Determinants of Doppler Flow Velocity Waveforms in the Uteroplacental and Umbilical Arteries

Non-invasive assessment of blood flow in the uteroplacental and umbilical arteries using Doppler velocimetry is a promising technique for the identification of fetal compromise and seems likely to be introduced widely into clinical practice. Nevertheless, many problems remain to be resolved regarding the potential use of Doppler in obstetrics. These problems arise in part from a lack of understanding of which indices are most appropriate for describing waveforms and of the factors determining abnormal waveforms. In a cross sectional study of 356 normal and complicated pregnancies the three most commonly used indices, the pulsatility index, resistance index and systolic/diastolic ratio were found to be highly correlated with each other and could be used interchangeably unless absence of end diastolic velocities was observed. The association between advancing gestational age and umbilical artery waveform indices, showing reduced placental vascular resistance, was confirmed although no such association was seen in uteroplacental patterns. Placental vascular pathology has been implicated in the production of abnormal uteroplacental and umbilical artery Doppler waveforms. It has been postulated that Doppler studies in mid-trimester might identify pregnancies at risk of later complications. Elevated maternal serum alphafetoprotein (AFP) in pregnancies with normally formed fetuses is also known to be associated with a high risk of perinatal complications and a placental pathology has recently been suggested as underlying this. To determine if Doppler might identify such a lesion, 40 patients with unexplained elevation of maternal serum AFP underwent Doppler studies in the mid trimester. Although there was a significant difference in the incidence of later complications from a matched group of control patients there was no difference from controls in Doppler indices from the uteroplacental or umbilical circulations. Consequently the hypothesis that Doppler might identify a placental lesion associated with later complications is rejected. Furthermore, despite a 40% incidence of complications in the elevated AFP group the values obtained were within previously published normal ranges for the gestation of study and the findings do not support a role for Doppler in screening in mid trimester for later complications. Abnormal waveforms have been reported in pregnancies in which the fetus is small-for-dates and in those complicated by hypertension but few controlled studies have been performed to determine the role played by these conditions in producing changes in waveform indices. A controlled study was performed to determine if uteroplacental and umbilical artery waveforms are altered in pregnancies in which the fetus is small-for-dates. Waveforms were obtained in 32 such pregnancies. No difference in uteroplacental waveform indices was identified. In contrast, the indices from the umbilical artery were significantly higher, though not necessarily abnormal, compared with matched controls. These data suggest that haemodynamic changes occur in such fetuses even when, as determined by Doppler, uteroplacental perfusion is maintained. It is known that in some small-for-dates fetuses blood viscosity is increased. From Pouseille's equation it is known that blood flow is dependent on blood viscosity and the need for intra-uterine intravascular transfusion in some cases of rhesus disease provided the opportunity to determine if an increase in blood viscosity following transfusion is associated with an increase in Doppler indices of resistance. Twenty women underwent a total of 35 percutaneous umbilical blood sampling procedures and on 22 occasions intra-vascular transfusion was indicated from the results obtained. The systolic/diastolic ratio was measured immediately before and 1 hour after sampling or, if performed, transfusion. A reduction in the systolic/diastolic ratio was seen whether transfusion was performed or not indicating that a reduction in resistance to blood flow occurred. This surprising finding suggests that umbilical cord puncture itself may result in the release of vasodilator substances and this must be investigated in further studies. To determine if pregnancy induced hypertension (PIH) and absolute levels of blood pressure influenced waveforms, a group of 48 untreated carefully defined patients were compared with an individually matched control group. In the hypertensive patients the Doppler indices of vascular resistance in the uteroplacental vessels were significantly higher than controls. In contrast, the results from the umbilical artery analysis showed increased resistance, reflected in higher values for waveform indices, only when significant proteinuria was present