Congenital heart disease
Congenital heart disease seen in 1% births. Most is due to abnormalities in the development of the heart. Usually the term is used when there are structural abnormalities but strictly it should include the inherited cardiomyopathies and channelopathies affecting cardiac function.
The heart begins to form on day 19 of the embryo's life. It is remarkable in that it is one of the few organs that are required to function almost immediately after they form. In this case rhythmic contractions have started by the time the embryo is 22 days old. By day 24 it will be circulating blood through the developing vascular system of the embryo.
The earliest precursor cells to the heart are in a cardiogenic area that spreads around the rostral and lateral edge of the neural plate. This part of the splanchnopleuric mesoderm not only leads to the endocardial tubes that form the heart but also to the dorsal aortae that will form the major portion of the vascular system.
Formation of the heart itself
The precursor cells congregate into two cardiogenic cords on the dorsal side of the pericardial coelom. These cords then canalise to form the endocardial tubes. Folding of the embryo brings the two endocardial tubes together into the midline of the embryo where they fuse to form a single heart tube.
By day 21 a series of constrictions (sulci) have appeared in the heart tube with the intermediate sections dilating appropriately. These sulci form, from the caudal end up, the sinus venosus, primitive atrium, ventricle, and bulbus cordis.
The sinus venosus is formed from the joining of the left and right sinus horns. These are the regions into which the cardinal and vitelline veins drain. The superior end of the bulbus cordis is often termed the conotruncus. From here will be formed the conus cordis and the truncus arteriosus. The truncus arteriosus will eventually split to form the ascending aorta and the pulmonary trunk.
The ventricle of the endocardial tube will go on mainly to form the left ventricle. Most of the right ventricle being formed from the inferior portion of the bulbus cordis. Therefore it is named either the bulboventricular sulcus or the interventricular sulcus.
During the 22nd day the forming heart is added to by further splanchnopleuric mesoderm that will later go on to form the myocardium and an acellular matrix known as cardiac jelly. The cardiac jelly separates the endocardial tubes from the myocardium.
The heart as we know it is not a tube and will undergo much folding to bring it to its final shape. Elongation begins on the 23rd day. This displaces the bulbus cordis to the right, ventrally and inferiorly. The ventricle moves to the left. The primitive atrium moves superiorly and posteriorly. By the 28th day this is complete. The heart chambers have assumed the correct spatial relationships and now all that remains is to separate the cardiac chambers properly.
The rotation of the chambers (as with other organs) is due to the action of cilia - defects of ciliary function lead to random lateralization, so 50% will have dextrocardia eg Kartagener syndrome.
Remodelling of the venous system
Remodelling of the heart coincides with a remodelled venous system. In the fourth and fifth weeks, blood flow becomes mainly diverted through the right sinus horn. The left sinus horn shrinks and becomes a small venous sac. This sac becomes the coronary sinus through which most of the coronary circulation of the heart will drain.
Through growth of the right side of the atrium the right sinus horn becomes incorporated into the atrial wall. The original atrial wall then gives rise to the right auricle. Through this process of intusussception of the right sinus venosus, the openings of the venae cavae are pulled into the atrial wall along with the opening of the coronary sinus.
A pulmonary vein sprouts from the primitive atrium. This vein bifurcates twice to form four pulmonary veins which grow towards the lungs, where they anastomose with veins developing from the mesoderm of the lung buds.
Intussusception occurs here during the fifth week when the pulmonary trunk and the first two branchings of the pulmonary vein system are taken into the posterior left wall of the wall of the primitive atrium. This leads to the draining of the pulmonary venous system into four orifices of the left atrium.
The atrium undergoes additional remodelling under the influence of the conotruncus. This produces a depression in the roof of the atrium on day 26 that by day 28 has begun to form a wedge of tissue known as the septum primum. As it grows down towards the ventricle it maintains a hole, known as the ostium (or foramen) primum, between the two atria.
In the atrioventricular canal, meanwhile, four endocardial ‘cushions’ develop (left, right, superior and inferior). The superior and inferior cushions fuse to form the septum intermedium by the end of the sixth week. The common atrioventricular canal that existed before is split into left and right atrioventricular canals. Initially, after folding, the atrioventricular canal connects only the right atrium to the left ventricle directly. Similarly it is the right ventricle and not the left that is continuous with the outflow path through the conus cordis and the truncus arteriosus. Therefore a movement of the atrioventricular canal occurs. Between the fourth and fifth weeks the septum intermedium moves across to the right side of the heart. By the time the endocardial cushions have fused to divide the atrioventricular canal into left and right canals the canals become aligned to their correct atrioventricular regions.
