obstetric dopplers
we would like to acknowledge ms. sarah dowthwaite for providing invaluable information for this section.
- oxygenated, nutrient rich blood placental blood enters the foetus via the umbilical vein.
- the umbilical vein joins the portal venous system in the liver, some umbilical blood is shunted directly to the ivc and right heart via ductus venosus.
- blood from the right heart pumps to the lungs through the pumonary artery. however it is already oxygenated so is bypassed through foramen ovale (right atrium to left atrium) and ductus arteriosus (pulmonary artery to aortic arch).
- foetal blood returns to the placenta from the iliac arteries into the umbilical arteries.
for a great explanation about the fetoplacental circulation pre and post partum, the following 5 minute video is extremely useful in assisting ultrasound practitioners understand the direction of blood flow.
fetoplacental circulation doppler
it is vitally important to minimise any measurement errors in obstetric doppler to ensure it is accurate and reproducible.
measurements include:
- peak systolic velocity (psv)
- end diastolic velocity (edv)
- pulsatility index (pi) (peak systolic velocity – end diastolic velocity)/ time averaged velocity
- resistive index (ri)
- systolic/ diastolic velocity ratio (s/d)
tips to improve accuracy
- measurements obtained during absence of fetal breathing and body movements. may need a maternal breath hold
- optimal insonation is complete alignment with the bloodflow (0deg angle)
- small doppler gate – approximately the width of the vessel being insonated
- vessel wall filter set to low 30-60hz -eliminates the noise from the vessel walls
- horizontal sweep shows 4-6 cardiac cycles
- scale (prf) adjusted to the vessel being studied – waveform fills at least 75% of the screen
- constitutionally small
- fetal size and weight at birth are less than expected
- estimated fetal weight (efw) less than the 10th centile
- fetal growth appropriate for parental size and ethnicity
- usually normal placental function
- other causes include chromosomal or intrauterine infections
fetal growth restriction/intrauterine growth restriction (fgr/iugr)
- decrease in fetal growth prevents fetus from reaching full growth potential
- placental insufficiency is the leading cause
- other causes: infection, aneuploidy, structural anomaly, maternal disease/smoking
- adaptive changes take place in placenta and fetus
- monitoring is vital to time delivery
umbilical cord anatomy
- there are initially four umbilical vessels – 2 arteries and 2 veins
- typically the right umbilical vein atrophies to leave a 3 vessel cord
- despite being extremely flexible , the cord protects it’s vessels from kinking or rupture with whartons jelly and collagen fibres.
- uas wrap around uv about 0.5 coils/cm.
- cord length varies – range of 35-70cm at birth.
- normal cord diameter is less than 2cm.
umbilical artery (ua)
indications for scanning the umbilical artery
provides information on the fetal side of the placenta
- suspected or known sga
- ac less than the 5th centile
- discrepancy between the hc and ac
- efw less than the 10th percentile
- decreased fetal movements
- maternal hypertensive disorders- preeclampsia
- indicator for cord pathology
scanning technique and equipment setup
- measure during fetal inactivity.
- identify free loop of cord on colour doppler
- high scale (prf) to reduce aliasing
- most accurate doppler angle= 0 degrees
- doppler gate 2-5mm
- optimise doppler baseline and prf to get a large waveform
- 5 consecutive waveforms recorded
- report the best measurement out of three taken, not an average.
normal – umbilical artery
umbilical artery pulsatility index (pi)
peak systolic velocity minus diastolic velocity divided by the mean velocity
pi=psv-dv/mv
pathology – umbilical artery
there should not be any reversal of blood in diastole.
the sd ratio , pi and ri should be assessed to determine if there is increased resistance.
this mother had pregnancy induced hypertension. this fetus was 23 weeks gestation ; growth restricted with reversal of the umbilical artery diastolic flow. there was fetal adaptation with cerebral redistribution. with these findings the fetus is at risk of premature delivery and a poor diagnosis.
