Umbilical Artery Doppler Assessment
What is Doppler?
Doppler is a method by which information can be obtained by evaluating the change in a waveform (sound, radar, light) in which the speed and direction of an object (blood, rain, stars) can be determined. In fetal medicine we use the Doppler principle to evaluate changes in sound waves which inform us about the direction and velocity of blood flowing through vessels and the heart. Using this technology and plotting it against time, characteristics of blood flow in the pregnant woman and the fetus can be measured.
What are the Umbilical Arteries?
The umbilical cord contains three vessels; two arteries and a vein. Blood flowing through the umbilical arteries originates from the fetus and enters the placenta. The flow of blood through the arteries is dependent upon the strength of the fetal heart contraction and the health of the placenta. Blood returning from the placenta goes through the umbilical vein to the fetus.Numerous medical studies conducted during the past few years have found that measurement of the umbilical artery using Doppler ultrasound identifies high-risk fetuses. When these fetuses are identified and management is altered by the physician, the fetal death rate as well as other severe complications are markedly reduced.
How is the Umbilical Artery Waveform Measured?
The umbilical artery is evaluated by measuring the blood flow velocity at peak systole (maximal contraction of the heart) and peak diastole (maximal relaxation of the heart). These values are then computed to derive a ratio. One of the most common ratios that is used is the Resistance Index. This is computed by measuring the peak of systole and then dividing it by the sum of measurements at peak systole and diastole. RI= systole/(systole+diastole) In early pregnancy the peak flow at diastole is less than later in pregnancy. Therefore, as the duration of pregnancy increases, the amount of blood flowing in the umbilical artery increases during diastole. This means that the placenta is less resistant to blood flow, thus providing more blood to flow from the fetus to the placenta. The following image illustrates Doppler waveforms at 20 and 36 weeks of pregnancy.
Systole (Sys) and diastole (D) are identified in green Note that diastole is less at 20 weeks (yellow ellipse)
than at 36 weeks (red ellipse).
What Is An Abnormal Doppler Waveform?
There are three types or degrees of abnormal waveforms.
Type I. Abnormal Resistance Index With Blood Flow Present During Diastole
Once the waveforms are obtained and measured, the results are plotted on graphs to determine if the amount of flow of blood during diastole is normal or abnormal. If the Resistance Index increases to a value above the upper range of normal, this identifies a fetus at risk or who has IUGR. If the Resistance Index that is measured from the Doppler waveform is above the red line, this is abnormal. The gold circle illustrates a measurement of the Resistance Index that is abnormal.
Type II. Absent Blood Flow During Diastole
This is a more serious form than Type I because blood flow is absent during diastole. Fetuses with this type of finding should be monitored closely in a hospital setting. The following ultrasound picture illustrates this type of Doppler waveform.This illustrates absent diastolic flow during diastole. When this occurs there is abnormal resistance in the placenta which results in a marked decrease in blood flow from the fetus to the placenta.
Type III. Reverse Blood Flow During Diastole
When the resistance in the placenta increases further, absent diastolic flow becomes reverse diastolic flow in which the Doppler waveform is observed to be below the baseline. When the fetus develops this type of abnormality, intense surveillance is required if the fetus is less than 32 to 34 weeks and delivery if it is greater than 32 to 34 weeks. The surveillance that is currently recommended is evaluation of the ductus venosus and/or inferior vena cava, in addition to traditional antepartum testing. The following illustrates reverse diastolic flow during diastole (blue circle). When this occurs there is abnormal resistance in the placenta which results in a marked decrease in blood flow from the fetus to the placenta.
Benefits of Umbilical Artery Surveillance
Recent studies have found that surveillance of high-risk fetuses with umbilical artery Doppler ultrasound results in a marked decrease in fetal death and morbidity when compared to traditional surveillance (non-stress test). For this reason, all fetuses with suspected intrauterine growth restriction should undergo umbilical artery Doppler evaluation.