TRANSPOSITION OF GREAT ARTERY

1. ABSTRACT

Transposition of the great arteries (TGA) is a congenital (present at birth) heart defect that occurs when the large vessels that take blood away from the heart to the lungs, or to the body, are improperly connected. Normally:

oxygen-poor (blue) blood returns to the right atrium from the body, travels to the right ventricle, then is pumped through the pulmonary artery into the lungs where it receives oxygen. Oxygen-rich (red) blood returns to the left atrium from the lungs, passes into the left ventricle, then is pumped through the aorta out to the body.

In transposition of the great arteries, the aorta is connected to the right ventricle, and the pulmonary artery is connected to the left ventricle-the exact opposite of a normal heart's anatomy:

Oxygen-poor (blue) blood returns to the right atrium from the body, passes through the right atrium and ventricle, then goes into the misconnected aorta back to the body. Oxygen-rich (red) blood returns to the left atrium from the lungs, passes through the left atrium and ventricle, then goes into the pulmonary artery and back to the lungs.

Two separate circuits are formed -- one that circulates oxygen-poor (blue) blood from the body back to the body, and another that recirculates oxygen-rich (red) blood from the lungs back to the lungs. Other heart defects are often associated with TGA, and actually may be necessary in order for an infant with transposition of the great arteries to live. Openings in the wall separating the left and right sides of the heart, called atrial septal defect or ventricular septal defect, will allow blood from one side to mix with blood from another, creating "purple" blood with an oxygen level somewhere between that of the oxygen-poor (blue) and the oxygen-rich (red) blood.

Babies with TGA have two separate circuits -- one that circulates oxygen-poor (blue) blood from the body back to the body, and another that recirculates oxygen-rich (red) blood from the lungs back to the lungs. Without an additional heart defect that allows mixing of oxygen-poor (blue) and oxygen-rich (red) blood, such as an atrial or ventricular septal defect, infants with TGA will have oxygen-poor (blue) blood circulating through the body, a situation that is critical. Even with an additional defect present that allows mixing, babies with transposition of the great arteries may not have enough oxygen in the bloodstream to meet the body's demands.

Even when a significant amount of mixing of oxygen-poor (blue) and oxygen-rich (red) blood occurs, other problems may be present. The left ventricle, which in TGA is connected to the pulmonary artery, is usually the stronger of the two ventricles since it normally has to generate a lot of force to pump blood to the body. The right ventricle, connected to the aorta in TGA, is considered the weaker of the two ventricles and may not be able to pump blood efficiently to the body. As a result, it will enlarge under the strain.

A patent ductus arteriosus (another type of congenital heart defect) will allow mixing of oxygen-poor (blue) and oxygen-rich (red) blood through the connection between the aorta and pulmonary artery. The "purple" blood that results from this mixing is beneficial, providing small, if not normal, amounts of oxygen to the body.

2. PREDISPOSITION AND EPIDEMIOLOGY

The heart is forming during the first eight weeks of fetal development. The problem occurs in the middle of these weeks, allowing the aorta and pulmonary artery to be attached to the incorrect chamber. Some congenital heart defects may have a genetic link, either occurring due to a defect in a gene, a chromosome abnormality or environmental exposure, causing heart problems to occur more often in certain families. Most of the time this heart defect occurs sporadically (by chance), with no clear reason for its development.

