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Atrial Septal Defect at a Glance
Introduction
Atrial septal defect (ASD) is an opening in the atrial septum allowing blood to shunt between left and right atria or shunt between two upper heart chambers.
ASD is a common type of congenital heart disease, though their precise incidence is not well understood. ASD is estimated about 10% of all congenital heart disease. An isolated ASD may found at any age, since the resulting murmur is often overlooked, and often asymptomativ through infancy and childhood.
In children with congenital heart disease, it is estimated that as many as 33-50% will have an associated ASD as part of their complex of disease. Interestingly, the prevalence of ASDs increases in populations living at higher altitudes.
Classifications
There are three kinds of ASD, that determine by location of the defect on the atrium wall.
- Primum ASDs belong to the family of atrioventricular canal defects, also referred to as endocardial cushion defects or atrioventricular septal defects (AVSDs). Those defects are typically moderate to large in size and therefore allow significant shunting. Like the complete atrioventricular septal defect, these defects are almost universally associated with abnormalities of the left atrioventricular valve including clefts, mitral regurgitation and other complicating features.
- Sinoseptal defects involve the area of the atrial septum derived from the sinus venosus. This includes defects at the junction of the superior vena cava and the right atrium. The right upper pulmonary veins typically enter the left atrium superiorly just to the left of the atrial septum and sinus venosus region. When a defect of the superior sinus venosus exists, the flow from these veins may be directed toward the right atrium through the sinus venosus defect. These defects are known as sinus venosus defects.
- The ostium secundum atrial septal defect is the most common type of atrial septal defect, and comprises 6-10% of all congenital heart diseases.
The secundum atrial septal defect usually arises from an enlarged foramen ovale, inadequate growth of the septum secundum, or excessive absorption of the septum primum.
Normal Physiology
In normal cardiac development, the septum primum grows downward from the top of the atria toward the central endocardial cushion tissue. As this process completes itself the primary foramen becomes smaller and is eventually closed. Prior to its closure however, perforations of the septum primum occur and these coalesce to form the ostium secundum in the central portion of the septum primum. The septum secundum then begins to grow downward to the right of the septum primum and toward the central portion of the heart. This growth continues until the septum secundum covers the superior rim of the ostium secundum or foramen ovale.
During fetal life, systemic venous return from the lower part of the body, including the newly oxygenated blood returning from the placenta, is directed toward the foramen ovale by a flap of tissue called the Eustachian valve. This allows the relatively well-oxygenated blood to enter the left heart circulation so that it may be delivered to the coronary arteries as well as those vessels supplying the developing central nervous system.
The foramen ovale is maintained patent in the fetus because right atrial pressure is higher than the left atrial pressure. At birth, the organ of oxygenation is transferred from the placenta, which is removed, to the lungs which become aerated. This results in a dramatic increase in the volume of blood delivered to the lungs. As a result, the volume of blood return from the lungs to the left atrium through the pulmonary veins increases, and this leads to an increase in left atrial pressure which forces the septum primum against the septum secundum, thus effectively closing the foramen ovale. This fusion is complete in most infants by a month of age, although as many as 20% of adults may have a probe patent, or intermittently patent foramen ovale.
In the transition from fetus to neonate, the relative compliance of the right ventricle and left ventricle are similar. As a result, the right and left atrial pressures are relatively equal and any flow that results from a communication between the right and left atrium may be bidirectional, with a slight net left to right shunt predominately.
During infancy, the right ventricle gradually becomes more compliant than the left due to the markedly lower pulmonary vascular resistance. This allows the more compliant right ventricle to accept more volume than the left ventricle and there is a marked increase in left to right shunting through any given atrial septal defect.
Clinical Manifestation
Patients with an ASD are generally asymptomatic through infancy and childhood. Only rarely are symptoms of pulmonary overcirculation, frequent respiratory infections and congestive heart failure seen in infants who have no other cardiovascular abnormality. Patients diagnosed later in childhood usually come to the cardiologist because of a systolic murmur appreciated by the primary care provider. Symptoms typically are more apparent in the older adolescent and young adult and these usually involve dyspnea on exertion, easy fatigue compared to peers, palpitations or sustained atrial arrhythmias.
The physical exam usually reflects the size of the ASD and the relative compliance of the right and left ventricle. Small ASDs are extremely unlikely to be associated with any historical or physical findings. If the defect is of moderate to large in size, the precordium may demonstrate a hyperactive impulse. With inspection, there may be a prominence of the left chest if the volume of shunt is large. On palpation of the chest wall, there may be a ventricular lift, particularly apparent with expiration, in the patient with a large volume shunt. The first heart sound may be accentuated or normal and the second heart sound is widely split without significant variation with respiration. This is in contrast to the normal child in whom the second heart sound splitting varies with respiration. It is thought that the wide split in the patient with an ASD results from a relative delay in emptying of the volume loaded right ventricle. The lack of variation of the split most likely results from the free communication between the two atria, which allows for equalization of the influence of respiration on both the right and left ventricle.
