its characteristic features and the appropriate transplacental therapy chosen. The types of arrhythmia could be characterised in accordance with postnatal ECG findings and irregular conduction was demonstrated in association with a CHD.

CONCLUSIONS

The use of the FMCG provides additional information to the common diagnostic tools that influence therapeutic decisions and thus contributes to optimal pre- and postnatal management.

Prenatal diagnosis of QT prolongation by magnetocardiography

Magnetocardiography constitutes a new tool for monitoring fetal cardiac activity. The fetal magnetocardiogram (FMCG) recorded noninvasively over the maternal abdomen is detectable with high temporal resolution and permits analysis of all parts of the PQRST waveform. In this way measurements of cardiac time intervals, including the QT interval, become possible. The following article constitutes the first report of antenatal detection of QT prolongation in two fetuses by FMCG.

Nonlinear analysis of biomagnetic signals recorded from the umbilical artery in normal and pre-eclamptic pregnancies

OBJECTIVE

In this study we investigated the hemodynamics of the feto-placental circulation in normal and pre-eclamptic near term pregnancies using the biomagnetometer SQUID. Thirteen abnormal and 25 normal pregnancies were included in this study.

STUDY DESIGN

The biomagnetic signals were analyzed using nonlinear analysis in order to differentiate these two types of pregnancies.

RESULTS

The application of nonlinear analysis reveal a clear saturation dimension value for pre-eclamptic and non-saturation for normal pregnancies. These findings were statistically significant and were correlated with fetal heart rate monitoring, pH and Apgar score: high biomagnetic cases (140-300 fT/square root Hz) were related with normal patterns, pH>7.25 and Apgar >7, while low biomagnetic recordings (50-110 fT/square root Hz) were connected with abnormal patterns, pH<7.25 and Apgar <7.

CONCLUSIONS

It is suggested that the biomagnetic measurements with SQUID and the application of nonlinear analysis, is a promising procedure in assessing fetal health, especially in high risk pregnancies.

Prenatal diagnosis of long QT syndrome using fetal magnetocardiography

We describe the detection of congenital long QT syndrome in a fetus at 37 weeks' gestation using magnetocardiography (MCG). The prenatal diagnosis was confirmed by standard electrocardiography (ECG) performed after birth. This is the first case report of fetal long QT syndrome detected by MCG. Fetal MCG may be useful in the prenatal diagnosis of congenital cardiac disease with abnormal ECG findings.

Noninvasive in utero assessment of PR and QRS intervals from the fetal magnetocardiogram

Fetal cardiac waveform intervals were assessed from fetal magnetocardiogram (FMCG) recordings taken from 59 pregnant women at 17-41 weeks gestation. Beyond 27 weeks' gestation PR and QRS intervals, measured from averaged waveforms, could be obtained from all subjects; however, prior to 21 weeks' gestation the success rate was 50% or less due to low signal-to-noise ratio. QT interval could not be assessed accurately in most subjects. Weak, but statistically significant, correlations with gestational age were found for PR interval (n = 145, R2 = 0.033, P = 0.028) and QRS interval (n = 145, R2 = 0.140, P < 0.0005). Abnormal waveform morphology was documented in several patients with cardiac malformations.

Fetal heart rate variability and complexity in the course of pregnancy

Aim of this study was the examination of fetal heart rate variability and complexity measures during pregnancy using fetal magnetocardiography. We registered 80 fetal magnetocardiograms in 19 healthy fetuses between the 16th and 41st week of gestation. On the basis of beat to beat intervals, mean RR interval (mRR), its standard deviation (SD), root mean square of successive differences (RMSSD), as well as complexity variables such as dimension (ApD1), entropy (ApEn), Lyapunov exponent (ApML) and trajectory divergence rate (p) were calculated for each recording. Dependency of these variables on gestational age was evaluated with correlation analysis. All variables changed consistently over time. RMSSD showed the strongest dependency on gestational age, followed closely by ApEn, SD and p. ApD1 and mRR showed only weak dependency. We conclude that magnetocardiography is well suited to register fetal cardiac activity with sufficient accuracy to permit detailed analysis of various heart rate variables during the second and third trimester of pregnancy. The observed increases in heart rate variability and complexity of fetuses most likely reflect differing but overlapping aspects of fetal development. They may be linked to the maturation of the autonomic nervous system and could aid in the timely identification of pathological conditions.

The use of non-linear analysis for differentiating the biomagnetic activity in ovarian lesions

In this study we investigated the biomagnetic activity measured with the superconducting quantum interference device (SQUID) in benign and malignant ovarian lesions using non-linear analysis. We used a single channel biomagnetometer SQUID in order to measure the magnetic field emitted from benign and malignant ovarian lesions. We can differentiate such biomagnetic activities using non-linear analysis. Using the application of non-linear analysis in the ovarian lesions together with the use of dimensional calculations we have observed a clear saturation value for the dimension of malignant ovarian lesions and non-saturation for benign ovarian lesions. The biomagnetic measurements with the SQUID and the application of non-linear analysis in benign and malignant ovarian lesions, is a promising procedure in assessing and differentiating ovarian tumours.

