RDCS Fetal Echo cheat sheet

Fetal heart lies in the left anterior thorax with the apex directed about 45 degrees leftward; a cardiac axis outside roughly 25-65 degrees from midline prompts evaluation for situs, conotruncal, or extracardiac causes.

Four-chamber view: both atria and ventricles are similar in size; the right ventricle is slightly larger and more trabeculated and contains the moderator band; the foramen ovale flap (septum primum) lies on the left atrial side.

The tricuspid valve inserts more apically than the mitral valve; a normal offset is under about 8 mm, and greater displacement indicates Ebstein anomaly.

Cardiothoracic area ratio = heart area divided by thoracic area on the four-chamber view; normal is roughly 0.25-0.35, and values above 0.35 indicate cardiomegaly.

Outflow tracts normally cross at approximately 90 degrees, with the pulmonary artery passing anterior to and over the aorta; loss of crossing suggests transposition or double outlet right ventricle.

Three-vessel view, left to right: pulmonary artery > aorta > superior vena cava, normally decreasing in caliber in that order.

Aortic arch shows a candy-cane shape and gives off head and neck vessels; the ductal arch shows a hockey-stick shape, is more anterior, larger, and gives off no head or neck branches.

Normal fetal heart rate is 110-160 beats per minute; situs is confirmed when the stomach and cardiac apex lie left and the aorta lies left of the spine.

At 11-14 weeks the four-chamber view and outflow tracts can be assessed; tricuspid regurgitation at this gestational age is a marker associated with aneuploidy.

Atrioventricular septal defect: a common atrioventricular valve with an ostium primum atrial septal defect and inlet ventricular septal defect (single crux); strongest association is trisomy 21.

Hypoplastic left heart syndrome: hypoplastic left ventricle with mitral and aortic atresia or stenosis; arch flow is retrograde and systemic output is ductal, so ductal closure is fatal.

Tetralogy of Fallot in fetuses shows a ventricular septal defect with aortic override plus a small pulmonary artery; pulmonary stenosis and right ventricular hypertrophy progress after birth.

Transposition of the great arteries: the great vessels are parallel rather than crossing, with the aorta arising anteriorly from the right ventricle.

Double outlet right ventricle: both great arteries arise predominantly from the right ventricle with an obligatory ventricular septal defect, subclassified by great vessel relationship (including the Taussig-Bing type).

Ebstein anomaly: apical displacement of the tricuspid septal leaflet with atrialization of the right ventricle, severe tricuspid regurgitation, and cardiomegaly; linked to maternal lithium exposure.

Cardiac rhabdomyoma is the most common fetal cardiac tumor; solid, often multiple or septal masses strongly suggest tuberous sclerosis (TSC1/TSC2).

Coarctation suspicion: right ventricle larger than left, small or narrow aortic isthmus, and continuous retrograde transverse arch flow on color Doppler.

Tricuspid atresia and pulmonary atresia with intact ventricular septum produce a hypoplastic right heart; ventricular size and the presence of a ventricular septal defect guide prognosis.

Heart rate thresholds: bradycardia below 110 beats per minute, tachycardia above 180; sustained supraventricular tachycardia is typically 220-260 beats per minute with 1:1 conduction.

Atrial flutter shows atrial rates of 300-500 beats per minute with variable atrioventricular block; flutter waves are demonstrated in the atrial wall by M-mode or Doppler.

Complete atrioventricular block shows dissociation, a normal atrial rate, and a ventricular rate of about 50-70 beats per minute; check maternal anti-Ro/SSA antibodies.

Myocardial performance index (Tei) = (isovolumetric contraction time + isovolumetric relaxation time) / ejection time; normal approximately 0.35-0.5 and rises with systolic or diastolic dysfunction.

Fractional shortening = (end-diastolic dimension - end-systolic dimension) / end-diastolic dimension; normal fetal values are roughly 28-40 percent.

Combined cardiac output is about 450 mL/kg/min, with the right ventricle contributing roughly 55-60 percent because it supplies the placenta through the ductus arteriosus.

Placental insufficiency Doppler pattern: umbilical artery pulsatility index rises, middle cerebral artery index falls (brain sparing), cerebroplacental ratio falls below 1, and the ductus venosus a-wave becomes absent or reversed.

The atrioventricular E/A ratio is below 1 early in gestation and increases with gestational age; failure of this trend suggests diastolic dysfunction.

Hydrops is defined by two or more of ascites, pleural or pericardial effusion, skin edema, and polyhydramnios, and is interpreted together with Doppler indices and myocardial performance index.

ALARA: use the lowest acoustic output and shortest dwell time that yield a diagnostic study; spectral Doppler delivers the highest intensities and should be minimized in the first trimester.

Thermal index (TIS for soft tissue, TIB when bone is in the beam) estimates heating; keep TI at or below 1 and limit exposure, particularly with prolonged Doppler.

Mechanical index estimates cavitation risk (peak rarefactional pressure divided by the square root of frequency); it is displayed and should be kept low even though the gasless fetus is at low risk.

Axial resolution equals spatial pulse length divided by two and improves with higher frequency; lateral and elevational resolution are always poorer than axial resolution.

Frequency selection: 5-8 MHz for most fetal echocardiography; 2-5 MHz when penetration is limited by maternal adiposity or a deep window.

Frame rate increases with a narrower sector, reduced depth, single focal zone, zoom, and lower line density; fetal echo requires at least 30-60 frames per second.

Nyquist limit = pulse repetition frequency / 2; velocities above it alias, corrected by raising the scale or PRF, shifting the baseline, or lowering the transmit frequency.

Power Doppler is angle-independent and does not alias but provides no velocity or direction information; color Doppler requires appropriate gain, wall filter, and scale settings.

Relative output ordering: B-mode < color Doppler < spectral Doppler, which has the highest dwell time and intensity; exposure time should be reduced accordingly.

Indications: family history of congenital heart disease, maternal diabetes, teratogen exposure (retinoids, ACE inhibitors, lithium, NSAIDs), aneuploidy markers, abnormal four-chamber view, arrhythmia, hydrops, monochorionic twins, and assisted reproduction.

Timing: transvaginal scanning at 10-14 weeks for early cardiac anatomy; complete transabdominal fetal echocardiography is best at 18-24 weeks.

Standard views: situs, four-chamber, left and right ventricular outflow tracts, three-vessel view, aortic and ductal arches, short axis, plus color Doppler of septa and arches.

Maternal anti-Ro/SSA or anti-La antibodies: serial fetal echocardiograms every 1-2 weeks from 16-28 weeks to detect PR interval prolongation and complete heart block.

Maternal diabetes: assess for asymmetric septal hypertrophy and associated anomalies; risk correlates with periconceptional glycemic control.

Third-trimester NSAID or indomethacin exposure can constrict the ductus arteriosus, producing right ventricular dilation and tricuspid regurgitation, so serial echoes are required.

Documentation should include two-dimensional sweeps through all views, color Doppler of the arches and septa, M-mode for rhythm assessment, and Doppler indices when function is questioned.

Refer for a dedicated fetal echocardiogram when the four-chamber view or outflow tracts cannot be adequately seen, or when extracardiac anomalies, hydrops, or an abnormal axis is identified.