The epidemiology of TGA are : Transposition is the most common cyanotic congenital heart lesion presenting in the

neonate. The overall annual incidence is 20-30 per 100,000 live births. It is more common in males than females, with a ratio of about 3:1. Transposition is rarely associated with syndromes or extracardiac malformationsFactors in the mother that may increase the risk of this condition include: Age over 40 Alcoholism Diabetes Poor nutrition during pregnancy (prenatal nutrition) Rubella or other viral illness during pregnancy

a. Frequency Despite its overall low prevalence, transposition of the great arteries is the most common etiology for cyanotic congenital heart disease in the newborn.1 This lesion presents in 5-7% of all patients with congenital heart disease. The overall annual incidence is 20-30 per 100,000 live births, and inheritance is multifactorial. Transposition of the great arteries is isolated in 90% of patients and is rarely associated with syndromes or extracardiac malformations. This congenital heart defect is more common in infants of diabetic mothers.

b. Mortality/MorbidityThe mortality rate in untreated patients is approximately 30% in the first week, 50% in the first month, and 90% by the end of the first year. With improved diagnostic, medical, and surgical techniques, the overall short-term and midterm survival rate exceeds 90%. Long-term complications are secondary to prolonged cyanosis and include polycythemia and hyperviscosity syndrome. These patients may develop headache, decreased exercise tolerance, and stroke. Thrombocytopenia is common in patients with cyanotic congenital heart disease leading to bleeding complications. Patients with a large ventricular septal defect, a patent ductus arteriosus, or both may have an early predilection for congestive heart failure, as pulmonary vascular resistance falls with increasing age. Heart failure may be mitigated in those patients with left ventricular outflow tract (pulmonary) stenosis.A small percentage (approximately 5%) of patients with transposition of the great arteries (and often a ventricular septal defect) develop accelerated pulmonary vascular obstructive disease and progressive cyanosis despite surgical repair or palliation. Long-term survival in this subgroup is particularly poor.

c. RaceNo racial predilection is known.

d. SexTGA has a 60-70% male predominance.

e. AgePatients with TGA usually present with cyanosis in the newborn period, but clinical manifestations and courses are influenced predominantly by the degree of intercirculatory mixing.

3. PATHOPHYSIOLOGY

Systemic and pulmonary circulations are completely separated. After returning to the right heart, desaturated systemic venous blood is pumped into the systemic circulation without being oxygenated in the lungs; oxygenated blood entering the left heart goes back to the lungs rather than to the rest of the body. This anomaly is not compatible with life unless desaturated and oxygenated blood can mix through openings at one or more levels. The pulmonary and systemic circulations function in parallel, rather than in series. Oxygenated pulmonary venous blood returns to the left atrium and left ventricle but is recirculated to the pulmonary vascular bed via the abnormal pulmonary arterial connection to

the left ventricle. Deoxygenated systemic venous blood returns to the right atrium and right ventricle where it is subsequently pumped to the systemic circulation, effectively bypassing the lungs. This parallel circulatory arrangement results in a deficient oxygen supply to the tissues and an excessive right and left ventricular workload. It is incompatible with prolonged survival unless mixing of oxygenated and deoxygenated blood occurs at some anatomic level. The following are 3 common anatomic sites for mixing of oxygenated and deoxygenated blood in transposition of the great arteries: Atrial septal defect, Ventricular septal defect, and Patent Ductus Arteriosus

4. CLINICAL MANIFESTATION

a. History

Infants with transposition of the great arteries (TGA) are usually born at term, with cyanosis apparent within hours of birth.

The clinical course and manifestations depend on the extent of intercirculatory mixing and the presence of associated anatomic lesions.

o Transposition of the great arteries with intact ventricular septum: Prominent and progressive cyanosis within the first 24 hours of life is the usual finding in infants.

o Transposition of the great arteries with large ventricular septal defect Infants may not initially manifest symptoms of heart disease, although mild

cyanosis (particularly when crying) is often noted. Signs of congestive heart failure (tachypnea, tachycardia, diaphoresis, and

failure to gain weight) may become evident over the first 3-6 weeks as pulmonary blood flow increases.

o Transposition of the great arteries with ventricular septal defect and left ventricular outflow tract obstruction

Infants often present with extreme cyanosis at birth, proportional to the degree of left ventricular (pulmonary) outflow tract obstruction.