middle cerebral artery (mca)
role of ultrasound /indications
mca peak systolic velocity
maternal-fetal isoimmunisation
- mum and baby with different blood types, mum’s immune system produces antibodies that pass back through placenta and attack baby’s red blood cells.
suspicion of fetal anaemia
- could be due to isoimmunisation, infection,syndromes, placental tumours , sacrococcygeal teratoma
- severe anaemia – the heart tries to overcome inadequate number or quality of red blood cells so pumps harder and can lead to heart failure
anaemia due to feto maternal haemorrhage
- most common antenatal presentation is decreased fetal activity or persistent maternal perception of decreased fetal movements.
- accidental trauma to the abdomen and uterus can lead to an acute bleed from the fetal chorionic villi into the maternal circulation.
- placental abruption
unexplained hydrops – severe oedema of the baby
- immune hydrops : mum’s immune system causes a breakdown in baby’s red blood cells.
- non immune hydrops : more common . diseases or complications interfere with baby’s ability to manage fluid.
mcda twins with suspicion of ttts or taps
- twin to twin transfusion syndrome (ttts)
- twin anaemia polycythemia syndrome(taps)
ttts : imbalance of fluid components of the fetal blood.
- donor twin compensates for blood loss by decreasing urine production.
- recipient twin compensates for the extra blood by increasing urine production.
taps : imbalance of the red blood cells.
- donor twin low blood count – anaemia.
- recipient twin thick blood.
- amniotic fluid levels remain normal.
mca pulsatility index
- all fetuses over 34 weeks
- abdominal circumference less than 10th centile
- estimated fetal weight less than the 10th centile
- afi less than 5cm or max vertical pocket less than 2cm
- umbilical artery pi greater than 95th centile
- mcda twin gestation with ttts
mca scanning technique
- measure during fetal inactivity.
- bpd view of the brain and move slightly caudal
- colour flow mapping to locate circle of willis
- coronal suture as an acoustic window.
- assess the mca closest to the transducer using hd zoom
- doppler angle 0 degrees
- small doppler gate 0.5-1mm
- measure 2mm from mca origin.
- optimise doppler baseline and prf to get a large waveform
- fewer than 10 consecutive waveforms recorded
- report the best measurement out of three taken.
mca limitations
- if the fetus’ head cannot be imaged in a decubitus position, then it is impossible to obtain an accurate velocity measurement with an angle greater than 60 degrees. therefore in the later stages of the pregnancy if the head is low and in an occipital anterior or occipital posterior position, the mca cannot be accurately measured.
- a transvaginal approach may be required.
normal – mca
normal pregnancy
- high resistance mca
- low diastolic flow
this image shows the middle cerebral artery at 90 degrees to the beam. the velocity is most accurate the closer the trace done to zero degrees.
the mca has a straight upstroke and is a high resistance waveform with forward flow in systole and diastole.
mca charts
ebbing c, rasmussen s, kiserud t. middle cerebral artery blood flow velocities and pulsatility
index and the cerebroplacental pulsatility ratio: longitudinal reference ranges and terms for
serial measurements. ultrasound obstet gynecol, 2007. 30(3): 287-9
if the mca is less than the 5th centile this is significant.
pathology – mca
pi less than 5th percentile is abnormal
iugr pregnancy
- placental resistance to blood flow
- decreased flow through ua
- decreased blood volume redistributes blood to vital organs such as brain, heart , kidneys
- mca vasodilation , increased diastolic flow
- decreased pi
fetal anemia
high mca velocities over the course of 45 minutes. ranging between 102 and 120cm/sec with normal pi.
this baby had come into hospital at 36 weeks 5 days with reduced fetal movements. she had a normal scan the week before. the size of the baby and the ductus venosus and cord dopplers were all normal. no other abnormalities were identified, however the baby did not move for 45 minutes. the obstetrician performed an emergency caesarian and the baby was anemic and required an immediate blood transfusion.
there is reversal of flow in the mca which increases the risk of cerebral haemorrhage and fetal death in utero in cases where it is sustained. there is a high pi also.
ductus venosus (dv)
role of ultrasound / indications
- the dv plays a critical role in delivering blood rich with oxygen and nutrients from the placenta to the fetal heart.
