Basic Cardiac Embryology & Fetal Circulation

Timeline of Events

Week 3 (21 days) - heart tube forms 23 days- heart beats Week 4 - cardiac loop forms Week 7 - heart fully developed

– the 1st organ to fully develop in the fetus– resembles adult heart except for foramen

ovale

approx. 3wks approx. 4 wks approx. 7 wks

Heart tube cardiac loop forms fully developed

Primitive Cardiac Tube(bulboventricular tube) Consists of 6 parts

functioning as a tubular pump:– sinus venosus– primitive atria– primitive ventricle– bulbus cordis– conus cordis– truncus arteriosus

Cardiac Loop

As the tube grows it bends in an anterior & rightward direction forming the bulboventricular loop.

Normal looping = dextro or ‘d-looping’

Abn. looping to left = levo or ‘l-looping’

What forms what

Sinus venosus (1) Primitive atria (2)

– 1 & 2 contribute to SVC, IVC, CS, RA & LA Atrioventricular canal (3)

– large area of connection between primitive atria & ventricle

– endocardial cushion will form here; will form parts of MV & TV

More...

Primitive ventricle (4) - forms into LV Bulbus cordis: 3 parts

– Primitive RV (5)– Conus cordis (6) - will form the ventricular

outflow tracts (crista supraventricularis)– Truncus arteriosus (7) - will form the aorta

& pulmonary artery trunk

Aortic arches

6 paired sets of arches form about week 4 These develop into the adult arterial

system– 3rd set forms common & internal carotid

arteries bilaterally

– 4th develops into aortic arch

– 6th develops into right & left pulmonary arteries & ductus arteriosus

Septation(septal development) Atrial septum:

– Atrial septum begins from the atrial roof; septum primum & septum secundum separate common atria into rt & lt halves

Endocardial cushions:– divide the AV canal into 2 orifices & parts of

MV & TV; help form part of the atrial primum septum & membranous ventricular septum

Valves

AV valves (mitral & tricuspid) are formed mainly from the internal ventricular muscular wall

Aortic & pulmonic roots are formed from a separation that occurs in the truncus arteriosus. The semilunar valves are formed from small tubercles in the truncus.

Fetal circulation (prenatal)

In the adult, the lungs provide oxygen and CO2 exchange

In the fetus, the lungs are basically collapsed & fluid-filled so there is high resistance to blood flow

The placenta provides oxygen for the fetus;delivers nutrients & removes wastes

Fetal Post-natal

Right-sided pressure higher

Higher pulmonary resistance

3 shunts exist Placenta provides

oxygenated blood IVC blood is O2 rich SVC,CS is O2 poor

Lungs inflate; lowers pulm resistance/ Rt-heart pressures

Lt-heart press. rises; closes foramen ovale

Ductus arteriosus closes w/in 48-72 hrs; becomes ligamentum

Ductus venosus closes as flow ceases; becomes ligamentum

Changes at birth

When newborn begins to breathe, the baby’s body gets higher levels of O2

Pulmonary vascular resistance decreases, blood flow into the lungs increases

LAP rises; closes foramen ovale Increases O2 levels, lower vascular

resistance closes ductus arteriosus

Continued..

Clamping umbilical cord ends placental function; closes umbilical vein & ductus arteriosus

In premature infants, PFO & PDA are common

Key to diagram1. Aortic arch 11. Portal sinus

2. Ductus arteriosus 12. Portal vein

3. Pulmonary trunk 13. Umbilical vein

4. Pulmonary veins 14. Umbilical arteries

5. LA 15. Placenta

6. SVC 16. Descending aorta

7. Foramen ovale

8. RA

9. IVC

10. Ductus venosus

Sources

CV Principles: A Registry Prep, Reynolds Echocardiography, 2nd edition, Allen Textbook of Diagnostic Ultrasound, 5th

edition, Hagan-Ansert DeWitt Feigenbaum’s Echocardiography, 6th

edition.