The basics of  congenital heart defects in children

A congenital heart defect (CHD) is a flaw in the structure of the heart and great vessels of a newborn baby which the baby is born with. Most such heart defects either obstruct the blood flow in the heart or major blood vessels near it or cause blood to flow through the heart in an abnormal fashion. Congenital heart defects are among the most common birth defects and are the leading cause of birth defect related deaths.

CHD affects about one out of every 125 babies born. Quite a number of them with more complicated defects and who are in resource-scarce poor developing countries will not survive their first year. Twice as many children die each year from a CHD than all forms of paediatric cancers combined. Just about eight common defects account for around 80 per cent of all congenital heart diseases while the remaining 20 per cent consist of many independently infrequent conditions or combinations of several defects. Ventricular septal defect (VSD) is generally considered to be the most common type of malformation, accounting for about one third of all significant congenital heart defects.

The incidence of CHD is higher when a parent or a sibling has a heart defect (4 to 5 per cent), in stillborns (3 to 4 per cent), in abortuses (10 to 25 per cent), and premature infants (about 2 per cent). With the current advances in treatment, the number of adults with problems connected to a congenital heart defect is rising and even passing the number of children with congenital heart defects in most Western countries. This group is referred to as grown-up congenital heart disease (GUCH) patients.

To comprehend the intricacies of congenital heart defects, it is helpful to understand how a healthy heart works. The human heart is an indefatigable muscular organ that is able to pump blood to all areas of the body throughout the life time of an individual. The heart, lungs, and blood vessels make up the circulatory system of the human body. The heart consists of four chambers, namely the left atrium and left ventricle and the right atrium and right ventricle. The heart also has four valves that direct the flow of blood through the chambers of the heart and then into the two great blood vessels or major arteries.

1. The left atrium or the upper chamber on the left side of the heart receives oxygen-rich blood from the lungs and then empties into the left ventricle, which is the left lower chamber, through the mitral valve.

2. The left ventricle pumps oxygen-rich blood out to the rest of the body. Blood leaves the left ventricle through the aortic valve and enters the aorta, the largest artery in the human body. It is the major blood vessel that carries oxygenated blood to the rest of the body. Blood then flows from the aorta into a network of many smaller arteries, providing the body’s organs and tissues with the oxygen and nutrients they need.

3. After oxygen in the blood is released to the tissues, the now deoxygenated or oxygen-poor blood returns to the heart through the veins which are the blood vessels that carry back the deoxygenated blood. This blood, which appears not as red as oxygenated blood, enters the right atrium which is the upper chamber on the right side of the heart and then travels across the tricuspid valve into the right ventricle which is the right lower chamber.

4. The right ventricle then pumps deoxygenated blood through the pulmonary valve into the lungs. The oxygen in the air we breathe binds to the haemoglobin of the red cells of the blood that is being pumped through the lungs. This oxygen-rich blood, which appears red, then returns to the left atrium and enters the left ventricle, where it is pumped out to the rest of the body once again.

5. The two atria as well as the two ventricles are completely separated from each other by muscular walls. The atrium on each side communicates with the ventricle on the same side through a one-way valve.

6. The right ventricle pumps blood into the lungs through the pulmonary artery and the left ventricle does the same through the aortic valve to send blood to the rest of the body through the aorta. The flow is controlled by these special one-way valves which allow blood to flow from the ventricles to the great vessels but does not permit any flow backwards into the ventricles.

This is the normal pathway that blood takes to travel through the heart and the body. However, abnormalities in the basic structure of the heart such as congenital heart defects can and does alter the dynamics of the process and affect its ability to function properly.

