EMBRYOLOGY OF HEART AND FETAL CIRCULATION

Dr.Archana Koshy

• Formation of the cardiovascular system begins during the 3rd week of gestation and is completed by the end of the 8th week

• In the third week of development, the germ disk has the appearance of a flat oval disk and is composed of two layers: THE EPIBLAST AND THE HYPOPLAST.

• The first faces the amniotic cavity and the latter faces the yolk sac.

• A prim itive groove, appears at approximately 16 days of development and extends half the length of the embryo. Serves as a conduit for epiblast cells that migrate inwards toward the hypoblast and replace it to form the endoderm

EMBRYONIC FOLDING • The flat germ disk transforms into a tubular structure during

the fourth week of development • Through a process of differential growth causing the embryo to

fold in two different dimensions:

1. Craniocaudal axis due to the more rapid growth of the neural tube forming the brain at its cephalicend. Growth in this direction will cause the embryoto become convex shaped.

2. Lateral folding causing the two lateral edges ofthe germ disk to fold forming a tube-like structure

• Prior to embryonic folding, angiogenic cell clusters coalesce to form capillaries in mesoderm of the germ disk.

• The blood vessels on either side of the neural tube join at their cranial end.

• As the embryo folds in its lateral dimension, it causes the embryo to acquire a tubular form

• The two outer endocardial tubes will start fusing cranially to caudally, thus forming a single median tube—the primitive heart tube.

PRIMITIVE HEART • The single tubular heart develops many

constrictions outlining future structures.

• Looping of the primitive heart occur on approximately day 23 of development .

• As the heart tube loops, the cephalic end of the heart tube bends ventrally, caudally, and slightly to the right.

• The bulboventricular sulcus becomes visible from the outside, and from the inside a primitive interventricular foramen forms.

• The bulbus cordis forms the right arm of the U-shaped heart tube and the primitive ventricle forms the left arm

1. The atria and inlet portion of the ventricle enlarge and the AV junction lags behind.

2. The sulcus tissue to invaginate into the ventricular cavity, forming a hanging flap.

3. The endocardial cushion tissue is located at the tip of this flap, which is formed from three layers.

4. The inlet portion of the ventricles then becomes undermined, forming the tethering cords holding the newly formed valve leaflets.

FETAL CIRCULATION

• Blood oxygenated in the placenta is returned by the umbilical veins which enters at the umbilicus and joins the course of the portal vein .

• DUCTUS VENOSUS - Provides a low resistance bypass between the Portal vein and Inferior Vena cava .

- Shunts most of the umbilical venous blood into IVC . • IVC directly enters the right atrium .

KEY POINTS 1. Presence of placental circulation which provides for

the gas exchange in the fetus .

2. Absence of gas exchange in the collapsed lungs

3. DUCTUS VENOSUS

4. Foramen ovale

5. DUCTUS ARTERIOSUS

POST NATAL CIRCULATION • Circulatory adjustments occur immediately

following birth • Brought about by a shift from placental

dependence for gas exchange in the fetus to pulmonary exchange in the neonate .

• Loss of placental circulation causes a sudden reduction of flow through the ductus venosus –closes off .

• Closure of Foramen ovale

NEONATAL CIRCULATION

CONGENITAL CARDIAC ANOMALIES

ATRIAL SEPTAL DEFECT Communication between

the right and left atria. Due to the low pressures

of the atria the lesion is typically asymptomatic until adulthood despite 2-4 times the normal pulmonary blood flow.

Gradual (high output) congestive cardiac failure eventually develops, usually becoming symptomatic by the age of 30.

PLAIN FILM (CXR)CAN BE NORMAL IN EARLY STAGES WHEN THE ASD IS SMALL

1. SIGNS OF INCREASED PULMONARY FLOW (SHUNT VASCULARITY)• Enlarged pulmonary vessels• Upper zone vascular

prominence• Vessels visible to the

periphery of the film• Eventual signs of pulmonary

arterial hypertension

2. CHAMBER ENLARGEMENT • Right atrium• Right ventricle

VENTRICULAR SEPTAL DEFECT

• 20%–40%

• Results in an abnormal hemodynamic communication between the right and left ventricles, causing a left-to-right shunt.

