Diagnostic ultrasound ( PDFDrive )

1274 PART IV Obstetric and Fetal Sonography
A
B
FIG. 37.1 Heart Position and Axis. (A) Normal position and axis of the heart. The heart is predominantly in the left side of the chest, with
the apex of the heart pointing leftward. Dual-screen image shows the stomach also on the left side. (B) Dextroposition of fetal heart caused by
a large, congenital pulmonary airway malformation. Transverse image through the fetal chest shows the heart displaced to the right, but the
apex (arrow) remaining leftward. LA, Left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle; S, stomach. Dual-screen image shows the
stomach on the correct left side.
across the foramen ovale to the let atrium and then into the let
ventricle, the aorta, and the fetal brain. Poorly oxygenated blood
from the superior vena cava (SVC) also enters the right atrium,
and mixes with the blood entering from the IVC, but continues
to the right ventricle and pulmonary artery. Most of this blood
is directed through the ductus arteriosus into the descending
aorta. hus these shunts function so that the majority of output
from both ventricles enters the systemic circulation, rather than
a substantial portion entering the pulmonary circulation, as in
the adult. Blood that enters the pulmonary vasculature via the
pulmonary artery is returned to the let atrium by the four
pulmonary veins. From the let atrium it enters the let ventricle
and ultimately the descending aorta, returning to the placenta
via the iliac and umbilical arteries. Normal values for measurements
of the fetal heart and great vessels are shown in Figs. 37.3
and 37.4.
Fetal echocardiography is best accomplished at 18 to 22
weeks of gestation. 28 Before 18 weeks, resolution is frequently
limited by the small size of the fetal heart. Ater 22 weeks the
examination may be compromised by progressive ossiication
of the fetal skull, spine, and long bones; the relatively smaller
amniotic luid volume; and unaccommodating fetal position.
Importantly, some congenital cardiac abnormalities progress in
utero and may be subtle or unrecognizable at or before 22 weeks
but more obvious closer to term. 29,30 Tachyarrhythmias may not
become apparent until the third trimester. 31 In some cases, irsttrimester
evaluation of the fetal heart may be accomplished with
transvaginal ultrasound as early as 11 to 14 weeks. 32-34 More
recently, diagnostic results have been possible with a transabdominal
approach at 11 to 13 weeks. 34 However, irst-trimester
fetal echocardiography is limited and should be considered an
adjunct to second-trimester evaluation, not a replacement.
Scanning the fetal heart requires a systematic approach,
beginning with determination of the position of the fetus within
the uterus and the heart within the fetal chest. A transverse view
through the fetal thorax above the level of the diaphragm