[Fetal magnetocardiography].

The electrical activities of the heart causes weak changes of the magnetic field, which can be recorded as magnetocardiogram (MCG). Fetal cardiac magnetic activity is measured in the order of less than 10 pT. An advance of the novel technology of a superconducting quantum interference device enabled the first recording of fetal MCG (FMCG) in 1974. In Japan, FMCG instrument (MC6400, Hitachi High-Technologies Ltd) was approved as a diagnostic tool by Japanese Government in 2003 owing to the cooperative studies of Tsukuba University, National Cardiovascular Center and Hitachi Ltd. FMCG offers similar information to a fetal electrocardiogram, which is difficult to be recorded because the fetal skin is covered with fatty caseous vernix of weak electrical conductivity in the second and third trimester of pregnancy. Magnetic flux can pass through the fat layer, and thus FMCG can measure the electrical activity of the fetal heart. Besides FMCG has far higher resolutions in time domain than echocardiography does. The amplitude of FMCG signals depends on the size of fetal heart and the distance between the sensors and the fetal heart. The amplitudes of the QRS, P and T waves increases with gestational age. Since the amplitudes of P and T waves are often weak, averaging of FMCG signals is needed to improve the signal-to-noise ratio. Current-arrow map is a useful mapping technique even in FMCG. FMCG has been applied in the prenatal diagnosis of fetal arrhythmias such as bradyarrhythmia (atrioventricular block, long QT syndrome, etc), tachyarrhythmia (supraventricular tachycardia, atrial flutter, atrial fibrillation and WPW syndrome, etc) and extrasystoles. Fetal cardiomegaly with myocardial abnormalities can be also diagnosed by FMCG. Applications of FMCG for fetal heart rate monitoring using beat-to-beat variability have been also studied to obtain better information on fetal well-beings.