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Fetal magnetocardiography (fMCG) has been shown to augment fetal ultrasound evaluation for high-risk conditions, but the clinical utility of fMCG depends on the reliability of the cardiac traces reconstructed. We performed a methodological study to examine the influence of gestational age on the properties of the fetal magnetocardiograms extracted with two methods of signal reconstruction: the template matching technique (TMT), which extracts the maternal components from the signal using only temporal information, and independent component analysis (ICA), which separates the fetal signals by using information on the spatial distribution of the mixed source signals in addition to higher order temporal statistics. Efficiency and accuracy were evaluated in terms of fetal beat detection, signal characteristics, and duration of cardiac time intervals (CTIs) on the averaged traces. ICA outperformed TMT with regard to beat detection and signal-to-noise ratio. The timing of the heartbeats and the duration of the CTIs were essentially the same, whereas some alterations in signal morphology were observed in the ICA traces. We conclude that ICA may be useful in early gestation when the signals are noisy, while TMT may be preferred when accurate beat morphology is required for diagnostic purposes.

Background: Heart rate variability (HRV) is becoming increasingly important in the analysis of prepartal cardiotocography (CTG). Dawes and Redmann have developed a computer algorithm which can calculate short-term variability on the basis of CTG data. In dealing with artefacts, CTG monitors average heart rate values over several beats which makes the use of standard measures of HRV such as the root mean square of successive differences (RMSSD) inappropriate. Fetal magnetocardiography (FMCG) enables the registration of signals similar to the electrocardiogram and this permits the precise determination of heart beat duration and, in consequence, of measures of fetal HRV., Methods: In this study we applied both methods--CTG and FMCG--sequentially and simultaneously in healthy pregnancies. Fetal short-term HRV was estimated on the basis of RMSSD values for both methods., Results: The RMSSD values of the FMCG data were generally higher and showed a wider dynamic range than those of the CTG. The direct comparison of the simultaneously acquired data demonstrated that the data processing of the CTG signal leads to a suppression of essential aspects of short-term HRV., Conclusion: FMCG permits a substantially more differentiated examination of fetal HRV and offers new possibilities in the analysis of fetal cardiac activity.

Fetal heart rate complexity was examined on the basis of RR interval time series obtained in the second and third trimester of pregnancy. In each fetal RR interval time series, short term beat-to-beat heart rate changes were coded in 8 bit binary sequences. Redundancies of the 2(8) different binary patterns were reduced by two different procedures. The complexity of these sequences was quantified using the approximate entropy (ApEn), resulting in discrete ApEn values which were used for classifying the sequences into 17 pattern sets. Also, the sequences were grouped into 20 pattern classes with respect to identity after rotation or inversion of the binary value. There was a specific, nonuniform distribution of the sequences in the pattern sets and this differed from the distribution found in surrogate data. In the course of gestation, the number of sequences increased in seven pattern sets, decreased in four and remained unchanged in six. Sequences that occurred less often over time, both regular and irregular, were characterized by patterns reflecting frequent beat-to-beat reversals in heart rate. They were also predominant in the surrogate data, suggesting that these patterns are associated with stochastic heart beat trains. Sequences that occurred more frequently over time were relatively rare in the surrogate data. Some of these sequences had a high degree of regularity and corresponded to prolonged heart rate accelerations or decelerations which may be associated with directed fetal activity or movement or baroreflex activity. Application of the pattern classes revealed that those sequences with a high degree of irregularity correspond to heart rate patterns resulting from complex physiological activity such as fetal breathing movements. The results suggest that the development of the autonomic nervous system and the emergence of fetal behavioral states lead to increases in not only irregular but also regular heart rate patterns. Using symbolic dynamics to examine the cardiovascular system may thus lead to new insight with respect to fetal development.

Background: In fetal surveillance, fetal heart rate changes are assessed using cardiotocography (CTG). Standard procedures used in the quantification of heart rate variability (HRV) are seldom applied as CTG does not deliver beat interval durations with the same accuracy as the electrocardiogram (ECG). Thus little is known about the interdependency of standard HRV measures prenatally., Materials and Methods: Using fetal magnetocardiography (FMCG: the magnetic equivalent of the fetal ECG), we investigated standard HRV measures in a homogenous group of 20 healthy fetuses. Ten HRV measures were analyzed in 60 five-minute RR interval time series derived from FMCG acquisitions (16th-40th week of gestation)., Results: Using regression analysis, we found a clear dependency on gestational age and mean RR interval for measures from the time and frequency domains as well as for a measure for complexity. Correlation analysis revealed a strong interdependency of the measures, in particular within the specific domains. An adequate description of fetal HRV may be achieved using the standard deviation from the time domain, the high frequency band (0.15-0.40 Hz) from the frequency domain and approximate entropy as a complexity measure., Conclusion: The results indicate that different characteristics of fetal HRV are embedded in the different domains.

Fetal magnetocardiography has shown that fetal P wave and QRS complex durations increase with gestational age, reflecting change in cardiac muscle mass. The latter should, in principle, be associated with an increase in signal strength. We examined two approaches for determining QRS signal strength in a healthy fetus on a weekly basis in the second and third trimester. Twenty-two fetal magnetocardiograms of the same fetus were obtained using a 61 channel Magnes 1300 biomagnetometer (20th-42nd week of gestation). In the signal averaged fetal beat produced at each week, signal strength was assessed on the basis of 1) peak-to-peak QRS signal amplitudes and 2) strength of an equivalent current dipole (ECD) computed at R peak. The results were assessed on the basis of correlation to week of gestation and by comparison to changes in QRS interval duration. All values increased with advancing gestation and regression analysis suggested a nonlinear dependency on age. ECD strength reflected gestational age slightly more reliably (r2=0.93) than signal amplitude values (mean, median, maximum: r2=089, 0.88, 0.85, respectively). ECD strength and mean signal amplitude also correlated well (r=0.97, p<0.0005) Values calculated from QRS complexes determined immediately before and after a clear change in fetal position (acquisition week 24) demonstrated a certain instability in both approaches. Nonetheless, the overall correlation of the amplitude to gestational age compared favorably with that of QRS complex duration. This indicates that not only magnetocardiographically determined fetal cardiac time intervals but also signal strength may be used to assess fetal growth.

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.