Prenatal diagnosis of QT prolongation by fetal magnetocardiogram - use of QRS and T-wave current-arrow maps

To determine the T wave of a fetal magnetocardiogram (FMCG), we have evalua ted the T/QRS ratio and obtained current-arrow maps that indicate weak curr ents. We measured FMCG signals for 52 normal fetuses and two abnormal fetus es with prolonged QT waves by using three superconducting quantum interfere nce device (SQUID) systems: a nine-channel system, a 12-channel vector syst em and a 64-channel system. The T/QRS ratio was calculated for all the norm al fetuses from the maximum magnitudes of the QRS complex and the T wave. C urrent-arrow maps of the QRS complex (R wave) and T wave were obtained by u sing the 64-channel system, and the phase differences of the total-current vectors were calculated by using the current arl-ow maps. The results showe d that the T/QRS ratio had a wide variability of 0.35 for the normal fetuse s. However, the magnitude of the prolonged T wave was as weak as the detect ion limit of the SQUID magnetometer Although the T/QRS ratios for the fetus es with QT prolongation were within the normal range (<0.35), the weak magn itude of the prolonged T wave could be evaluated. On the other hand, by com paring the current-arrow maps of the R and T waves for the normal fetuses, we found that the maximum-current arrows were indicated as either in the sa me direction or in opposite directions. These patterns could be identified clearly by the phase differences. Very weak prolonged T waves for the two a bnormal Fetuses could be determined by using these current-ar-row maps and phase differences. Consequently, although the T/QRS ratios of FMCG signals have a wide distribution, we have concluded that the current-arrow map and phase difference can be used to determine the T wave of an FMCG signal.