Magnetocardiography is valuable for monitoring the fetal heart

Fetal magnetocardiography (FMCG) is a non-invasive technique that can be used to study the heart. It measures the magnetic field produced by electrical activity of cardiac contractions that occur from day 22 in the embryo. Traditionally, this has been monitored using fetal electrocardiography (FECG). However, FECG recordings from non-invasive abdominal leads show a high level of noise, so it is difficult to obtain detailed information even using advanced computer models. FMCG has a better signal-to-noise ratio and thus provides significant advantages in gathering clinically valuable data on the developing baby.¹

FMCG measures the magnetic field from a position above the maternal abdomen, and the waveform produced demonstrates the typical features of an electrocardiogram including a P wave, QRS complex, and T wave (see Figure 1). We used FMCG throughout pregnancy to demonstrate the valuable information it provides. We began by studying a 12-week old fetus and found the amplitude of the FMGG was in the order of the noise of the measuring system (see Figure 2). By the 14th week the signal-to-noise ratio was significantly better and we could see the QRS complex, and this was further improved by using averaging to reduce the noise. At week 16 the QRS complex and P wave could clearly be seen, and from the 18th week all FMCGs showed a complete cycle of P wave, QRS complex, and T wave sufficient to allow medical evaluation of the heart.

Figure 1. The measuring position and waveform of the FMCG.

Figure 2. An FMCG recording of fetal heart activity at 12 weeks.

We used a subject in the 28th week to demonstrate alternative measuring positions above the abdomen (see Figure 3). At this stage of pregnancy, the QRS complexes and P waves are clearly visible without using computer processing to eliminate noise. When we recorded from below the umbilicus we saw only the signal from the fetal heart (see Figure 3a). However, measuring from a point above the umbilicus the FMCG also showed maternal activity in positive (upwards) QRS complexes (see Figure 3b). The fluctuation of the isoclines was caused by the movement from the mother's breathing.

Figure 3. Measuring FMCG from different positions provides different information; a) measuring position below umbilicus showing fetal heart activity only; b) measuring position above umbilicus showing fetal heart activity downwards and maternal heart activity upwards.

From our recordings we calculated the ratio of the amplitudes of different parts of the waveform to analyze heart activity. We found the ratio of the P wave to the R wave was higher than that of the T to R, a feature observed in all our subjects up to the 39th week of pregnancy. In most recordings the T wave was small or not visible, both in the original waveform (see Figure 4), and when we averaged the signal by synchronizing QRS complexes of individual heartbeats (see Figure 5). The waveform parameters can also be used to detect fetal heart disorders such as arrhythmia or an atrioventricular block,^2–4 the latter diagnosed from abnormal P wave and QRS complex rates (see Figure 6).

Figure 4. The FMGG waveform can be used to calculate wave amplitude ratios.

Figure 5. A signal averaged by QRS-synchronization, used to analyse wave amplitude ratios.

Figure 6. FMCG showing abnormal P wave and QRS complex rates indicates an atrioventricular block.

We also compared the change in waveform amplitude of the FMCG signal throughout pregnancy with that of the FECG (see Figure 7). FECG amplitude varies disproportionately with fetal age, and following an initial increase it drops for several weeks around week 28 before rising to its maximal value at birth. This strong decrease in mid-pregnancy is caused by the insulating effect of the vernix caseosa coating formed on the fetus.^5,6 However, the FMCG remains proportional to age throughout pregnancy, a function demonstrated by plotting the highest amplitude measured at each stage using different symbols for different subjects. FMCG is unaffected by the formation of the vernix caseosa and therefore allows significantly improved monitoring of fetal development.

Figure 7. Unlike FECG, the FMCG signal amplitude remains proportional to fetal age throughout pregnancy.

Our studies have demonstrated that FMCG provides valuable information about the fetal heart, allowing analysis of the shape and duration of all parts of the waveform, and calculation of P wave, QRS complex, and T wave conduction intervals.⁷ Unlike FECG it is unaffected by noise and remains proportional to age throughout pregnancy, and therefore FMCG provides a valuable alternative for monitoring fetal cardiac health and development.