What is Doppler Ultrasound?
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 Uterine Arteries?
The uterine arteries provide blood to the uterus. Two arteries are present, one on the left and one on the right side of the uterus. Blood from the uterine arteries supplies blood to the muscles of the uterus and the placenta.
How is the Uterine Artery Waveform Measured During Pregnancy?
The uterine artery can be evaluated by direct visualization, i.e. examining the characteristics of the waveform too determine if notching is present or absent (see below), or by quantifying the waveform 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. The most common approach is to measure the Resistance Index (RI) in which the peak of systole is divided by the sum of systole and 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.
Systole (S) and diastole (D) are identified by the yellow and green box outlines, respectively. Notice that during diastole there is continuous blood flow which is approximately 2/3 the height of systolic flow. This means that there is continuous flow of blood to the uterus during each heart beat of the mother.
What Is An Abnormal Doppler Waveform?
To understand the changes in the uterine artery waveform, it is important to compare the non-pregnant and pregnant state.
Non-Pregnant Uterine Artery Waveform
Because of a high resistance to blood flow in the non-pregnant uterus, the waveform demonstrates notching at the beginning of diastole with low flow at the end of diastole. Once pregnancy occurs vessels in the placenta develop, resulting in a low resistance to blood flow with a concomitant increase in the height of the diastolic waveform. The following ultrasound image demonstrates blood flow in the uterine arteries in a non-pregnant patient.
This image illustrates a Doppler waveform from a non-pregnant women. Notice that after the sharp spike (A), the waveform has a notch (B), followed by a flattened waveform (C).
Type I. Abnormal Resistance Index
Once the waveforms are obtained and measured, the results are plotted on graphs to determine if blood flow 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 may be undergrown (too small). To determine the Resistance Index the peak of systole (Sys Meas) is divided by the sum of the systolic (Sys Meas) and diastolic measurements (Dias Meas). A value greater than 0.58 is considered to be abnormal. One of the problems with this measurement is a higher false-positive rate than if one uses the presence or absence of notching (see below). For this reason, Dr. DeVore prefers using the presence or absence of a "notch" to determine if the waveform is normal or abnormal.
The resistance index is measured to determine if it is abnormally high. In this example the low diastolic flow results in an abnormal Resistance Index.
Type II. Mild Notching of the Uterine Artery
This is a more serious form than Type I because there is a "notch" at the beginning of diastole. The notch is the result of an increase in resistance to blood flowing into the placenta. The reason for this is because the blood vessels in the placenta are not enlarging or dilating as they should. When this occurs, notching is present in the Doppler waveform. However, as in this example, blood flow at the end of diastole appears to be normal, thus giving a normal Resistance Index. The presence of a notch, even with a normal Resistance Index, places the patient at high-risk for adverse fetal outcome.
This illustrates NOTCHING with a normal Resistance Index. The presence of a notch places the patient at high-risk for adverse pregnancy outcome.
Type III. Severe Notching with an Abnormal Resistance Index
When the Resistance Index is abnormal (low-diastolic flow) and a notch is present, this places the patient at the highest risk for adverse pregnancy outcome.
This illustrates NOTCHING with an abnormal Resistance Index. The combination of these two findings places the patient at increased risk for adverse pregnancy outcome.
Benefits of Uterine Artery Surveillance
Recent studies have found that surveillance of high-risk fetuses with abnormal uterine blood flow may decrease morbidity and mortality. The majority of the earlier studies examined the uterine arteries during the second trimester of pregnancy (20-24 weeks) (Medical Literature). However, recent studies have suggested that identification of abnormal Doppler waveforms during the first-trimester and subsequent treatment with low-dose aspirin may be more beneficial than waiting until the second-trimeter to evaluate these vessels . The main problems with abnormal uterine artery blood flow that persists throughout the pregnancy is that there is an increased risk for pre-eclampsia (high blood pressure, protein in the urine) during the late second and third trimesters of pregnancy. When this occurs, the only treatment is delivery. In addition, fetuses of mothers who have an abnormal uterine artery Doppler waveform have an increased risk for abnormal fetal growth resulting in an undergrown, or small fetus with all of its consequences. For this reason, Dr. DeVore recommends initiation of low-dose aspirin therapy (81 mg/day) when unilateral or bilateral notching is identified during the first-trimester of pregnancy. He then re-evaluates these vessels during the second-trimester. If the notching persists, the aspirin is continued until 32 weeks of gestation.