**The Basis of Management of Congenital Heart Disease**

Krishnan Ganapathy Subramaniam and Neville Solomon

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/53654

**1. Introduction**

#### **1.1. Cardiac embryology — The development of the heart**

#### *1.1.1. Early heart development and the folding of the primitive tube*

properly cited.

By the 3rd week of development the heart has developed from the cardiogenic region, a horseshoe-shaped structure, at the cranial end of the embryo, when it is about the size of a raisin. By day 21 the primitive heart tube has moved below the head region and by day 22 it fuses and moves into the future thoracic cavity and it is from this time that it begins to beat. The tube now starts to bend and twist and over the next 8 days, various chamber of the heart begin to develop and by the end of 2 months it bears a superficial resemblance to the fetal heart. [1]

The tube is anchored at one end by the arterial trunks and at the other end, by the various venous channels draining into it. Being fixed at both ends, the cardiac tube grows rapidly in length and begins to twist and bend. The embryonic ventricle is bent in to a loop to the right of the midline and the ventricle grows rapidly to cover the atrium and the great veins (figure 1). The sacculations projecting laterally will become the right atrium and the left atrium. The future left ventricle lies to the left of interventricular groove and the right ventricle or the bulboconus region communicates with the truncus arteriosus. A four chambered structure is formed from this convoluted tube by development of 3 septa which partitions the atria, ventricle and the truncus arteriosus. [2]

The septae develop simultaneously at about the same time between the 28 to 42nd day. The atria and ventricle are separated by a deep groove, the atrio-ventricular groove which appears like an invagination from inside.This forms the atrio-ventricular canal, which becomes divided

The interventricular foramen is obliterated by (a) masses of *endocardial tissue* at the interven‐ tricular septum, (b) masses of '*endocardial cushion'* tissue and (c) spiral septum. The partitioning of the heart is now complete. The aortopulmonary septum rotates 180 degree and fuses with the superior margin of the interventricular septum. This accounts for the manner in which the ventricular outflow tracts are aligned in a fully-developed heart- the aortic blood flowing posteriorly to anteriorly and the pulmonary blood flowing anterior to the aorta first and then

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The events which occur during this period accounts for a majority of congenital heart disease. Atrial septal defect (which is most commonly a secundum defect) occurs due to defect of septum primum. Inadequate development of septum secundum (which forms by invagination of the developing superior vena cava and pulmonary veins) accounts for the sinus venosus

The development of ventricular septum helps in understanding the predominance of peri‐ membranous ventricular septal defects as three different regions have to fuse in a coordinated fashion to completely obliterate the interventricular communication. Uneven spiral partition‐ ing of the outflow tracts can explain the occurrence of Tetralogy of Fallot (TOF) and double outlet right ventricle(DORV) and failure of the spiral pattern of aortopulmonary septum results in transposition of great arteries (TGA), when the aortopulmonary septum grows directly towards the interventricular septum. Failure of the septum to develop results in truncus arteriosus and the contribution of this septum to the interventricular septum explains

Failure of the endocardial cushions to develop properly results in the spectrum of endocardial cushion defects varying from ostium primum defects to partial and complete AV septal de‐

Initially the blood coming from the lung buds drain into the splanchnic plexus which connects to the paired common cardinal and umbilicovitelline veins. The right common cardinal system forms the right SVC and azygous vein, the left common cardinal veins becomes the left superior vena cava and coronary sinus. The umbilicovitelline system becomes the inferior vena cava,

At 27 – 29 days the primitive pulmonary vein appears as an endothelial out pouching from superior left atrial wall and this joins the pulmonary venous plexus by 30 days. The common pulmonary vein enlarges and incorporates into the left atrium. The pulmonary venous part of the splanchnic plexus gradually loses its connection with cardinal and umbilicovitelline veins. Knowledge of the normal development of pulmonary venous pathway facilitates understand‐

Aortic arches are series of six paired embryological structures connecting the ventral to the dorsal aorta.The ventral aorta at the level of 4- 6th arch fuses to form the truncus arteriosus

ing of various types of anomalous pulmonary venous connections. [4]

the almost invariable association of truncus arteriosus defect with outlet VSD.

posteriorly.

type of defects.

fects. [3]

*1.1.2. Pulmonary venous development*

ductus venosus and portal vein.

*1.1.3. Aortic arches*

**Figure 1.** Development of the heart tube in the pericardial sac.

by the cushions which grow towards the junction. The endocardial cushions grow from opposite sides of the atrio-ventricular aperture and fuse to separate the atrium and ventricle.

From the interventricular ridge a proliferating muscular septum advances across the common ventricle towards the base of the heart. Simultaneously interatrial septum is formed by the septum primum growing rapidly towards endocardial cushions, leaving a foramen primum. Before the foramen primum becomes obliterated a new opening appears high on the interatrial septum- the foramen secundum-which allows shunting of blood from the right atrium to the left. The septum secundum develops from a ridge to the right of septum primum and grows like a curtain over the foramen secundum, the edge of the septum secundum forming the foramen ovale and the septum primum acting like a unidirectional valve, allowing blood to flow only from the right to the left.