There is often a systolic murmur (grade 3 or less) that has an outflow quality mimicking that of pulmonary stenosis, and may in fact emanate from the turbulence across the pulmonary valve resulting from the increased flow imposed by the ASD. The murmur is typically loudest at the mid to upper left sternal border, and may radiate posteriorly. With a large left to right shunt through an ASD, there may be an early to mid diastolic rumble at the lower left sternal boarder, which results from turbulence across the tricuspid valve.
Supporting Diagnosis
Chest X-ray may demonstrate a normal cardiac size in the patient with a small or moderate atrial septal defect. Pulmonary vascularity is likely to be increased in these patients. With increasing size of the atrial shunt, the right atrium and right ventricle may appear dilated. The former is best seen on the posteroanterior view, while the latter is most evident on the lateral view with a noticeable reduction in the retrosternal clear space. As the volume of left to right shunting increase, the pulmonary vascularity will become more prominent and the main pulmonary artery segment will be enlarged.
ECG typically shows a sinus rhythm with a normal P wave axis in patients with a secundum ASD. The P wave axis may be abnormal when there is a sinus venosus ASD. The normal regression of relative right ventricular forces with age does not typically occur in children with an ASD. There is usually right axis deviation and there is often an incomplete right bundle branch block. In older children and adults, complete or incomplete right bundle branch block pattern is seen in 90% of patients. Patients with a primum atrial septal defect typically have a superior frontal QRS axis in addition to the findings noted above.
Once an atrial septal defect has been detected, it should be closed at a convenient time for the family. Usually this is prior to school age for young children. Exceptions to this are the primum atrial septal defect and sinus venosus defects that may be associated with morbidity by preschool age, and are thus usually closed in the first few years of life. The primary indication to close an atrial septal defect is to prevent eventual right ventricular dysfunction from chronic volume overload, risk of paradoxical thrombus and pulmonary vascular obstructive disease. Ostium secundum defects in isolation do not require antibiotic prophylaxis.
Treatment
Two methods of closure currently exist for the common secundum atrial defect - surgical or transcatheter closure. Surgical repair should carry an extremely low mortality and should yield few incomplete closures. More recently, techniques employing transcatheter devices have been developed. With this procedure, patients can undergo ASD closure on an outpatient basis or with a short inpatient stay. Factors negating the transcatheter approach include large atrial defects, close proximity of the defect to the atrioventricular valves and sinoseptal defects. In addition, patients must be of sufficient size and weight to accommodate the transvenous sheath delivery system.
Atrial septal defect (ASD) is an opening in the atrial septum allowing blood to shunt between left and right atria or shunt between two upper heart chambers.
ASD is a common type of congenital heart disease, though their precise incidence is not well understood. ASD is estimated about 10% of all congenital heart disease. An isolated ASD may found at any age, since the resulting murmur is often overlooked, and often asymptomativ through infancy and childhood.
In children with congenital heart disease, it is estimated that as many as 33-50% will have an associated ASD as part of their complex of disease. Interestingly, the prevalence of ASDs increases in populations living at higher altitudes.
Classifications
There are three kinds of ASD, that determine by location of the defect on the atrium wall.
- Primum ASDs belong to the family of atrioventricular canal defects, also referred to as endocardial cushion defects or atrioventricular septal defects (AVSDs). Those defects are typically moderate to large in size and therefore allow significant shunting. Like the complete atrioventricular septal defect, these defects are almost universally associated with abnormalities of the left atrioventricular valve including clefts, mitral regurgitation and other complicating features.
- Sinoseptal defects involve the area of the atrial septum derived from the sinus venosus. This includes defects at the junction of the superior vena cava and the right atrium. The right upper pulmonary veins typically enter the left atrium superiorly just to the left of the atrial septum and sinus venosus region. When a defect of the superior sinus venosus exists, the flow from these veins may be directed toward the right atrium through the sinus venosus defect. These defects are known as sinus venosus defects.
- The ostium secundum atrial septal defect is the most common type of atrial septal defect, and comprises 6-10% of all congenital heart diseases.
The secundum atrial septal defect usually arises from an enlarged foramen ovale, inadequate growth of the septum secundum, or excessive absorption of the septum primum.
Normal Physiology
In normal cardiac development, the septum primum grows downward from the top of the atria toward the central endocardial cushion tissue. As this process completes itself the primary foramen becomes smaller and is eventually closed. Prior to its closure however, perforations of the septum primum occur and these coalesce to form the ostium secundum in the central portion of the septum primum. The septum secundum then begins to grow downward to the right of the septum primum and toward the central portion of the heart. This growth continues until the septum secundum covers the superior rim of the ostium secundum or foramen ovale.
