Doppler Evaluation in Obstetrics Obstetrical Sonography 2 Lecture 22 Holdorf Doppler Evaluation in Obstetrics

Outline The Use of Doppler in IUGR Umbilical Artery Placental Insufficiency Middle Cerebral Artery Anemia Fetal Thoracic Aorta Fetal Renal Artery Uterine Artery What are the safety considerations for the usage of Doppler in obstetrics? The two most useful clinical applications of Doppler US in obstetrics More Doppler Ultrasound Applications in Obstetrics The Single Umbilical Artery

The use of Doppler in IUGR While a great deal of information regarding the use of Doppler in obstetrics had been published, unfortunately, no one technique has been proven diagnostic or IUGR. The predictive value of Doppler in IUGR has been shown to be low (20-40%). When Doppler findings suggest an abnormality, additional prenatal monitoring, such as non-stress testing or biophysical profile, should be done. While many protocols interrogating various fetal vessels have been studied, measurements of umbilical artery resistance hare most widely accepted.

Umbilical Artery Doppler is very important in the management of IUGR Power Spectral Density (PSD) as a routine use of detecting IUGR is not proven to be better than fetal biometric parameters. It has, however, been proven to be a good indicator of fetal outcome Umbilical artery (UA) Doppler in suspected IUGR is performed. If UA is normal, then no other Doppler investigations need to be performed. If UA is abnormal, then other vessels should be evaluated.

During normal pregnancy, there is an increase in placental size and blood flow volume, resulting in a decrease in resistance (impedance) to maternal blood flow, leading to LOW blood resistance. Suboptimal placental growth and function, generally referred to as placental insufficiency, is the common cause of fetal growth restriction. The more severe the placental insufficiency, the worst the fetal consequence.

With placental insufficiency, placental impedance is higher than normal and results in diastolic abnormalities such as decreased, absent, or reversed end- diastolic flow in fetal vessels. These diastolic abnormalities in flow are best characterized by the standard Doppler indices (S/D ratio, RI, and PI).

Doppler evaluation of placental insufficiency.

The diastolic flow reversal in umbilical arteries signifies severe placental insufficiency and increased placental vascular resistance, which is bad news for the fetus.

The Umbilical artery-may be evaluated anywhere along the length of the cord. Normal UA Doppler waveform has a rapid upstroke to peak systole and a gradual decline during diastole.

Normal Umbilical Artery Doppler Waveform S/D 2.2

Normally progressive decrease in resistance during the course of pregnancy. Systolic/diastolic ratio >3.0 is abnormal after 3o weeks Ratios are higher if measured closer to fetal cord insertion, so measurements should be taken in UA close to placental cord insertion site Absent or reversed diastolic flow in umbilical artery is ALWAYS an ominous sign

Abnormal UA Doppler waveform depends on the severity of disease.

Rule of Thumb: An umbilical artery S/D ratio >3 is recognized as being abnormal beyond 30 weeks. Therefore: a Combination of sonographic IUGR findings AND an abnormal UA Doppler waveform usually points to placental insufficiency as the cause of the growth restriction.  

Fetal Middle Cerebral Artery The MCA has surfaced as the favored vessel in assess the fetal cerebral circulation. Looking to asses the PSV-Peak systolic velocity. Normal MCA blood flow shows continuous forward flow throughout the cardiac cycle with low end diastolic flow resulting in relatively high PI and RI values. The Pulsatility Index (PI) of the MCA in normal fetuses decreases during late gestation.

The Pulsatility Index A measure of the variability of blood velocity in a vessel, equal to the difference between the peak systolic and minimum diastolic velocities divided by the mean velocity during the cardiac cycle.

The circle of Willis

Normal MCA in the fetus

IUGR fetuses may be experiencing chronic hypoxemia (decreased blood oxygen concentration) . With fetal hypoxemia, there is increased blood flow to vital organs (brain and myocardium) and reduced flow to the gastrointestinal tract and kidneys. Thus, the PSV of a hypoxic fetus will show an increase in the MCA velocity.

MCA and ANEMIA Middle Cerebral artery (MCA) Doppler can help determine likelihood of fetal anemia. MCA is examined close to its origin from the internal carotid artery. the angle of the ultrasound beam and the direction of blood flow should be zero degrees Risk of anemia is highest in fetuses with a pre-transfusion peak systolic velocity of 1.5 times the median or higher.

An abnormal MCA.

Anemic on left, nml on right

Fetal Thoracic Aorta Should be performed in the descending aorta, just above the diaphragm. In Normal fetuses, the Thoracic Aorta waveform is characterized by continuous forward flow throughout the cardiac cycle.

