The most important heart diseases in children are congenital cardiac malformations
Most cardiac malformations are manifest at or shortly after birth, usually by some
manifestation of cardiac failure such as cyanosis, breathlessness, feeding difficulties,
and failure to thrive. Certain maternal factors are known to increase the incidence of 
congenital cardiac malformations, particularly maternal rubella infections and chronic 
alcohol abuse, but in the majority of cases no teratogenic factors can be identified.

Congenital cardiac defects may be divided into two main groups: lesions that cause
obstruction to blood flow, and those that cause abnormal shunting of blood 
between the two sides of the heart.

The latter group normally cause a left-to-right shunt because of the higher pressure 
in the left side of the heart. This is not associated with clinical cyanosis.

If there is an increase in resistance to blood moving on from the right side of the
heart (either from obstruction of the right ventricular outflow or from pulmonary hypertension),
there is a right-to-left shunt; with blood bypassing the lungs to enter the systemic circulation,
cyanosis develops.
The most common congenital heart defects cause left-to-right shunts

The most common group of malformations result in a shunt of blood from the left side of
the heart to the right.

Atrial septal defects (ASDs) are due to a defect in the interatrial septum. The lesion
is usually located at the level of the fossa ovalis, which is incompletely closed 
(ostium secundum defect).

Ventricular septal defects (VSDs) are due to a defect in the interventricular septum.
The larger defects involve the muscular wall of the septum, but small defects are often
confined to the tiny membranous area 
(maladie de Roger)

Patent ductus arteriosus (PDA) is due to persistent patency in the ductus arteriosus,
an embryological connection between the aorta and the pulmonary trunk or left main 
pulmonary artery. In intrauterine life the ductus is an important channel, allowing blood 
oxygenated in the placenta to bypass the lungs, but it closes at or shortly after birth,
when the lungs become aerated and expand. Persistence of the duct is most common in females,
and there is a recogonised association with maternal rubella.
The severity of the symptoms depends on the size of the shunt between the left and right
sides of the heart or main vessels. In the case of small ASDs or VSDs the flow can be minor
and largely asymptomatic, sometimes presenting in mid-adult life with unexplained right
heart failure. Patent ductus usually requires treatment in childhood.

Defects leading to permanent right-to-left shunts are less common

The most important and common congenital abnormality of heart to cause right-to-left shunting is 

Fallot's tetralogy which consists of:
 • VSD.}
 • An overriding aorta, which sits astride the VSD, so that it receives blood 
   from both right and left ventricles.
 • Pulmonary stenosis, usually due to thickening of the subvalvar muscle in the 
   pulmonary outflow tract, but sometimes associated with fused stenotic valve cusps.
 • Right ventricular hypertrophy.

Because the abnormal aorta receives blood from the right ventricle as well as 
the left, the systemic circulation contains deoxygenated blood,and the patient 
has clinical cyanosis.
The pulmonary stenosis leads to inadequate perfusion of the lungs. 
Affected individuals develop a right to left shunt and have cyanosis.
With growth, the pulmonary stenosis remains fixed and eventually there is severe right 
ventricular outflow obstruction. 
Most cases are surgically corrected.

Surgical correction of this complex disorder is usually aimed at closing the VSD,
rechannelling the flow into the aorta from the left ventricle only, and relieving the 
pulmonary stenosis. This technique has greatly increased survival.

Complications include bacterial endocarditis, and consequent cerebral infarction or 
brain abcess.

Some congenital malformations are not associated with significant shunting of blood
Of those congenital malformations not associated with significant shunting, the most
common are aortic stenosis, coarctation of the aorta, and transposition of the great 
vessels.

With aortic stenosis there is stenosis of the valve, which is usually bicuspid.
Symptoms tend to appear later in life, when fibrosis and calcification 
of the abnormal valve lead to functional stenosis and incompetence.
Less commonly the stenosis occurs in the muscular tissue below the aortic valve 
(subvalvar stenosis), or in the aortic valve (supravalvar stenosis).
These two variants may be associated with coarctation of the aorta.
In cases of coarctation of the aorta, there is a stenotic narrowing of the aorta,
usually located at or just beyond the site of the ductus arteriosus,
which appears closed.

The stricture produces hypertension proximal to the stenosis.

Transposition of the great vessels is a complex malformation in which the connections
between the right and left ventricle and aorta and pulmonary artery are disordered,
the aorta emanating from the right ventricle and the pulmonary artery from the atria.