A Lecturer; Week 10 MED1022; Anatomy
Cardiac defects are 20% of all congnital defects. Fetal circulation only becomes a problem after birth. Defects can be cyanotic or acyanotic. Cyanotic (more than 3g/dL of reduced haemoglobin) is when blood is not sufficiently oxygenated and gives a purple or blue tinge in areas of dense superficial capillary circulation for example where there is a right to left shunt. In week 3, the heart tube begins to form, and is easily affected by teratogens. It occurs when lateral plate mesoderm gives rise to precardiogenic mesoderm. At day 19, a pair of endocardial tubes develop in the cardiogenic region, aorta begins to develop. Late in week 3, the two endocardial tubes are brought into the thoracic region where they meet at the midline and fuse, with apoptosis occuring between the two. Leads to formation of the first aortic arch. Initially, 6 vessels empty into the heart (cardinal, vitelline, umbilical).
By day 21, constrictions (sulci) and expansions appear which divide the heart into rough regions that become the chambers. Upper portion is bulbis cordis, lower is primitive atria, middle is primitive ventricles. By day 22, splanchopleuric mesoderm invades and differentiates into cardiac jelly and myocardium as well as epicardium. By day 28, the primary heart tube is suspended in the primitive pericardial cavity by dorsal mesocardium and the four chambers are brought into spatial arrangement with each other. At day 28 the dorsal mesoderm which held the heart in position is ruptured, heart is then suspended by vessels.
Heart looping results in an S shape, if loops wrong heart can end up on the right side of the body (dextrocardia). In the fourth week there is partitioning between the atria and ventricles. Over the next two weeks, endocardial cushions form the AV valves, and the atrium divides to two by the formation and modification of two septa. At the end of the 4th week the heart begins to contract. Septum primum forms a primordial AV septum between the cushions, before fusion there are perforations from apoptosis, the foramen secundum is formed during fusion of primum. In the 5/6th week this forms the foramen ovale. There is left to right shunt through ovale. At this time the ventricles are being partitioned by a median muscular ridge. By the seventh week the ventricles divide and the pulmonary trunk and aorta are matched to their chambers. As the heart develops, it twists.
One defect is transposition where the pulmonary artery attaches to the left ventricle and aorta to the right ventricle. This will lead to death because of insufficient oxygenation. Valves can be malformed (stenosis/bicuspid/regurgitation), causes murmurs. Atresia is where the opening is occluded, this results in death. Shunts can be inserted but this is associated with poor function.
Ductus venosus allows oxygenated blood from umbilical cord to the heart, foramen ovale between left and right atria, bypassing the pulmonary circuit, ductus arteriosus is where blood in right atrium mixes with poorly oxygenated blood from SVC dumping it to the aorta.
At birth the foramen ovale closes due to increased pressure in the left atria compared to the right because of opening of lungs, ductus arteriosus constricts, and umbilical vessels including the ductus venosus constricts.