The clinical history may be similar to that of an infant with tetralogy of Fallot.

o Transposition of the great arteries with ventricular septal defect and pulmonary vascular obstructive disease

Progressively advancing pulmonary vascular obstructive disease can prevent this rare subgroup of patients from developing symptoms of congestive heart failure, despite a large ventricular septal defect.

Most often, patients present with progressive cyanosis, despite an early successful palliative procedure.

b. Physical

Newborns with transposition of the great arteries are usually well developed, without dysmorphic features. Physical findings at presentation depend on the presence of associated lesions.

Transposition of the great arteries with intact ventricular septumo Infants typically present with progressive central (perioral and periorbital) cyanosis.o Other than cyanosis, the physical examination is often unremarkable.

Transposition of the great arteries with large ventricular septal defecto Cyanosis may be mild initially, although it is usually more apparent with stress or

crying.o Upon presentation, infants often have an increased right ventricular impulse, a

prominent grade 3-4/6 holosystolic murmur, third heart sound, mid-diastolic rumble, and a gallop rhythm.

o Hepatomegaly may be present. Transposition of the great arteries with ventricular septal defect and left ventricular outflow

tract obstructiono Cyanosis is prominent at birth, and the findings are similar to those of infants with

tetralogy of Fallot.o A single, or narrowly split, diminished second heart sound and a grade 2-3/6 systolic

ejection murmur may be present.o Hepatomegaly is rare.

Transposition of the great arteries with ventricular septal defect and pulmonary vascular obstructive disease

o Progressive pulmonary vascular obstructive disease is not always evident from physical examination findings.

o Cyanosis is usually present and can progress despite palliative therapy in the newborn period.

o No murmur (despite the ventricular septal defect) or early short systolic ejection sounds are heard.

o The second heart sound is often single, with increased intensity.o In later childhood or adolescence, a high-pitched, blowing, early decrescendo

diastolic murmur of pulmonary insufficiency and a blowing apical murmur of mitral insufficiency are evident.

5. DIAGNOSIS

A pediatric cardiologist and/or a neonatologist may be involved in your child's care. A pediatric cardiologist specializes in the diagnosis and medical management of congenital heart defects, as well as heart problems that may develop later in childhood. A neonatologist specializes in illnesses affecting newborns, both premature and full-term. Cyanosis is the major indication that there is a

problem with your newborn. Your child's physician may also have heard a heart murmur during a physical examination. A heart murmur is simply a noise caused by the turbulence of blood flowing through the openings that allow the blood to mix.

Other diagnostic tests are needed to help with the diagnosis, and may include the following: Chest X-ray — A diagnostic test that uses invisible electromagnetic energy beams to produce images of internal tissues, bones, and organs onto film.

In the normal anatomy, the aorta is anterior to and at the right of the pulmonary artery . In transposition of the great arteries, the pulmonary artery is situated to the right of its normal location and is obscured by the aorta on frontal chest radiographs. This malposition, in association with stress-induced thymic atrophy and hyperinflated lungs, results in the apparent narrowing of the superior mediastinum on radiographs, the most consistent sign of transposition of the great arteries. The cardiovascular silhouette varies from normal in the first few days after birth to enlarged and globular, with the classic egg on a string appearance.

Electrocardiogram (ECG or EKG) — A test that records the electrical activity of the heart, shows abnormal rhythms (arrhythmias or dysrhythmias) and detects heart muscle stress.Echocardiogram (echo) — A procedure that evaluates the structure and function of the heart by using sound waves, recorded on an electronic sensor, that produce a moving picture of the heart and heart valves.

Cardiac Catheterization — A procedure that gives very detailed information about the structures inside the heart. Under sedation, a small, thin, flexible tube (catheter) is inserted into a blood vessel in the groin and guided to the inside of the heart. Blood pressure and oxygen measurements are taken in the four chambers of the heart, as well as in the pulmonary artery and aorta. Contrast dye is injected to more clearly visualize the structures inside the heart.Cardiac Magnetic Resonance Imaging (MRI) — A non-invasive test that uses three-dimensional imaging technology produced by magnets to accurately determine blood flow and functioning of the heart as it is working.

