**5. The transition from fetal to neonatal circulation**

**d.** In AV canal defects the AV node and the coronary sinus are displaced inferiorly, and the AV node is placed in between the coronary sinus and the crest of interventricular septum at the so called 'NODAL triangle' rather than at the usual apex of the triangle of Koch. [12]

Transient damage to the conduction system can occur as a result of myocardial protection during cardiopulmonary bypass, which should usually recover in less than 7 days. A perma‐

The circulatory system evolved as the simple process of diffusion of nutrients from amniotic

The placenta does the function of oxygenation in the fetus as the lungs are bypassed by presence of ductus. The arrangement of circulation in the fetus is such that the more oxygen‐ ated blood goes to the head and the less oxygenated blood goes through the ductus into

The umbilical vein brings the oxygenated blood which bypasses the liver through the ductus venosus and this blood coming into the IVC is preferentially streamed by the Eustachian valve into the left atrium through the foramen in the atrial septum and this oxygenated blood enters

The more deoxygenated blood from the SVC goes into the right ventricle which is pumped by the RV into the PA and as the fetal lungs are collapsed with very high resistance in the pulmonary circulation, the blood bypasses the lungs and goes into the descending aorta

A few lesions provide insights, which can evolve into more complicated congenital heart pathologies. Absence of foramen is one such lesion, which has been linked to the development of hypoplastic left heart syndrome, the development of which can be prevented by dilating

Atresia of the pulmonary and aortic valve can also be intervened by ballooning to prevent the development of its sequel which can be hypoplastic left heart and RV dependent coronary

Absence of ductus arteriosus is a condition which is associated with absent pulmonary valve The absence of ductus could be the primary condition which leads to RV output regurgitating back into ventricle due to high fetal pulmonary vascular resistance, with the VSD partially decompressing the right ventricle. The increased right ventricular output causes the main and the branch pulmonary arteries to dilate. This could extend into the lungs, compressing the airways, and thereby causing severe respiratory compromise in neonatal period as seen in

circulation in cases of pulmonary atresia with intact ventricular septum.

extreme cases of TOF with absent pulmonary valve.

nent pacemaker is usually needed if sinus rhythm does not return in 9-10 days

sac is no longer able to meet the metabolic needs of the growing embryo.

**4. Fetal and neonatal circulation**

232 Principles and Practice of Cardiothoracic Surgery

descending aorta and into the umbilical arteries.

through the ductus.

the foramen ovale.

the left ventricle and from there to the aorta and arch vessels.

The first breath causes the lungs to expand and oxygen content in the blood to increase and this provides the impetus for the ductus to constrict and close. The right ventricular output goes to the lungs and reaches the left atrium, the increased pressure in the left atrium causing the flap valve to shut, closing the foramen ovale. The umbilical vein and ductus venosus regress to leave the vestigial ligamentum teres and the ligamentum venosum.

Maintaining the patency of ductus is critical to many potential fatal neonatal conditions. These can be any of the conditions in which the output of the heart through either the aorta or the pulmonary artery is critically reduced and the patent ductus maintains the flow from one great artery to the other. These are hypoplastic left heart syndrome, critical aortic stenosis and pulmonary atresia. The presence of ductus is also useful in mixing lesions like TGA though the degree of mixing is much better in the presence of atrial level communication. Patency of ductus can be maintained by Prostaglandin E1 infusion, the availability of which has enabled the stabilization of many sick neonates before subjecting them to surgery [13].