The opening between the ventricular cavities -the interventricular foramen- persists, the closure of which depends on the development of a spiral septum which partitions the truncus and conus region into aorta and pulmonary artery.

The truncus arteriosus gives rise to the aortic arches, the 4th aortic arch forming the aorta and the 6th forming the origin of the pulmonary artery. A pair of ridges forms at the bifurcation which fuse and spiral down towards the interventricular foramen.

The interventricular foramen is obliterated by (a) masses of *endocardial tissue* at the interven‐ tricular septum, (b) masses of '*endocardial cushion'* tissue and (c) spiral septum. The partitioning of the heart is now complete. The aortopulmonary septum rotates 180 degree and fuses with the superior margin of the interventricular septum. This accounts for the manner in which the ventricular outflow tracts are aligned in a fully-developed heart- the aortic blood flowing posteriorly to anteriorly and the pulmonary blood flowing anterior to the aorta first and then posteriorly.

The events which occur during this period accounts for a majority of congenital heart disease. Atrial septal defect (which is most commonly a secundum defect) occurs due to defect of septum primum. Inadequate development of septum secundum (which forms by invagination of the developing superior vena cava and pulmonary veins) accounts for the sinus venosus type of defects.

The development of ventricular septum helps in understanding the predominance of peri‐ membranous ventricular septal defects as three different regions have to fuse in a coordinated fashion to completely obliterate the interventricular communication. Uneven spiral partition‐ ing of the outflow tracts can explain the occurrence of Tetralogy of Fallot (TOF) and double outlet right ventricle(DORV) and failure of the spiral pattern of aortopulmonary septum results in transposition of great arteries (TGA), when the aortopulmonary septum grows directly towards the interventricular septum. Failure of the septum to develop results in truncus arteriosus and the contribution of this septum to the interventricular septum explains the almost invariable association of truncus arteriosus defect with outlet VSD.

Failure of the endocardial cushions to develop properly results in the spectrum of endocardial cushion defects varying from ostium primum defects to partial and complete AV septal de‐ fects. [3]

#### *1.1.2. Pulmonary venous development*

by the cushions which grow towards the junction. The endocardial cushions grow from opposite sides of the atrio-ventricular aperture and fuse to separate the atrium and ventricle.

From the interventricular ridge a proliferating muscular septum advances across the common ventricle towards the base of the heart. Simultaneously interatrial septum is formed by the septum primum growing rapidly towards endocardial cushions, leaving a foramen primum. Before the foramen primum becomes obliterated a new opening appears high on the interatrial septum- the foramen secundum-which allows shunting of blood from the right atrium to the left. The septum secundum develops from a ridge to the right of septum primum and grows like a curtain over the foramen secundum, the edge of the septum secundum forming the foramen ovale and the septum primum acting like a unidirectional valve, allowing blood to

The opening between the ventricular cavities -the interventricular foramen- persists, the closure of which depends on the development of a spiral septum which partitions the truncus

The truncus arteriosus gives rise to the aortic arches, the 4th aortic arch forming the aorta and the 6th forming the origin of the pulmonary artery. A pair of ridges forms at the bifurcation

flow only from the right to the left.

and conus region into aorta and pulmonary artery.

**Figure 1.** Development of the heart tube in the pericardial sac.

222 Principles and Practice of Cardiothoracic Surgery

which fuse and spiral down towards the interventricular foramen.

Initially the blood coming from the lung buds drain into the splanchnic plexus which connects to the paired common cardinal and umbilicovitelline veins. The right common cardinal system forms the right SVC and azygous vein, the left common cardinal veins becomes the left superior vena cava and coronary sinus. The umbilicovitelline system becomes the inferior vena cava, ductus venosus and portal vein.

At 27 – 29 days the primitive pulmonary vein appears as an endothelial out pouching from superior left atrial wall and this joins the pulmonary venous plexus by 30 days. The common pulmonary vein enlarges and incorporates into the left atrium. The pulmonary venous part of the splanchnic plexus gradually loses its connection with cardinal and umbilicovitelline veins. Knowledge of the normal development of pulmonary venous pathway facilitates understand‐ ing of various types of anomalous pulmonary venous connections. [4]

#### *1.1.3. Aortic arches*

Aortic arches are series of six paired embryological structures connecting the ventral to the dorsal aorta.The ventral aorta at the level of 4- 6th arch fuses to form the truncus arteriosus which forms the distal end of the developing heart tube. The dorsal aorta on the right usually disappears and the dorsal aorta on the left forms the descending aorta.

