Describe the development of the heart. Explain the septation of the atria and ventricles and add a note on the congenital anomalies.

Early Development

• The smooth parts of the atria come from absorbed venous structures — the right from the sinus venosus, the left from the absorbed pulmonary veins; the rough parts are the original atrium
• The crista terminalis marks the junction of the smooth and rough parts in the right atrium — visible on the interior of the adult heart

Septation of the Atria

• Blood can pass right to left before birth but not left to right, because the flap opens only one way — the whole point of the arrangement
• Probe patency of the foramen ovale persists in about 25% of adults without any functional consequence

Septation of the Ventricles and Outflow

• Muscular interventricular septum grows up from the apex, leaving the interventricular foramen
• The membranous part closes it last, formed from the right and left bulbar ridges and the inferior endocardial cushion, at about the 7th week
• TRUNCO-conal (aorticopulmonary) septum — formed by neural crest cells, it grows in a spiral, which is why the aorta and pulmonary trunk twist round each other
• Endocardial cushions form the atrioventricular valves and contribute to both the atrial and ventricular septa

Fate of the Aortic Arches

• The asymmetry of the recurrent laryngeal nerves follows from this — each hooks round the sixth arch of its side; on the right the sixth disappears so the nerve ascends to the fourth arch, the subclavian artery, while on the left it stays with the ductus arteriosus, hooking round the arch of the aorta

Development of the Venous System

• The venous system is at first symmetrical and becomes asymmetrical by the development of left-to-right anastomoses — the blood is shunted to the right side, and the left channels regress
• This is why the left brachiocephalic vein is longer and crosses the midline
• Anomalies include a persistent left superior vena cava draining into the coronary sinus, and a double or left-sided IVC — important to recognise before retroperitoneal surgery

Applied Aspects

• Ventricular septal defect — the commonest congenital cardiac anomaly; a left-to-right shunt, and if large, pulmonary hypertension and eventual eisenmenger syndrome with shunt reversal and cyanosis
• Atrial septal defect — ostium secundum is the commonest; ostium primum is a defect of the endocardial cushions and is associated with Down syndrome
• Tetralogy OF FALLOT — from unequal division of the truncus by an anteriorly displaced conal septum: pulmonary stenosis, VSD, overriding aorta and right ventricular hypertrophy. The commonest cyanotic lesion beyond infancy
• Transposition of the great arteries — the aorticopulmonary septum fails to spiral, so the aorta arises from the right ventricle; incompatible with life unless a shunt exists
• Persistent truncus arteriosus and other outflow anomalies are neural crest defects, and occur with DiGeorge syndrome (22q11 deletion)
• Dextrocardia results from looping to the left; harmless when part of complete situs inversus, but serious with isolated dextrocardia
• Coarctation of the aorta — narrowing near the ductus; gives upper limb hypertension with weak, delayed femoral pulses and rib notching from collateral intercostal arteries
• Patent ductus arteriosus — the sixth arch derivative fails to close; a continuous machinery murmur, common after congenital rubella and in prematurity
• Double aortic arch and aberrant right subclavian artery — vascular rings from persistence of arch segments that should regress; they compress the trachea and oesophagus, causing stridor and dysphagia lusoria
• Ostium primum defect with an atrioventricular canal is a failure of the endocardial cushions and is strongly associated with Down syndrome
• Ectopia cordis — failure of the ventral body wall to close, so the heart lies outside the thorax; usually fatal
• The heart is the organ most often malformed, affecting about 8 per 1000 live births, because its development is so complex and so compressed in time
• Fetal echocardiography at 18 to 22 weeks detects most major structural lesions and allows delivery to be planned where surgery is available
• Maternal diabetes and rubella are the two environmental causes most often identified, and both are preventable