fetal heart – normal
the heart is a rounded structure and should be assessed from multiple angles and positions to ensure that even the smallest abnormality is not missed. the size of the heart more than doubles from 13 weeks to 19 weeks therefore there are anomalies which may not be diagnosed at the early structural scan.
heart beat
transvaginal ultrasound can detect a heart beat as early as 34 days of gestation. before
6 weeks, the heart rate averages between 100 and 115 beats per minute.
the embryonic heart-rate peaks at 8 weeks gestation between 144 and 159 bpm, and after 9 weeks the rate plateaus at 137 to 144 bpm. the normal fetal heart rate is between 115 and 160 bpm.
(ref 1,2,3,4,5,6)
pulsed doppler may be used to assess the fetal heart rate in second trimester as long as the alara principle is adhered to. fetal rhythm abnormalities, which include fetal heart rates that are irregular, too fast or too slow, occur in up to 2% of pregnancies.(7)
m-mode should be used in fetuses less than 11 weeks gestation due to the alara principle.
situs
it is important at the commencement of any scan following visualisation of a heart beat is the determination of the left side of the fetus and the correct position of the heart axis and the stomach. demonstrated above is the dual images labelling the left side, stomach and heart.
the right ventricle should be the most anterior part of the heart and it can be determined by looking for the moderator band.
fetal heart anatomy colour
four chamber heart
la= left atrium
lv= left ventricle
ra= right atrium
lv left ventricle
the transverse view should only contain 1 rib. if you can only get a 4 chamber view with more than this it could be a sign of cardiac pathology. the heart should occupy approximately 1/3 of the chest. the apex is orientated 45 degrees to the left. the right ventricle contains the moderator band.
anatomy of the heart
the atrioventricular valves are seen closed with the tricuspid valve on the right between the right atrium and rt ventricle, the bicuspid or mitral valve is the valve on the left between the left atrium and left ventricle.
in a fetus there is a shunt that bypasses the lungs and it is called the foramen ovale. this shunt moves blood from the right atrium of the heart to the left atrium.
outflow tracts
the pulmonary artery must bifurcate whereas the aorta does not bifurcate until the level of the neck vessels. these images show the pulmonary artery tracking behind the ascending aorta.
the lvot must cross the heart beneath the rvot as it is ascends. the probe is angled cephalad.
3 vessel view
the 3 vessel view (3vv) or pav (pulmonary artery ascending aorta and superior vena cava) view.
the 3 vessels should be positioned in this sequence of reducing size.
.the pulmonary artery (pa), left and right pulmonary artery (lpa and rpa), superior vena cava (svc) and aorta (ao) are all positioned normally in this view.
the right ventricular outflow tract (rvot) should come directly posteriorly from the rt ventricle towards the spine.
the transverse portion of the aorta turns back to the right and joins the rvot forming an “arrowhead” hence the name. the aorta should be seen descending on the left side anterior to the spine. the oesophagus should be seen (red dot) lateral to the transverse aorta.
interventricular septum
ivs (inter-ventricular septum):
should be assessed when the foetus is in a decubitus position so the ultrasound beam is perpendicular to the septum.
this will avoid anisotropy and a false positve for septal defect.
it should be assessed in both bmode and colour doppler.
this image shows the foramen ovale closed between the atria and the interventricular septum in orange.
a colour or power doppler image should be taken after watching the heart dynamically and moving up and down the septum to assess for any holes .
the foramen ovale should be assessed for both the septum primum and secundum with colour flow traversing between the atria.
arches
the aortic arch arch arises from the central heart and is a “walking stick” shape. the neck vessels can be seen arising from the transverse portion of the aorta.
the ductal arch is seen in a sagittal view from the rt ventricle anteriorly . it is a “hockey stick” shape with the ductus making the superior portion appear flat . whereas, the aortic arch is a “cane stick” shape with the aorta arising from the left ventricle in the centre of the heart.
scan protocol
role of ultrasound
to confirm normal anatomy to the best of our ability.
to progress, or elaborate on, known foetal heart pathology.
limitations
with patience, the difficulties posed by foetal position can usually be overcome.
patient preparation
there is no need for the patient to have a full bladder. ensure they are comfortable.
equipment setup
depending on the gestational age and maternal habitus, a curvilinear probe between 3-9mhz.a linear probe may be used on thin patients.
if 3rd trimester with very large maternal habitus, a 2.5mhz annular array may be needed.
low dynamic range b-mode
high prf colour and doppler settings with low persistance and a high frame rate.
