A comparison of Doppler ultrasound waveform indices in the umbilical artery--I. Indices derived from the maximum velocity waveform.

Various Doppler waveform indices have been used for assessment of the fetal circulation. Comparisons were made to show what relations exist between the indices, and to identify any differences or difficulties which might arise from using one as opposed to another in clinical practice. Both normal pregnancy and cases of fetal growth failure were studied. Indices were obtained from the maximum velocity envelope of the umbilical artery waveform using a curve fitting technique. The values were very reproducible for all indices. The FHR, which varied over the entire normal range, did not significantly affect the values of any index. The downstream impedance indices calculated included the AB ratio, pulsatility index (PI) and Pourcelot ratio. These all gave very closely correlated results for normals but discrepancies occurred in the at risk group, where values were elevated. This could be attributed to differences in the underlying distributions. The indices suggested for cardiac contractility were not as closely related to each other, and moreover the differences between them showed no clear pattern. None of the indices varied independently of the others. The rising slope, which is by definition related to the PI, was more highly correlated with the downstream indices than the relative flow rate index.