Umbilical waveform variations in the acutely distressed fetus: an experimental model in the fetal sheep.

In previous experimental studies on the pregnant ewe, umbilical arterial and venous waveform variations, produced by maternal aortic and umbilical cord occlusions of different degrees, were investigated in healthy fetuses (pH > 7.30). In our study, the fetal pH in 10 pregnant ewes was first brought down to below 7.20 with a preliminary series of maternal aortic and umbilical cord occlusions; then the 10 ewes were again submitted to a similar series of maternal aortic and umbilical cord occlusion. There were 24 experimental sessions with series of umbilical occlusions and 27 sessions with maternal aorta occlusions. The differences in waveform responses between these two types of fetuses are characteristic. The acidotic fetus produces more protracted drops in S, D, and venous velocity and, at the end of occlusion, the return to normal is slower with no reactive overflow waveform. In addition, there is a typical venous undulating pattern and no reappearance of blood flow velocity during the umbilical cord occlusion (always observed in healthy fetuses). Another important observation is that the fetal heart rate responses (bradycardia or tachycardia or no fetal heart rate variations) are more erratic in the acidotic fetus and do not relate to the intensity of the occlusion or to the fetal pH. The possible clinical implications of these experimental findings are discussed.