The molecular mechanism behind the complex development is being now slowly unravelled. The consistent rightward looping of the heart suggests a highly conserved molecular control mechanism, The beating cilia on the node beats in a counter clockwise direction causing a leftward flow of fluid across the Hensen's node. The unidirectional movement is based on the inherent molecular chirality of the ciliary protein machinery. Pitx-2, a homeodomain contain‐ ing protein plays a role in development of laterality. The Nkx2.5 and Tbx5 gene are expressed in atria and in conduction system. Irx4 is expressed only in ventricular chambers. The HAND 1 gene is expressed on the left ventricle and HAND 2 is expressed in the right ventricle.

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The vertebral heart develops from precardiac mesoderm, anterior heart field and cardiac neural crest cells also play a critical part in myocardium and normal outflow tract develop‐ ment. The cardiac neural crest cell stabilizes the arch vessels and prevents their regression. A subpopulation of cardiac neural crest cells participates in the septation of outflow tract and contributes to the formation of semilunar valves. Retinoic acid is believed to have a role in signalling neural crest migration and cardiac development. Understanding the molecular and

The heart is enclosed in a pericardial sac. The pericardial cavity is the space between the inner lining of the fibrous pericardium and the surface of the heart.There are two recesses in the pericardial cavity lined by serous pericardium, the first is the transverse sinus behind the great arteries in front of the atria and is in free communication with the pericardial cavity on either

The cardiac mass is 1/3rd to the right and 2/3rd to the left of midline. The ventricle is a three sided pyramid with diaphragmatic, anterior and left surfaces. The right sided margin is the

The right atrium has 3 components-the appendage, venous sinus and the vestibule. (figure 3)The junction between the appendage and the venous sinus is marked by the prominent terminal groove. The groove internally corresponds to the crista terminalis, from which the pectinate muscle originates. The extensive array of pectinate muscles serves as one of the markers of the morphologic right atrium. Parallel and posterior to the groove is the second deeper groove between the right atrium and the right pulmonary veins. Dissection into this deep interatrial groove (Waterston's or Sondergaard's groove) permits incisions to be made

The sinus node lies in the subepicardial position at the cranial part of the terminal groove, and is a spindle-shaped structure which lies lateral to the superior cavoatrial junction. The artery to the sinus node arises from right coronary artery (55%) or cirumflex coronary artery (45%).

side. The second is the oblique sinus, a blind ending cavity behind the left atrium.

genetic mechanism may help in future fetal cardiac interventions. [6]

**2. Surgical anatomy of the heart**

acute margin and the left is the obtuse margin [7].

into the left atrium (the *classic posterior approach* to the left atrium).

**2.1. The morphological right atrium**

The first arch disappears and the 2nd persists as stapedial artery which is not of clinical significance. The third arch forms the internal carotid artery on both the sides and is called the carotid arch. The 4th arch on the right forms the right subclavian artery as far as the origin of the internal mammary branch and the left 4th arch forms the arch of aorta between the left carotid artery and the termination of ductus arteriosus. The 5 th arch disappears on both sides. The proximal part of the right 6th arch forms the right pulmonary artery and the distal part disappears. The proximal part of the left 6th arch forms the left pulmonary artery and the distal part persists as the ductus arteriosus.(figure 2)

**Figure 2.** Development of aortic arches

Double aortic arch which is the commonest arch anomaly causing trachea and oesophageal compression occurs as a result of persistence of dorsal aorta. The recurrent laryngeal nerve loops around the 6th arch and hence goes around the ductus arteriosus on the left side and the subclavian artery on the right side. Persistence of the ductus arteriosus results in patent ductus arteriosus (PDA) and its excessive resorption can result in coarctation of aorta or stenosis of left pulmonary artery. [5]

The molecular mechanism behind the complex development is being now slowly unravelled. The consistent rightward looping of the heart suggests a highly conserved molecular control mechanism, The beating cilia on the node beats in a counter clockwise direction causing a leftward flow of fluid across the Hensen's node. The unidirectional movement is based on the inherent molecular chirality of the ciliary protein machinery. Pitx-2, a homeodomain contain‐ ing protein plays a role in development of laterality. The Nkx2.5 and Tbx5 gene are expressed in atria and in conduction system. Irx4 is expressed only in ventricular chambers. The HAND 1 gene is expressed on the left ventricle and HAND 2 is expressed in the right ventricle.

The vertebral heart develops from precardiac mesoderm, anterior heart field and cardiac neural crest cells also play a critical part in myocardium and normal outflow tract develop‐ ment. The cardiac neural crest cell stabilizes the arch vessels and prevents their regression. A subpopulation of cardiac neural crest cells participates in the septation of outflow tract and contributes to the formation of semilunar valves. Retinoic acid is believed to have a role in signalling neural crest migration and cardiac development. Understanding the molecular and genetic mechanism may help in future fetal cardiac interventions. [6]