The normal fetal thoracic aorta

Previous slide Flow velocity waveform in the fetal descending thoracic aorta in an anemic fetus demonstrating high velocities and Doppler ‘window’ for low velocities during systole (top). After blood transfusion, there is a decrease in peak systolic velocity and the Doppler ‘window’ has disappeared (bottom).

Fetal renal artery The normal renal artery PI decreases with advancing gestation, Indicating an increase in renal perfusion. The normal Doppler flow waveform is characterized by continuous forward flow throughout the entire cardiac cycle, similar to the UA.

The fetal renal arteries

A normal Fetal Renal Artery Waveform

Uterine Artery Low resistance in second and third trimesters is normal Appearance of a ‘notch’ in Doppler waveform is abnormal and thought to predict insufficiency & IUGR

Uterine Artery: Normal impedance to flow the uterine arteries in 1º trimester

Normal impedance to flow the uterine arteries in early 2ºtrimester

Normal impedance to flow the uterine arteries in late 2º and 3º trimester

Doppler Waveform ‘NOTCH’

Previous slid Ultrasound Doppler spectrum of uterine artery blood velocity. Upper panel: Normal blood velocity. Lower panel: Decreased blood velocity with a characteristic notch in early diastole as a sign of increased placental vascular impedance.

What are the safety considerations for the usage of Doppler in obstetrics?   The safety of Doppler ultrasound remains a concern. In particular the use of pulsed Doppler involves the use of higher intensities compared to diagnostic ultrasound, and may cause significant tissue heating and other thermal effects. These thermal effects depend on the presence of tissue/air interface and may therefore not be clinically significant in obstetric ultrasound examinations.  

The following is a brief overview of the two most useful clinical applications of Doppler US in obstetrics A) Uterine Arteries- Pregnancy is associated with the physiologic changes at the level of the uterine vasculature, resulting in a progressive decrease in impedance with advancing gestation. Each uterine vessel can be demonstrated by color Doppler as it crosses over the hypogastric artery and vein just before it enters the uterus at the uterine-cervical junction. The presence of a notch in the waveform and an increase in the impedance index after 22 weeks of gestation characterizes an abnormal uterine circulation. Pregnancy complications include fetal growth restriction, preeclampsia, preterm delivery and non-reassuring fetal status in labor.

B) Umbilical Arteries-The umbilical arterial circulation is normally a low impedance circulation, with an increase in the amount of end-diastolic flow with advancing gestation. Umbilical artery Doppler waveforms reflect the status of the placental circulation, and the increase in end-diastolic flow that is seen with advancing gestation is a direct result of an increase in the number of tertiary stem villi that takes place with placental maturation. Doppler waveforms of the umbilical arteries can be obtained from any segment along the umbilical cord.

More Doppler applications… The middle cerebral artery is the most accessible cerebral vessel to ultrasound imaging in the fetus, and it carries more than 80% or cerebral blood flow. The cerebral circulation is normally a high-impedance circulation with the continuous forward flow throughout the cardiac cycle. The right and left middle cerebral arteries represent major branches of the circle of Willis in the fetal brain. The circle of Willis is supplied by the internal carotids and vertebral arteries and can be imaged with color flow Doppler ultrasound in a transverse plane of the fetal head obtained at the base of the skull.

Blood flow through the umbilical vein carrying oxygenated blood to the fetus can be assessed either within its entrance into the fetal abdomen or adjacent to the fetus within the amniotic fluid.

Ductus Venosus Doppler waveforms can be easily obtained from a transverse view of the abdominal circumference. By color Doppler to grayscale image, the ductus venosus can be identified as it branches from the portal vein. High velocity flow and turbulence is commonly seen within the ductus venosus given its narrow lumen.

IVC Doppler waveforms can be obtained from a coronal plane of the chest and abdomen. In this view, the IVC can be imaged as it enters into the right atrium, joined by the ductus venosus and the left hepatic vein. The IVC waveforms are triphasic in shape, with the first phase corresponding to ventricular systole, the second phase to early diastole, and the third phase to late diastole or the atrial kick.

Why is the detection of a single umbilical artery a concern for dealing with prenatal diagnosis?   Anywhere from half to two-thirds of babies born with single artery umbilical cords are born healthy and with no chromosomal or congenital abnormalities. Of the remaining babies with SUA, some studies suggest that about 25 percent have birth defects, including chromosomal and/or other abnormalities. These can include Trisomy 13 or Trisomy 18. However, the most common pregnancy complications that occur in infants with SUA are heart defects, gastrointestinal tract abnormalities and problems with the central nervous system. The respiratory system, urinary tract, and musculoskeletal system may also be affected. One in five babies affected by SUA will be born with multiple malformations.

The End