Protocols for High-Risk Pregnancies. Группа авторов
Maternal‐Fetal Medicine, University of Colorado School of Medicine, Colorado Fetal Care Center, Aurora, CO, USA
Overview
Doppler ultrasound depends upon the ability of a pulsed ultrasound beam to be changed in frequency when encountering moving objects such as red blood cells (RBC). The change in frequency (Doppler shift) between the emitted reflected beams is proportional to the velocity of the RBC and dependent on the angle between the ultrasound beam and the vessel. Pulsed‐wave Doppler velocimetry provides a flow velocity waveform from which information can be obtained to determine three basic characteristics of blood flow that are useful in obstetrics: velocity, resistance indices, and volume blood flow. Doppler velocimetry is applied in a broad number of clinical circumstances in high‐risk pregnancies including diagnostic fetal echocardiography, fetal growth restriction (FGR), fetal anemia, adverse pregnancy outcome assessment, twin‐twin transfusion syndrome (TTTS), twin anemia polycythemia sequence (TAPS), and preterm labor (ductus arteriosus assessment for indomethacin tocolysis). Pulsed‐wave Doppler velocimetry is also used to evaluate the ductus venosus (DV) in first‐trimester risk assessment for Down syndrome but is not discussed in this protocol.
Pathophysiology
Normal fetal circulation
The umbilical vein is a conduit vessel bringing oxygen and nutrient‐rich blood from the placenta to the fetus. The umbilical vein enters the umbilicus, courses anteriorly along the abdominal wall prior to entering the liver, and becomes the hepatic portion of the umbilical vein. The umbilical vein eventually becomes the portal vein, but first gives off the left inferior and superior portal veins, the DV, and finally the right portal vein. Approximately 50% of umbilical vein blood is directed into the DV and then to an area under the diaphragm referred to as the subdiaphragmatic vestibulum. The subdiaphragmatic vestibulum also receives blood from the inferior vena cava and blood exiting the liver via the right, middle, and left hepatic veins.
The process of preferential streaming begins in the subdiaphragmatic vestibulum with blood from the DV and the left and middle hepatic veins preferentially shunted across the foramen ovale into the left atrium and left ventricle so that the heart and head receive the most oxygenated and nutrient‐rich blood. In contrast, blood coming from the inferior vena cava and right hepatic vein are preferentially streamed into the right atrium and right ventricle. Then, after exiting through the pulmonary artery, this blood is shunted to the descending aorta via the ductus arteriosus. Blood leaves the fetus via two umbilical arteries arising from the hypogastric arteries which course around the lateral aspects of the bladder in an anterior and cephalad direction, exiting the umbilicus, returning oxygen‐reduced blood and waste products back to the placenta.
There are three primary fetal circulatory shunts that require closure after delivery for normal newborn cardiopulmonary transition to occur and for the subsequent adult circulation to be established. As mentioned above, the DV shunts blood from the umbilical vein toward the heart. The ductus arteriosus shunts approximately 90% of the blood in the main pulmonary artery to the descending aorta, leaving only 10% of pulmonary artery blood to reach the fetal lungs. The third shunt is the foramen ovale, which is maintained in a patent state in utero to allow the process of preferential streaming to occur from the right atrium to the left. Failure of any one of these shunts to close properly may result in adverse cardiopulmonary transition in the newborn.
Fetal growth restriction
Fetuses that fail to reach their genetically determined growth potential due to uteroplacental dysfunction may develop abnormal resistance to blood flow in the placenta. This abnormal resistance is due to numerous placental vascular abnormalities (poor villous capillarization, reduced number and branching of stem arteries, luminal reduction, and wall hypertrophy), which can be detected with Doppler velocimetry in the umbilical artery located upstream from the placenta. Progression of placental disease with concomitant worsening of blood flow resistance may lead to additional Doppler velocimetry changes in the central nervous system, and eventually in the precordial venous system or the heart. Once the fetus decompensates to that level of Doppler abnormality, acidemia is nearly always present.
Fetal anemia
In Rh disease, a fetal RBC antigen enters the maternal bloodstream and stimulates antibody production against that RBC antigen. An amnestic response may occur in a subsequent pregnancy if the same RBC antigen is presented to the mother’s immune system and this may lead to a series of events that include fetal anemia, extramedullary hematopoiesis, hydrops fetalis, and fetal death. Historically, the degree of fetal anemia and need for fetal RBC transfusion involved an amniocentesis to determine the amniotic fluid ΔOD450 to assess the degree of RBC‐derived hemoglobin breakdown products and to estimate anemia. Isoimmunization with the Kell antibody also results in fetal anemia but does so through bone marrow suppression rather than hemolysis and thus, the ΔOD450 will not be abnormal. Fetal anemia can also result from infections such as parvovirus B19. Doppler velocimetry of the middle cerebral artery (MCA) is now used to determine the risk for moderate to severe anemia supplanting the previously used amniocentesis. If moderate to severe anemia is suspected, the fetus should undergo a fetal blood sampling and transfusion.
Preterm labor
Although the pathophysiology of preterm labor is still largely unknown, tocolytic use is widespread. Use of agents that inhibit prostaglandin synthesis can result in premature closure of the ductus arteriosus and oligohydramnios. Doppler velocimetry is useful in assessment of ductus arteriosus closure by determining the peak systolic velocity as well as whether there is continuous flow throughout diastole.
Cardiac abnormalities
Fetuses with known cardiac abnormalities including congenital or structural heart disease, arrhythmias and congestive failure may have intracardiac and outflow tract flow velocity abnormalities that can be detected with Doppler velocimetry. Depending on the nature of the abnormality, this can affect other flow velocity waveforms including the DV, hepatic veins, inferior vena cava, and umbilical vein.
Diagnosis
Doppler techniques and measurements
As mentioned in the overview, pulse‐wave Doppler velocimetry can be used to obtain the following information from a flow velocity waveform.
Velocity of the blood – requires an angle of insonation of zero degrees between the transducer and the vessel of interest (Figure 7.1). The angle correction function available on most ultrasound machines can be used but the actual angle between the vessel and the ultrasound beam should be less than 30°. For middle cerebral artery peak systolic velocity (MCA PSV), the sample volume should be placed in the proximal third of the MCA as it branches from the circle of Willis.
Resistance indices (systolic/diastolic or S/D ratio, resistance index, pulsatility index) – these are angle‐independent measurements such that the value obtained for any one of these indices is not dependent upon the angle between the transducer and the vessel being interrogated (Figure 7.2).
Volume blood flow (milliliters per minute) – this is determined by obtaining the velocity of the blood and multiplying it by the cross‐sectional area of the vessel (obtained by two‐dimensional ultrasound) times 60 seconds:
Volume flow (mL/min) = velocity (cm/sec) × cross‐sectional area (cm2) × 60 seconds