The human heart starts its development in the unborn baby as early as the third week of fetal life. There are several phases of the development of the fetal heart. It starts as just a simple hollow tube of tissue. This tube goes through an amazing process of growth and maturation to ultimately produce the robust organ that we know as the human heart. The original tube grows so fast that it needs more space and to accommodate this phenomenon of rapid growth it bends and twists back, forming the familiar shape. During the next phase the two atria partly separate but there is just one big ventricle. The final stage of development begins when the two atria are completely separated and the ventricles are just beginning to separate. Ultimately the ventricles separate completely and the complete and efficient human heart is formed. During this process there is specialised development of the four chambers of the heart, the valves of the heart and the major blood vessels that enter and leave the heart. Congenital heart defects occur because of incomplete or abnormal development of the fetal heart during the very early weeks of pregnancy.

There are quite a few known causative factors for congenital heart defects. Some of these are genetic while others are environmental. Known genetic causes of CHD include chromosomal abnormalities such as Down syndrome and other types of more exotic genetic and chromosomal aberrations. More recently, a range of newly recognised genetic point mutations, point deletions and other genetic abnormalities have also been acknowledged as causative factors. Known antenatal environmental factors include maternal infections such as Rubella, drugs such as alcohol, hydantoin, lithium and thalidomide and maternal illness like diabetes mellitus, phenylketonuria, and systemic lupus erythematosus. However, in a considerable proportion of cases, no definitive causative factor could be identified and the occurrence of CHD seems to be just a natural quirk of Mother Nature. Many parents of children with congenital heart defects worry and feel guilty that somehow the problems in the child are due to some abnormality in either the father or the mother. In the vast majority this is definitely not the case and in those instances, there is no reason to believe that the problem was caused by a physical or genetic abnormality in one or both parents.

The fundamentals of the abnormalities seen in congenital heart defects are of several types. In some cases it is due to some sort of failure of development of the intact walls that separate the chambers of the heart. Normally, the two atria as well as the two ventricles are separated by muscular walls. These walls are referred to as septa. When these walls develop in an abnormal fashion, it leads to "holes" in these walls. Thus between the atria, there may be a communication known as an Atrial Septal Defect (ASD) and when it is between the ventricles, it is known as a Ventricular Septal Defect (VSD). In these defects there is abnormal mixing of blood either from the left side of the heart to the right or vice versa. All the problems from these defects emanate from the size of the defects and the type of mixing of blood. Broadly, the mixing or shunting of the blood is the main reason for the development of symptoms. Very small defects with inconspicuous and insignificant mixing of blood may not cause any symptoms at all.

Another type of defect is the narrowing of the valves or the main blood vessel itself. In extreme cases, the structure concerned does not develop at all and is known by the term atresia. Examples are pulmonary atresia and tricuspid atresia. This type of defect produces symptoms in the patient due to the chamber immediately behind the obstruction having to work much harder than normal to pump the blood and is usually combined with septal defects that try to circumvent the severe obstruction.

In other situations the valves may be defective and are unable to close properly. All four valves in the heart are one-way valves and if they cannot close properly, then there is a back-rush of blood every time the pumping action of the heart occurs. In such situations, the chamber immediately in front as well as the chamber immediately behind the abnormal valve, has to work harder than normally.

In some more complicated defects, there are positional changes in some of the structures of the heart. These usually lead to profound changes, unnatural mixing of considerable amounts of blood and marked symptoms. These markedly distort the dynamics of blood flow through the heart and lead to major attempted compensatory changes.

In the more esoteric types of defects, several anomalies are combined together to produce certain well known combinations of malformations of the heart. In some of these, narrowing of valves, maldevelopment of vessels and positional changes may all be combined to produce a recognised complex of aberrations. Some of the names given to these complex disorders of congenital heart diseases reflect the different components of the combinations while others are labelled after the person or persons who described these initially.

All these fundamentals are taken into account when a child with a congenital cardiac defect is evaluated. It is essential not only to identify the exact structural changes but also to ascertain the functional derangements as well. With the amount of amazing progress in the field of paediatric cardiology, particularly during the last few decades, there is quite a lot that could be done for these children who are born with congenital cardiac diseases.

The writer would appreciate some feedback from the readers. Please e-mail him at bjcp@sltnet.lk

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