During fetal life, systemic venous return from the lower part of the body, including the newly oxygenated blood returning from the placenta, is directed toward the foramen ovale by a flap of tissue called the Eustachian valve. This allows the relatively well-oxygenated blood to enter the left heart circulation so that it may be delivered to the coronary arteries as well as those vessels supplying the developing central nervous system.
The foramen ovale is maintained patent in the fetus because right atrial pressure is higher than the left atrial pressure. At birth, the organ of oxygenation is transferred from the placenta, which is removed, to the lungs which become aerated. This results in a dramatic increase in the volume of blood delivered to the lungs. As a result, the volume of blood return from the lungs to the left atrium through the pulmonary veins increases, and this leads to an increase in left atrial pressure which forces the septum primum against the septum secundum, thus effectively closing the foramen ovale. This fusion is complete in most infants by a month of age, although as many as 20% of adults may have a probe patent, or intermittently patent foramen ovale.
In the transition from fetus to neonate, the relative compliance of the right ventricle and left ventricle are similar. As a result, the right and left atrial pressures are relatively equal and any flow that results from a communication between the right and left atrium may be bidirectional, with a slight net left to right shunt predominately.
Clinical Manifestation
Patients with an ASD are generally asymptomatic through infancy and childhood. Only rarely are symptoms of pulmonary overcirculation, frequent respiratory infections and congestive heart failure seen in infants who have no other cardiovascular abnormality. Patients diagnosed later in childhood usually come to the cardiologist because of a systolic murmur appreciated by the primary care provider. Symptoms typically are more apparent in the older adolescent and young adult and these usually involve dyspnea on exertion, easy fatigue compared to peers, palpitations or sustained atrial arrhythmias.
The physical exam usually reflects the size of the ASD and the relative compliance of the right and left ventricle. Small ASDs are extremely unlikely to be associated with any historical or physical findings. If the defect is of moderate to large in size, the precordium may demonstrate a hyperactive impulse. With inspection, there may be a prominence of the left chest if the volume of shunt is large. On palpation of the chest wall, there may be a ventricular lift, particularly apparent with expiration, in the patient with a large volume shunt. The first heart sound may be accentuated or normal and the second heart sound is widely split without significant variation with respiration. This is in contrast to the normal child in whom the second heart sound splitting varies with respiration. It is thought that the wide split in the patient with an ASD results from a relative delay in emptying of the volume loaded right ventricle. The lack of variation of the split most likely results from the free communication between the two atria, which allows for equalization of the influence of respiration on both the right and left ventricle.
There is often a systolic murmur (grade 3 or less) that has an outflow quality mimicking that of pulmonary stenosis, and may in fact emanate from the turbulence across the pulmonary valve resulting from the increased flow imposed by the ASD. The murmur is typically loudest at the mid to upper left sternal border, and may radiate posteriorly. With a large left to right shunt through an ASD, there may be an early to mid diastolic rumble at the lower left sternal boarder, which results from turbulence across the tricuspid valve.
Supporting Diagnosis
Chest X-ray may demonstrate a normal cardiac size in the patient with a small or moderate atrial septal defect. Pulmonary vascularity is likely to be increased in these patients. With increasing size of the atrial shunt, the right atrium and right ventricle may appear dilated. The former is best seen on the posteroanterior view, while the latter is most evident on the lateral view with a noticeable reduction in the retrosternal clear space. As the volume of left to right shunting increase, the pulmonary vascularity will become more prominent and the main pulmonary artery segment will be enlarged.
ECG typically shows a sinus rhythm with a normal P wave axis in patients with a secundum ASD. The P wave axis may be abnormal when there is a sinus venosus ASD. The normal regression of relative right ventricular forces with age does not typically occur in children with an ASD. There is usually right axis deviation and there is often an incomplete right bundle branch block. In older children and adults, complete or incomplete right bundle branch block pattern is seen in 90% of patients. Patients with a primum atrial septal defect typically have a superior frontal QRS axis in addition to the findings noted above.
Once an atrial septal defect has been detected, it should be closed at a convenient time for the family. Usually this is prior to school age for young children. Exceptions to this are the primum atrial septal defect and sinus venosus defects that may be associated with morbidity by preschool age, and are thus usually closed in the first few years of life. The primary indication to close an atrial septal defect is to prevent eventual right ventricular dysfunction from chronic volume overload, risk of paradoxical thrombus and pulmonary vascular obstructive disease. Ostium secundum defects in isolation do not require antibiotic prophylaxis.
Treatment
Two methods of closure currently exist for the common secundum atrial defect - surgical or transcatheter closure. Surgical repair should carry an extremely low mortality and should yield few incomplete closures. More recently, techniques employing transcatheter devices have been developed. With this procedure, patients can undergo ASD closure on an outpatient basis or with a short inpatient stay. Factors negating the transcatheter approach include large atrial defects, close proximity of the defect to the atrioventricular valves and sinoseptal defects. In addition, patients must be of sufficient size and weight to accommodate the transvenous sheath delivery system.
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