[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.