Heart development

Heart tube and looping

• Forms from cardiogenic splanchnic (lateral plate) mesoderm in front of the oropharyngeal membrane; lateral folding fuses paired tubes into one.
• Beats from about day 22: the first organ to function.
• Segments from tail to head: sinus venosus, primitive atrium, primitive ventricle, bulbus cordis, truncus arteriosus.
• The tube loops to the right (days 23–28), bringing the atrium up and behind the ventricle. Abnormal looping gives dextrocardia; with situs inversus and ciliary dysfunction this is Kartagener syndrome.

Atrial septation

• Septum primum grows down towards the endocardial cushions; the gap below it is the ostium primum. Before that gap closes, holes appear in its upper part by apoptosis to form the ostium secundum.
• Septum secundum, a thicker crescent, grows to the right of the primum and leaves an opening, the foramen ovale. The thin septum primum acts as a flap valve letting blood pass right to left only.
• Secundum ASD (commonest ASD): excessive resorption of septum primum or a short septum secundum, at the fossa ovalis. Gives wide fixed splitting of S2 and a pulmonary flow murmur.
• Primum ASD: endocardial cushion defect; part of the atrioventricular septal defect typical of Down syndrome.
• Probe-patent foramen ovale in about 25% of adults allows paradoxical embolism.

Ventricular septum

• The muscular septum grows up from the floor; the membranous part closes the gap using tissue from the endocardial cushions and conotruncal ridges.
• VSD is the commonest congenital heart defect and usually membranous. It gives a harsh pansystolic murmur at the lower left sternal edge; small muscular VSDs often close spontaneously.

Outflow tract and conotruncal defects

Neural crest cells form the conotruncal ridges, which fuse in a spiral as the aorticopulmonary septum, so the aorta ends up connected to the left ventricle and the pulmonary trunk to the right.

• Tet spells and squatting: in tetralogy, cyanosis depends on the balance between pulmonary outflow resistance and systemic vascular resistance. Squatting kinks the femoral arteries and raises systemic resistance, so less blood shunts right to left across the VSD and more goes to the lungs. Phenylephrine works the same way; vasodilators worsen spells.
• Ebstein anomaly: tricuspid valve displaced down into the right ventricle (atrialised RV); linked to maternal lithium.

Aortic arch arteries

• Normally the right dorsal aorta disappears below the right subclavian. If it persists, a double aortic arch forms a complete vascular ring around trachea and oesophagus: stridor from birth worse with feeds, dysphagia, bilateral oesophageal indentation on barium swallow.
• If the right 4th arch and right dorsal aorta above it disappear instead, the right subclavian arises last from the arch as an aberrant right subclavian artery passing behind the oesophagus (dysphagia lusoria); it is not a complete ring.
• Coarctation of the aorta: narrowing near the ductus, usually just distal to the left subclavian. Associations: Turner syndrome, bicuspid aortic valve, berry aneurysms. Signs: arm hypertension, weak and delayed femoral pulses (radio-femoral delay). Collaterals through the internal thoracic and dilated posterior intercostal arteries erode the inferior borders of ribs 3–8, causing rib notching; ribs 1 and 2 are spared because their intercostal arteries come from the costocervical trunk.

Fetal circulation

• Oxygenated blood leaves the placenta in the umbilical vein, which carries the most oxygenated blood in the fetus (about 80% saturated, PO₂ about 30–35 mmHg).
• About half bypasses the liver through the ductus venosus into the IVC, then the right atrium.
• The crista dividens and IVC valve direct this stream through the foramen ovale into the left atrium, left ventricle and ascending aorta, so the heart and brain receive the best oxygenated blood.
• SVC blood goes to the right ventricle and pulmonary trunk, then mostly through the ductus arteriosus into the descending aorta, because pulmonary resistance is high (collapsed, hypoxic lungs).
• Two umbilical arteries (from the internal iliacs) return blood to the placenta.
• Fetal blood has a lower PO₂ than maternal blood but HbF (binds 2,3-BPG poorly) has higher oxygen affinity, and fetal haemoglobin concentration is higher.

Changes at birth

• Patent ductus arteriosus: prematurity, congenital rubella, high altitude; continuous machinery murmur below the left clavicle. Closed with indometacin or ibuprofen (prostaglandin synthesis inhibitors).
• Duct-dependent lesions: when systemic flow depends on the duct (hypoplastic left heart, critical coarctation) the baby collapses with grey colour and weak femoral pulses as the duct closes on day 2–5; when pulmonary flow or mixing depends on it (pulmonary atresia, transposition) cyanosis worsens. Both need prostaglandin E₁ (alprostadil) to reopen the duct; watch for apnoea.

Remember Spiral septum fails: transposition; septum pushed forward: Fallot; septum absent: truncus.

Exam trap The coronary sinus comes from the left horn of the sinus venosus.

Exam trap Indometacin closes the duct; prostaglandin E₁ keeps it open in duct-dependent lesions.

How FCPS asks about this

• Cause of secundum ASD: excessive resorption of septum primum; fixed split S2.
• Commonest congenital heart defect and site: VSD, membranous part.
• Embryology of tetralogy and why squatting helps: anterior displacement of the infundibular septum; squatting raises systemic resistance.
• Collapsed neonate with tiny left ventricle, or worsening cyanosis in transposition: start prostaglandin E₁.
• Rib notching in coarctation: dilated intercostal collateral arteries eroding ribs 3–8.
• Double aortic arch causing stridor and dysphagia: persistence of the right dorsal aorta.
• Highest oxygen saturation in the fetus: umbilical vein; ductus arteriosus becomes the ligamentum arteriosum.