Ultrasonic measurement of human fetal blood flow.

Pulsed Doppler technique was combined with real-time B-mode ultrasonography for non-invasive measurement of human fetal blood flow in utero. The fetal vessel, e.g. aorta or umbilical vein, was identified in the real-time image and the real-time transducer put parallel to the vessel. The 2 MHz pulsed Doppler transducer insonated the vessel at a known angle, as the transducers were firmly attached to each other at an angle of 52 degrees. The Doppler instrument processed the Doppler shift signals and estimated the mean and maximum blood velocity. For calculation of blood flow, fetal vessel diameter must be measured. Three ultrasonic techniques (real-time B-mode, TM-mode, Time-distance recording) were applied for diameter measurements. The accuracy of the real-time B-mode technique was tested in vitro on 8 glass tubes with various diameters; the ultrasonically measured diameter was within +/- 0.4 mm of the diameter measured with vernier calipers. The reproducibility of diameter measurement was tested in vivo; the standard deviation of the difference between measurement was 0.15 mm for the fetal aorta and 0.20 mm for the umbilical vein. Fetal blood flow was measured in 38 normal late pregnancies. Blood flow in the thoracic part of the descending fetal aorta ws 185 ml min-1 kg-1 when based on mean blood velocity, and 261 ml min-1 kg-1 when based on maximum blood velocity. In the intra-abdominal part of the umbilical vein the mean blood flow was 115 ml min-1 kg-1.