08-19-2018, 04:56 PM
The common congenital heart diseases leading to left-to-right shunts include:
• Ventricular septal defect (VSD)
• Atrial septal defect (ASD)
• Patent ductus arteriosus (PDA)
Ventricular septal defect is characterized by a harsh holosystolic murmur that occurs at the left lower sternal border.
Ventricular septal defect is the second-most common congenital cardiac defect (bicuspid aortic valve is the most common).
The majority of ventricular septal defects occur in the membranous interventricular septum.
Children with Down syndrome often have an endocardial cushion defect with VSD, ASD, or AV septal defects. Unlike other children, these kids usually have a low baseline heart rate.
Another rare congenital heart defect associated with Down syndrome is Tetralogy of Fallot.
Eisenmenger’s syndrome describes a condition in which a left-to-right shunt reverses to become a right-to-left shunt in the presence of progressive pulmonary hypertension secondary to increased pulmonary circulation.
Late cyanosis with clubbing and polycythemia often accompany Eisenmenger’s syndrome.
The common congenital heart diseases leading to right-to-left shunts include:
1. Truncus arteriosus (1 vessel)
2. Transposition of great vessels (2 vessels switched)
3. Tricuspid atresia (Tri = 3)
4. Tetralogy of Fallot (Tetra = 4)
5. Total anomalous pulmonary venous return (TAPVR = 5 letters)
Note that the right-to-left shunt conditions all begin with the letter T, and thus can be remembered as the 5 T’s.
The right-to-left shunts cause early hypoxia, so the patients manifest cyanosis in early childhood, or even at birth.
Generally, right-to-left shunts result from a high pulmonary venous resistance and low systemic vascular resistance. Increased pulmonary vascular resistance (PVR) (e.g., crying, hypoventilation, and acidosis) or decreased systemic peripheral resistance (SVR) (e.g., hypotension, histamine release, sepsis) will increase the shunting and worsen the hypoxia.
Persistent truncus arteriosus is caused by abnormal neural crest cell migration, leading to incomplete fusion of the AP septum and failure of the truncus arteriosus to divide.
In persistent truncus arteriosus, a single large vessel leaves the heart. This vessel receives blood from both ventricles, effectively causing a right to left heart shunt.
Patients with persistent truncus arteriosus typically present with cyanosis.
Tetralogy of Fallot is characterized by four congenital abnormalities that include (mnemonic: PROVe):
• Pulmonic stenosis
• Right ventricular hypertrophy
• Overriding aorta
• Ventricular septal defect
The severity of tetralogy of Fallot is dependent upon the degree of pulmonic stenosis.
Tetralogy of Fallot typically presents as early childhood cyanosis; it is the most common congenital cyanotic
heart disease after the neonatal period. An associated feature is the presence of “tet spells”. When crying increases pulmonary resistance, the increase in right ventricular pressure leads to increased blood flow from the right to left ventricle via the VSD, resulting increased cyanosis.
A compensatory mechanism children with tetralogy of Fallot during a "tet spell" often use involves crouching down, which increases systemic vascular resistance. This leads to increased left ventricular pressure and lessens the effect of the right-to-left shunt, decreasing cyanosis.
Tetralogy of Fallot can lead to right ventricular hypertrophy, which classically appears as a “boot-shaped” heart on x-ray.
The definite treatment of tetralogy of Fallot involves surgical repair, which consists of VSD patch closure and right ventricular outflow tract reconstruction.
Tetralogy of Fallot is caused by the abnormal migration of neural crest cells during development, which leads to the anterior and cephalad (aka anterosuperior) malalignment of the infundibular septum.
Patent ductus arteriosus (PDA) is characterized by persistence of the ductus arteriosus, which serves as a connection between the pulmonary artery and aorta. The ductus arteriosus functions to divert blood away from the pulmonary circuit before birth. Upon birth, the systemic pressure increases, and causes blood to leak from the aorta, through the PDA and into the pulmonary circulation.
Patent ductus arteriosus is characterized by a continuous machine-like murmur.
Patent ductus arteriosus is often associated with congenital rubella.
Patent ductus arteriosus can be treated pharmacologically with COX inhibitors, such as NSAIDs (e.g., indomethacin), or may require invasive procedures in refractory cases.
The murmur associated with patent ductus arteriosus can best be heard over the left infraclavicular region.
Coarctation of the aorta is aortic narrowing near the insertion of the ductus arteriosus that is divided into preductal and postductal forms, with respect to the ductus arteriosus.
Adult coarctation of the aorta is commonly associated with a bicuspid aortic valve.
Adult coarctation of the aorta presents as hypertension in the upper extremities and hypotension with weak pulses in the lower extremities.
Adult coarctation of the aorta leads to increased collateral circulation over the intercostal arteries, these enlarged arteries cause progressive "notching of ribs" on x-ray.
Infantile coarctation of the aorta commonly presents as lower extremity cyanosis in infants, generally found at birth.
Infantile coarctation of the aorta is often associated with Turner syndrome.
D-transposition of the great vessels is a condition in which the aorta originates from the right ventricle and the pulmonary artery originates from the left ventricle. It is the most common congenital cyanotic heart disease presenting in the neonatal period.
Maternal diabetes mellitus is a risk factor for D-transposition of the great arteries.
D-transposition of the great vessels is caused by failure of the aorticopulmonary septum to spiral, often due to defective neural crest cell migration.
Neonates with D-transposition of the great vessels classically present with cyanosis within 24 hours of life that is not improved by supplemental O2. While the defect itself does not cause a murmur, associated murmurs (e.g. VSD, PDA) can be heard. Auscultation reveals a single S2 while chest x-ray shows a heart with an “egg on a string” appearance.
Initial management of D-transposition of the great vessels includes intravenous prostaglandin E1 (alprostadil) to maintain patency of the ductus arteriosus, which allows for some mixing of arterial and venous blood.
Definitive treatment of D-transposition of the great vessels involves surgical correction. In this surgery the great vessels are removed distal to the arterial valves and switched. The blood supply of the coronary arteries must be transferred to the new location of the aorta as well.
Atrial septal defect is characterized by a loud S1 with a wide, fixed split S2 that is best heard at the upper left sternal border.
The wide, fixed split S2 is due to the right ventricle having to pump more blood during systole due to blood flow from the high pressure left atrium to the low pressure right atrium. This delays closing of the pulmonic valve and causes the split S2.
There is a systolic ejection murmur due to increased blood flow across the pulmonary valve because of the left to right shunt.
ASDs are classified by the location of the defect; the two most common locations are ostium secundum (most common) and ostium primum.
Ostium secundum ASDs most commonly occur as an isolated defect, while ostium primum ASDs are often associated with other cardiac defects.
Patients with an uncorrected ASD or patent foramen ovale have an increased risk of stroke due to a paradoxical embolism (embolism of venous origin).