The volume conductor may act as a temporal filter on the ECG and EEG

The influence of the volume conductor on the EEG, MEG, fetal ECG and fetal MCG is studied by means of simulations. The assumption that the Maxwell equations can be used in a quasi-static approximation is reconsidered and the fact that the conductivity of human tissue is frequency dependent is taken into account. It is found that displacement currents have a substantial effect on the fetal ECG and to a lesser degree on the fetal MCG. Moreover, the frequency dependence of the conductivity of the tissues within the head may have a considerable effect on the EEG.

[Fetal magnetocardiography]

Fetal magnetocardiography is a new, alternative method for prenatal surveillance. The fetal magnetocardiogram (FMCG) registers the magnetic field produced by conduction currents in the fetal heart. Compared to the fetal electrocardiogram, the propagation of magnetic fields is relatively undisturbed by surrounding tissue. The FMCG thus has the advantage of a higher signal-to-noise ratio and can be acquired earlier pregnancy. Also, the high temporal resolution of the signal permits a significantly more precise determination of fetal heart rate parameters than fetal ultrasound. FMCG registration using a biomagnetometer is noninvasive and can be performed as of the second trimeter. It can be used to examine signal morphology, cardiac time intervals, heart rate variability as well as cardiac magnetic fields. To date, arrhythmic activity has been observed in the form of supraventricular and ventricular ectopies as well as atrial flutter, atrio-ventricular block, atrial tachycardia and Torsades de Pointes tachycardia. We also report here on the presence of short episodes of bradycardia in the second trimester of normal pregnancy. Measurement of the magnetic field strength at various locations above the abdomen has allowed the reconstruction of the fetal cardiac magnetic field and the determination of its relation to the position of the fetus. Signal averaging has permitted the precise examination of signal amplitude and cardiac time intervals and has shown that they increase in the course of pregnancy. Heart rate variability could be quantified in the time and frequency domain as well as using parameters of nonlinear dynamics. The results demonstrated an increase of variability and complexity over gestational age. Furthermore spectral analysis of fetal heart arte data could be associated with sympathetic and parasympathetic activity as well as, with respiration. Although the studies presenting these results have involved only limited numbers of observations, they demonstrate the potential of the method in the examination of the fetal conductive system, arrhythmias, congential defects, growth, development of the autonomic system, acidosis and distress. Furthermore, first results in pathological cases indicate that it may become a valuable tool in prenatal diagnostics. Improvements in instrumentation as well as prospective multicenter studies with larger numbers of appropriate subjects are required to determine whether magnetocardiography will establish itself as a new tool in clinical fetal, surveillance.

The hemodynamics of the umbilical artery in normal and pre-eclamptic pregnancies. A new application of SQUID biomagnetometry

This study was set to investigate the hemodynamics of the feto-placental circulation in normal and pre-ecclamptic pregnancies using the biomagnetometer SQUID. Thirteen women with pre-eclampsia and 26 healthy women were studied. All were "near term". Biomagnetic signals (waveforms) were recorded from the umbilical arteries. After statistical Fourier analysis, the findings were assigned arbitrarily in terms of spectral amplitudes as high (200-300 fT/ root of Hz), low (80-150 fT/ root of Hz) and borderline (151-199 fT/ root of Hz). In all cases the frequencies considered were distributed in the range 2-7 Hz. Interestingly, the umbilical artery waveforms and the corresponding spectral densities were of high amplitudes in most (88%) normal pregnancies of low amplitudes in most (84.6%) pregnancies complicated with pre-eclampsia. These findings were of statistical significance and were correlated with fetal heart rate (FHR) monitoring, pH and Apgar score at 1 and 5 minutes; high amplitude cases were related with normal FHR monitoring, pH > 7.25 and Apgar score > 7, while low amplitude recordings were connected with abnormal FHR patterns, pH < 7.25 and Apgar score < 7. It is suggested that biomagnetic measurements of the umbilical artery flow, which is an entirely new application of SQUID technology, is a promising procedure in assessing fetal health, especially in high risk pregnancies.

The fetal electrocardiogram

Advances in microprocessing technology have made fetal ECG analysis a feasible adjunct to fetal surveillance. Time interval and morphology changes of the FECG occur during fetal hypoxia. The use of these changes to detect a fetus at risk of intrapartum asphyxia awaits validation in terms of both future and ongoing clinical trials. Recognition of FECG changes during decelerations may improve the sensitivity of EFM. Antepartum FECG analysis has potential for the detection of a number of pathological fetal conditions, including intrauterine growth retardation, but remains hampered by low signal-to-noise ratios, rendering successful signal acquisition unreliable.