• A large VSD is easily diagnosed on the four-chamber view alone.

TRANSPOSITION OF THE GREAT VESSELS

• The most common cyanotic congenital heart lesion found in neonates, accounts for 5%–7% of congenital cardiac malformations.

• This malposition, in association with stress-induced thymic atrophy and hyperinflated lungs, results in the apparent narrowing of the superior mediastinum on radiographs.

• Produced by a ventriculoarterial discordance in which the aorta arises from the morphologic right ventricle and the pulmonary artery arises from the morphologic left ventricle.

• The volume of the pulmonary flow may be normal in the first few days after birth, but it increases with closure of the ductus arteriosus.

• This increase may be mild to severe, depending on the size of the communication.

• A large communication also leads to enlargement of the heart unless the shunt is balanced or impeded by an obstruction of the pulmonary artery.

TOTAL ANOMALOUS PULMONARY VENOUS RETURN

• Occurs when the pulmonary veins fail to drain into the left atrium and instead form an aberrant connection with some other cardiovascular structure.

1. TYPE I(55%)-The anomalous pulmonary veins terminate at the supracardiac level

2. TYPE II(30%)- involves a pulmonary venous connection at the cardiac level; join either the coronary sinus or the right atrium.

3. TYPE III-always accompanied by some degree of obstructed venous return causes cyanosis early and severe congestive heart failure

4. Type IV- Anomalous venous connections at two or more levels.

• The structure in which the anomalous vein terminates appears dilated.

• All the systemic venous and pulmonary venous blood enters the right atrium .

• The only path for its exit to the left heart is, usually a large atrial septal defect or patent foramen ovale.

• This right-to-left shunt is essential for survival

• The right heart is prominent in TAPVR because of the increased flow volume, but the left atrium remains normal in size.

PARTIAL ANOMALOUS PULMONARY VENOUS RETURN

• Anomalous pulmonary vein that drains any or all of the lobes of the right lung.

• Scimitar vein curves outward along the right cardiac border and empties into the inferior vena cava .

• SCIMITAR SYNDROME(a) Hypoplasia of the right lung with dextroposition of the heart(b) Hypoplasia of the right pulmonary artery, and(c) Anomalous arterial supply of the right lower lobe from the abdominal aorta.

• Many patients with a scimitar vein are asymptomatic and have a normal or near-normal life span.

• Symptoms generally do not occur unless 50% or more of the pulmonary flow shifts from the left to the right.

ENDOCARDIAL CUSHION DEFECTS(4%) • Results from interruption of

the normal development of the endocardial tissues during gestation.

• The endocardial cushion normally forms the lower portion of the atrial septum, the upper portion of the interventricular septum and the septal leaflets of the mitral valve and the tricuspid valve.

GOOSE NECK DEFORMITY : • Deficiency of both the conus and sinus portions ofthe interventricular septum, with narrowing of theleft ventricular outflow tract.•

TETRALOGY OF FALLOT( 10%–11%)

• Uplifting of the cardiac apex because of right ventricular hypertrophy and concavity of the main pulmonary artery.

• Pulmonary oligemia

• More severe the obstruction of the right ventricular outflow tract, the more pronounced that deformity

COARCTATION OF AORTA/DUCTUS ARTERIOSUS • Produced by a deformity of the aortic media and intima, which

causes a prominent posterior infolding of the aortic lumen.• Occurs at or near the junction of the aortic arch and the

descending thoracic aorta.• Luminal narrowing in turn obstructs the flow of blood from the left

ventricle.

CLINICAL MANIFESTATIONS• Congestive heart failure in infancy to hypertension with differential

pressures between the upper and lower extremities in adulthood.

• TWO TYPES : 1. Localized coarctation –Post ductal ( adult ) 2. Tubular hypoplasia- Pre ductal ( infantile )

EBSTEIN’S ANOMALY • Downward displacement of the septal leaflets and

posterior leaflets of the tricuspid valve into the inflow portion of the right ventricle.

• Results in the formation of a common right ventriculoatrial chamber and causes tricuspid regurgitation

• The right atrium becomes enlarged, and a R-L shunt is seen in most patients.

• Cyanosis is caused primarily by the right-to-left shunt.