Use of an Indwelling Doppler Probe to Study Acute Changes in Umbilical Vein Waveforms in the Fetal Sheep

A chronic sheep model for Doppler umbilical vascular analysis with indwelling probes was used for the investigation of umbilical vein velocity waveforms. Maternal aortic occlusions produced a delayed drop but never eliminated umbilical vein velocity. With umbilical cord occlusion, there was immediate umbilical vein waveform response. Sudden cord occlusion and release showed reactive venous overflow velocity at the beginning and end of occlusion. Total cord occlusion produced zero velocity, but within seconds, the flow velocity was seen despite persistent occlusion in 65% of the cases. A characteristic undulating venous waveform pattern synchronous with fetal heart rate in over half of the cases was often observed. Reduction of blood flow by uteroplacental insufficiency may be differentiated from umbilical cord occlusion.