- the dv is a branchless, hourglass‐shaped vessel narrowing to 1–2 mm, approximately one third of the width of the uv so 25% of blood is shunted to the ivc under pressure , propels blood into the ra and directly into the la.
scanning technique
in early morphology scanning
- a right mid-sagittal view of the fetal trunk , colour flow mapping used to demonstrate the uv, dv and heart.
- sampled at the inlet to dv
- small sample volume 0.5-1mm
- doppler angle less than 30 degrees
limitations
- if the ductus venosus is parallel to the ultrasound beam it is impossible to gain an accurate spectral doppler trace.
normal – ductus venosus 1st and 3rd trimester
- because of turbulent flow, the dv often has a “sparkling” appearance which is the result of aliasing.
- should have continuous forward flow towards the heart
the dv waveform relates to atrial pressure and volume changes throughout the entire cardiac cycle, producing a multiphasic flow pattern
pathology – ductus venosus
there is obvious reversal of the a-wave in this 22 week pregnancy.
uterine arteries
role of ultrasound / indications
provides important information on the conversion process of spiral arteries into uteroplacental arteries
provides flow resistance information on the maternal surface of the placenta
preeclampsia screening in the first and second trimester
fetal growth restriction (iugr) in early pregnancy
patients with pregnancy induced hypertension
early sga fetuses at 20 weeks
preeclampsia is a major cause of maternal and perinatal morbidity and mortality , affecting 2-3 % of all pregnancies.
in a normal pregnancy
placental cells invade the inner third of the myometrium –
the entire length of the maternal spiral arteries dilate to optimise delivery of oxygen and nutrients to the fetus.
in women with preeclampsia
there is a failure of trophoblast invasion
the spiral arteries maintain muscle elastic coating so impedance to blood flow persists.
limitations
- there should not be any limitations to scanning however a half full bladder improves the visualisation of the uterine arteries.
patient preparation
- at least 200mls in the bladder will improve visualisation in the first trimester.
scanning technique
trans abdominal scanning in first trimester
- mid sagittal on the uterus and cervix (half full bladder)
- keep probe in the midline and angle slightly lateral until paracervical vascular plexus is seen.
- measure ua as it turns cranially to make its ascent to the uterine body.
- psv greater than 60cm/s (to make sure not measuring the arcuate arteries)
- doppler angle less than 30 degrees , (it is easy to get it in complete alignment with blood flow)
- doppler gate 2mm wall filter 30-60 hz
- show 3 consecutive waveforms to measure pi.
trans abdominal scanning – 2nd/3rd trimester
- probe is placed longitudinally in the lower lateral quadrant of the abdomen and angled medially.
- colour flow mapping is used to identify the uterine artery as it is seen crossing the external iliac artery.
- sample volume placed 1cm downstream from this crossover point.
- if the uterine artery bifurcates , measure before this.
- with advancing maternal age, the uterus rotates clockwise so left side not as lateral as the right side.
- doppler angle less than 30 degrees
- doppler gate 2mm wall filter 30-60 hz
- show 3 consecutive waveforms to measure pi.
uterine artery – normal
in the non-pregnant state there is rapid rise and fall in uterine artery flow velocity during systole.
there is a notch in the descending waveform in early diastole.
at 12 weeks (diastolic notch up to 16 weeks)
normal uterine artery flows at 19 weeks gestation
normal uterine artery waveforms in 3rd trimester. note the rising diastolic flow
uterine artery – pathology
preeclampsia- there is a notch which persists and low diastolic flow, showing increased impedance.
preeclampsia – note the reversed flow in early diastole due to the very high resistance to flow.