Tracking of fetal breathing movements by pulsed Doppler ultrasound

Fetal breathing movements form a useful means of monitoring fetal conditions and functional development. Precise quantitative evaluation of fetal breathing movements requires the development of a continuous automated technique. A preliminary study of Doppler signals due to fetal breathing movements was carried out using a cardiotocograph in which the normal high-pass 'thump' filter had been removed. A pulsed Doppler system was then modified to produce five simultaneous outputs representing movement at depths separated by 1.5-cm intervals. These outputs represent the displacement inside tissue at the various depths. Signal processing was executed on a 32-bit computer with an analog-to-digital converter. On-line processing using a zero-crossing method and an off-line high-accuracy displacement estimation technique using an arc-tangent method were applied. Using computerized signal processing, it was possible to detect movements of a few hundred microns representing fetal breathing movements while rejecting gross movements due to maternal breathing, etc. Although further studies are required before clinical application, it appears that quantitative assessments of fetal breathing movements should be possible with this technique.