Significance of Obstetric Dopplers in Pregnancy Screening
MedPiper Technologies and JournoMed conducted a World Pre-eclampsia Week Webinar Series along with National IRIA Preventive Radiology and Samrakshan Committees which addressed the use of Obstetric Dopplers in Pregnancy.
Dr Rijo Mathew and Dr Praveen Kumar from IRIA spoke about how doppler can change pregnancy outcomes in India. During the second and third trimesters, most obstetricians prefer the biophysical and growth scan through which they can predict pregnancy complications such as pre-eclampsia, foetal growth restrictions, preterm births and avoid perinatal mortality and morbidity.
Another speaker, Dr Sandhya Dhankhar, spoke about the Obstetric Doppler and its significance in detecting various maternal and foetal blood vessels. She stressed on the importance of color doppler in Obstetrics, the exact technique of performing obstetric doppler, how to interpret the findings and the clinical implications.
What is the use of an obstetric doppler?
Doppler helps in detecting the hemodynamics of the vessel. The Pulsative Index is the most important factor for calculating foetal hemodynamics, which includes foetal systolic, diastolic and mean arterial pressure.
The importance of color doppler in Obstetrics
To understand the importance of color doppler in Obstetrics, knowledge of the following is required:
- Foetal Maternal and Foetoplacental circulation
- Identifying both normal and abnormal anatomic structures like cord insertion loops around the foetal neck, umbilical cord knot etc.
- In the case of multiple gestations, TTS (Twin to twin transfusion syndrome) and velamentous insertion or unequal sharing of the placenta may be seen. Knowing about this can help in diagnosing foetal growth restrictions and pre-eclampsia.
The safety of the Obstetric doppler can be understood by Alara Principle which measures two indices i.e. Mechanical Index and Thermal Index.
Basic rules for Color Doppler
- Identify area where color doppler needs to be performed with B mode
- Zoom on the vessel of interest where color doppler is performed
- Identify the presence of flow and its direction
- Reduce pulse repetition frequency (PRF)
- Use probe positioning /beam steering to obtain the vessel angle/ satisfactory beam
- The angle of insonation should be less than 30 degrees
- Avoid foetal movements to obtain good images
- Choose triplex scanning with B mode which decreases the temporal resolution
The spectral doppler/the pulse wave doppler is used to identify the velocity parameters in the blood vessels and the Power Doppler is a sensitive tool to look for low velocities and flows.
Foetal maternal circulation
The uterine arteries supply the blood to the placenta and the umbilical vessels supplies the blood from the placenta to the foetus. Deoxygenated blood flows through the umbilical artery whereas oxygenated blood flows through the umbilical vein. There are three important shunts responsible for foetal maternal circulation. One of which is Ductus venosus and it supplies the blood to the vital organs of the foetus.
Vessels to assess in Foetoplacental circulation
The vessels sampled to assess the foetal placental circulation in the first trimester are uterine arteries, ductus venosus and tricuspid valve. In 2nd and 3rd trimesters, specialists scan different vessels to understand foetal growth. These vessels include Middle cerebral artery (MCA), Uterine artery, Ductus Venosus, Aortic Isthmus, Tricuspid Flow, Umbilical Venosus and Inferior vena cava (IVC).
Obstetric Doppler in the first trimester
Uterine arteries are scanned using the obstetric doppler. This artery is a branch of the internal iliac artery. Doppler helps to understand placental bed development and uterine artery placement. The difference in the waveforms of the uterine artery, normal or abnormal, will indicate if the patient has pre-eclampsia, placental abruption or has foetal growth restriction conditions. The mean value of uterine artery PI is always plotted in the graph.
Ductus venosus is a small trumpet-shaped connection between the Umbilical Vein and IVC. It carries the oxygenated blood from the placenta directly into the right atrium of the foetus and is easily identified by a typical doppler sound.
The ductus venosus directly reflects the physiological status of the right ventricle regarding ventricular preload, myocardial compliance and right ventricular end-diastolic pressure. There is an increased risk of aneuploidy if the ductus PI value is less than 1. Sagittal or tranverse views of the ductus venosus are observed in the 2nd trimester during which there should be minimal foetal movements.
Agenesis of Ductus Venosus: Agenesis of ductus venosus is a rare condition where the vessel is associated with cardiac and extra cardiac malformations. This could lead to poor outcomes.
Vessels in the 2nd and the 3rd trimester
Umbilical artery is the signature vessel and it tells about the blood flow within the placenta. Diastolic flow is a measure of placental vascular bed impedance. This artery also has different waveforms for each trimester. The absence or reversal of the diastolic flow is an indicator of perinatal mortality.
Middle cerebral artery (MCA)
The middle cerebral artery (MCA) is the main predictor of fetal hypoxia and anemia. It is important to screen the vessel. Whenever the mother arrives for a growth scan, scan for both the umbilical artery and the middle cerebral artery. Usually, an umbilical doppler scan is recommended along with the growth scan to detect foetal abnormalities, brain sparing and thalassemia.
Cerebral-Placental Ratio (CPR): The Cerebral-placental ratio was described by Gramel -lini et al in 1992.
Cerebral placental ratio = Middle cerebral artery Pulsatility index /Uterine artery Pulsatility index
In normal pregnancies, the ratio is always greater than 1.
Foetal aorta / aortic isthmus
Foetal aorta give an idea about cardiac output and peripheral resistance of systemic circulation. The sample of this vessel is obtained via a sagittal view and color doppler is done to look for the aortic branches. The sample flow must be in the forward direction in the aortic isthmus.
The aortic isthmus doppler (AoI) is associated with increased foetal mortality and neurological morbidity in early-onset foetal growth restriction. This vessel reflects the balance between the impedance of the brain and the systematic vascular systems. A reverse AoI flow indicates advanced foetal deterioration and results in prenatal cerebral damage.
Umbilical Vein and IVC
The Umbilical vein is the last vessel to scan. In severe foetal hypoxia, reversal and pulsatile flow is seen, which results in adverse perinatal outcomes.
Significance of screening
For predicting foetal growth restriction, screening is required in all three trimesters. Samrakshan describes performing a multi-disciplinary approach to diagnose pre-eclampsia and foetal growth restriction during the first or second trimester. Maternal demographics, mean arterial pressure and uterine artery pulsatile index were should be calculated to predict the risks of pregnancy. A low dose of Aspirin and follow-up visits to detect doppler findings and foetal growth restrictions can resolve the issue to some extent. Color doppler also helps find other abnormalities such as single umbilical artery, vasa previa etc. All of these could predispose foetal growth restrictions.
Performing growth scan alone is not enough. Dopplers are necessary and are an integral part of growth scans, as per Samrakshan Protocol. Foetal hypoxia and acidosis can be detected by using these doppler vessels. Perinatal hypoxia conditions and intrapartum complications can also lead to cerebral palsy or delayed neurodevelopmental disorders.
Through dopplers, pre-eclampsia, and foetal growth restriction screening, all the possible pregnancy complications can be dealt with. The main focus of Samrakshan is on integrating doppler in all three trimesters which can significantly change the perinatal outcomes of the pregnancy.