(most machines will have an adequate factory preset, fetal heart setting)
common pathology
- an interventricular septal defect
scanning technique
ten steps in imaging heart
1st step: check the heart is beating. m-mode heart rate – should be between 120 and 180 beats per minute
2nd step: situs- check which is the left side of fetus then do a dual image in a tranverse axial plane of the fetus with firstly the thorax showing the heart apex orientated to the left at an angle of approximately 45degrees. the transverse section should only contain 1 rib. the second image showing the stomach on the left ensuring the left and right side is labelled.
3rd step: four chamber view- angling cephalad from a transverse axial view of the abdomen.
the heart should occupy approximately 1/3 of the chest.the ventricles should be of similar size and the atria should be of similar size. the moderator band is in the right ventricle. the left ventricle extends more apically.
assess the av valves (atrioventricular) ie the tricuspid valve on the right is more apical than the mitral (on the left)valve insertion onto the interventricular septum. (the “offset cross” appearance)
watch ,in real time, the opening and closing of the valves in systole and diastole.
the pulmonary venous connections can be identified.
4th step: lvot
from the 4chamber view, angle further cephalad to see the left ventricle and the aorta (left outflow tract) in the same view.
the aorta will be coursing to the right posterior direction.it should be assessed in colour doppler also looking for any stenosis.
5th step: rvot from the lvot view, the probe is angled further towards the head and slightly towards the fetal left shoulder.
this show the pulmonary trunk heading directly posteriorly towards the spine. it will divide into the pulmonary arteries.
image and look in bmode and colour doppler.
6th step: 3 vessel view this view is a slightly oblique, axial view.
it cuts the upper part of the arches and transversally the superior vena cava.
this is an image with
a full length view of the pulmonary artery (p) which arises from the right ventricle.
a cross section of the ascending aorta (a)
the superior vena cava (v) or (s)
it is commonly labelled pav or pas on the image.
it is important to have the 3 vessels in line with each other in order of largest (p) to smallest (v).
the aorta and pulmonary artery must be perpendicular to each other, otherwise there is a serious heart defect such as transposition of the great vessels.
7th step : arrowhead
the pulmonary artery and the transverse aorta will merge posteriorly and should be an acute angle not curved/rounded.
8th step: interventricular septum
ivs (inter-ventricular septum):
should be assessed when the foetus is in a decubitus position so the ultrasound beam is perpendicular to the septum.
this will avoid anisotropy and a false positve for septal defect.
it should be assessed in both bmode and colour doppler.
9th step: aortic arch
the ‘arches’ are best assessed when the foetus is prone.
aortic arch: turn the probe 90degrees to a para-sagittal plane on the foetus.
the aortic arch arises from the centre of the heart and is commonly referred to as a “cane”.
coarctations may be visualised in this view.
10th step: ductal arch
this is the ductus arteriosis: the junction between the pulmonary trunk and the aorta.
utilise a similar scan plane to the aortic arch.
the ductal arch is referred to as a “hockey stick” appearance, with the arch arising from the anterior of the heart. bmode and colour assessment.
references
1. coulam cb, britten s, soenksen dm. early (34-56 days from last menstrual period) ultrasonographic measurements in normal pregnancies. hum reprod1996; 11:1771-74. 61
2. laing fc, frates mc. ultrasound evaluation during the first trimester of pregnancy. in callen, pw (ed): ultrasonography in obstetrics and gynecology (4th ed), philadelphia: wb saunders 2000; 105-45.
3. merchiers eh, dhont m, de sutter pa, beghin cj, vandeker- ckhove da. predictive value of early embryonic cardiac activity for pregnancy outcome. am j obstetgynecol1991; 165:11-14.
4. britten s, soenksen dm, bustillo m, coulam cb. very early (24-56 days from last menstrual period) embryonic heart-rate in normal pregnancies. hum reprod1994; 9:2424- 26.
5. doubilet pm, benson cb. embryonic heart-rate in the early first trimester: what rate is normal? j ultrasound med 1995; 14:431-34.
6. pildner von steinburg s, boulesteix a, lederer c, grunow s, schiermeier s, hatzmann w, schneider km, daumer m. 2013. what is the “normal” fetal heart rate? peerj 1:e82 https://doi.org/10.7717/peerj.82)
7.hornberger, l. k., & sahn, d. j. (2007). rhythm abnormalities of the fetus. heart (british cardiac society), 93(10), 1294–1300. https://doi.org/10.1136/hrt.2005.069369