Obliteration of the ostium primum
The ostium primum is obliterated when the septum primum grows down to and fuses with the septum intermedium. If this event occurred on its own there would be big problems in the circulatory system. This is because in the uninflated lungs the pulmonary blood flow is severely restricted. In the adult all blood flows first from the right atrium and ventricle to the lungs and back to the left atrium. From the left atrium it is taken in and pumped round the body by the left ventricle. In the new born infant there will only be significant blood flow once the new born infant has taken his first breath and allowed the pulmonary blood vessels to expand to their proper size. Before this time, to enable proper circulation of the embryonic blood supply a right to left shunt exists through the ostium primum. Thus the lungs, which have no function in utero, are bypassed. Obviously were the ostium primum to close then there should be a blood supply problem. This is avoided by the programmed cell deaths of a portion of the septum primum forming the ostium secundum just before the ostium primum closes.
Once the septum primum is well on its way to completion a more muscular septum secundum forms adjacent to it. This septum never completely closes and a foramen ovale remains patent until birth. At birth the opening of the pulmonary blood vessels causes a reversal of the pressure gradient between the two atria. The thin membranous septum primum is pressed against the septum secundum. Because of the staggering of the two foramina this causes a closure of the shunt between the two atria.
A common congenital disease is a failure of the septum secundum to completely occlude the ostium primum. The massive shunting of blood that occurs between from the left to the right atria can be completely asymptomatic in the infant. Eventually however it can lead to enlargement of the right ventricle and the pulmonary trunk. Cardiac failure may follow in later life.
The two ventricles begin their separation at the end of fourth week. The bulboventricular sulcus begins to protrude into the cardiac lumen between the presumptive left and right ventricular chambers. This causes growth of a muscular interventricular septum. This septum does not finish its growth and join with the septum intermedium however as to do so too early would block the outflow of the left ventricle. The final growth stage will occur when the atrioventricular canals have assumed their final position either side of the interventricular septum.
Formation of the valves
Alongside the growth of this septum is the formation of the atrioventricular valves. These are formed by the undermining of the myocardium surrounding the atrioventricular canals. This leads to a posterior and an anterior leaflet or cusp being formed on the respective sides of the canals. In the left atrioventricular canal these form the bicuspid (or mitral, because of its resemblance to a bishop's hat) valve. The right atrioventricular will usually develop a third cusp on its septal edge. It is therefore known as the tricuspid valve. The free edges of the cusps are held in place by thin sinews known as chordae tendinae. These insert into small hillocks of cardiac muscle, known as papillary muscles, for extra support.
With most of the heart in place we cannot neglect its connection to the rest of the embryo. The outflow from the heart is initially through the conus cordis and truncus arteriosus. The outflow must then be divided completely between that to the pulmonary circulation and that to the rest of the embryo. This separation starts when two (or, it is alternatively suggested, three pairs of) swellings appear on opposite sides of the walls of conus cordis and truncus arteriosus. These then fuse across the midline to form a septum that divides the contruncal lumen. The pulmonary trunk separates from the ascending aorta through a split in this septum. Final separation of the two arterial systems occurs when the interventricular septum completes its growth; fusing with the inferior endocardial cushion at the same time as the truncoconal swellings fuse with it. The most common congenital heart defect is caused by a mis-fusion of these septa.
The truncoconal swellings are also responsible for the formation of the semilunar valves. At the inferior ends of the swellings two tubercles grow. A second pair of tubercles grow on the anterior and posterior borders of the truncus arteriosus. As the truncus arteriosus is split by the septum the lateral tubercles also become split as well. This means that each of the two vessels formed has three tubercles. The tubercles then develop into the cusps of the semi lunar valves that prevent backflow of blood from the arterial system into the ventricles. By the end of 9th week these valves are fully formed.
Electrical conduction system of the heart
The final important characteristic of the heart is its ability to maintain its own rhythm without nervous stimulation. The electrical impulse that causes contraction of the heart is myogenic. From about day 22 the heart starts to generate its electrical rhythm in the ventricle. Control of the rhythm is however quickly taken over by a group of cells derived from either the right cardinal vein or right sinus venosus. This develops into the sinoatrial node. Shortly after this, a group of cells form the atrioventricular node. The AV node controls the beating of the two ventricles. It acts on impulses received from the SA node through the cristae terminalis. The signal is transferred through to the ventricles through the bundle of His which develops at about the same time as the AV node.
Changes in Circulation in newborn
For further information see also fetal circulation. Decreased pulmonary resistance (hours - days) Closure of foramen ovale Closure of ductus arteriosus (days - weeks)
- Atrial septal defect
- Ventricular septal defect
- Patent ductus arteriosus
- Tetralogy of Fallot
- Coarctation of the aorta
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