6. TREATMENT

Specific treatment for transposition of the great arteries will be determined by physician based on:

child's age, overall health and medical history extent of the disease child's tolerance for specific medications, procedures or therapies how child's doctor expects the disease to progress opinion or preference The children most likely will be admitted to the intensive care unit (ICU) or special care

nursery once symptoms are noted. Initially, your child may be placed on oxygen or a ventilator to assist his/her breathing. Intravenous (IV) medications may be given to help the heart and lungs function more efficiently.

Other important aspects of initial treatment include the following: A cardiac catheterization procedure can be used as a diagnostic procedure, as well as an initial treatment procedure for some heart defects. A cardiac catheterization procedure will usually be performed to evaluate the defect(s) and the amount of blood that is mixing. As part of the cardiac catheterization, a procedure called a balloon atrial septostomy may be performed to improve mixing of oxygen-rich (red) and oxygen-poor (blue) blood. 1. A special catheter with a balloon in the tip is used to create an opening in the atrial

septum (wall between the left and right atria). 2. The catheter is guided through the foramen ovale (a small opening present in the atrial

septum that closes shortly after birth) and into the left atrium. 3. The balloon is inflated. 4. The catheter is quickly pulled back through the hole, into the right atrium, enlarging the

hole, allowing blood to mix between the atria. An intravenous medication called prostaglandin E1 is given to keep the ductus arteriosus from closing.All patients require antibiotic prophylaxis prior to dental and indicated surgical procedures in order to reduce the risk of subacute bacterial endocarditisWithin the first 1 to 2 weeks of age, transposition of the great arteries is surgically repaired.

The procedure that accomplishes this is called a "switch," which roughly describes the surgical process. The surgical correction of TGA is carried out through an incision in the middle of the chest. The breast bone is split in the middle and spread apart to expose the heart. A heart-lung machine is used to do the work of the heart while the heart is cooled, stopped, emptied and opened. The aorta and pulmonary arteries are disconnected and reconnected to their proper ventricles. The coronary arteries must be transferred to the newly positioned aorta as well, or "blue" blood will supply the muscle of the heart. Associated holes between the chambers of the heart are closed. The heart is then restarted as the heart-lung machine is withdrawn.

7. COMPLICATIONS

Congestive heart failure Arrhythmias Right ventricular dysfunction in long-term survivors Eisenmenger's syndrome Polycythaemia and hyperviscosity syndrome Seizures may occur in about 5% of patients before surgery2

Thrombocytopenia

8. PROGNOSIS

Many infants who undergo TGA surgical repair will grow and develop normally. After TGA repair, however, infants will need to be followed periodically by a pediatric cardiologist who will make assessments to check for any heart-related problems, which can include the following:

fast, slow or irregular heart rhythms leaky heart valvesnarrowing of one or both of the great arteries at the switch connection site(s)narrowing of the coronary arteries at their switch connection site The mortality rate in untreated patients is approximately 30% in the first week, 50% in the first month, and 90% by the end of the first year.Death is usually due to anoxia, acidosis, heart failure and complications associated with polycythaemia, including thromboembolic events.The overall survival rate following arterial switch operation is approximately 90% at 15 years of age. However, exercise performance, cognitive function and quality of life may be impaired.Low gestational age and a high preoperative lactate are the most important predictors of poor developmental outcome.

SOURCES :

1. Children’s Hospital Boston online : TGA2. eMedicine Pediatrics:Cardiac Disease and Critical Care_TGA3. http://www.wikipedia.org 4. Merck Manual Professional : Congenital Cardiovascular Anomalies_TGA5. Medline Plus Medical Encyclopedia_TGA6. Doctor and Patients UK_TGA7. American Journal of Roentgenology online