Atrial septal defects (ASDs) and ventricular septal defects (VSDs)

Atrial septum defect (ASD)

An atrial septal defect (ASD) is a congenital defect in the interatrial septum allowing a direct communication between the atria. It is common, about 10% of all congenital heart disease cases, more common in females. (male to female ratio of 1:2).
There are three main types of ASDs:
Secundum ASD, the most common type (70-75% of ASDs) is located in the central portion of the interatrial septum.
Primum ASD, about 20% of all ASDs, (or partial endocardial cushion defect ) is in the lower part of the septum, near the atrioventricular valves. This is the second most common type. 
Sinus venosus defect is located near the entrance of the superior vena cava (SVC) or the inferior vena cava IVC to the right atrium and is a less common type (about 5-10% of all ASDs), commonly associated with a partial anomalous pulmonary venous return. 
An ASD causes an abnormal flow between the atria, called a left to right (L-R) shunt, i.e. flow of blood through the defect with a direction towards the right heart chambers. This results in a volume overload to the right atrium and right ventricle and an increase in blood flow through the pulmonary arterial circulation. The magnitude of the L-R shunt depends upon the size of the defect and also the relative pressures on the left and right sides of the heart.
The patients are asymptomatic in the majority of cases but large defects can be associated with recurrent respiratory infections, shortness of breath on exertion (dyspnea), easy fatiguability, or atrial arrhythmias (palpitations). Patients with large defects, late in the course can develop complications, such as right heart failure, or pulmonary arterial hypertension PAH (see also chapter Pulmonary Hypertension). Another rare complication of an ASD is a stroke due to paradoxical embolism (caused by a thrombus reaching the right atrium from the venous circulation and then crossing the ASD and entering the left cardiac chambers and the arterial circulation).
In a moderate or large ASD, physical examination findings usually include a widely split and fixed second heart sound (S2) and a systolic ejection murmur of low intensity, (grade 2 to 3/6) at the upper left sternal border (at the area of the pulmonary artery due to the increased blood flow through the right heart and the pulmonary artery).
In case of a large ASD with a large L-R shunt, a mid-diastolic rumble may be audible at the left lower sternal border, i.e. at the tricuspid area due to the increased flow through the valve.
The typical auscultatory findings often are not present in infants and toddlers, even with a large defect, because at this age the right ventricle often is not compliant enough to allow a large L-R shunt.
The ECG usually shows a RBBB, or an incomplete RBBB with an rsR′ pattern in V1 (also see chapter The Electrocardiogram -ECG . In case of a moderate to large ASD, right QRS axis deviation (frontal axis between +90 to +180°) and/or other indications of right ventricular hypertrophy may be present , but in the case of an ostium primum defect there is an RBBB pattern with a left QRS axis.
Chest radiographs if the shunt is moderate or large, show cardiomegaly (with right atrial and right ventricular enlargement), increased vascular markings in the lungs and a prominent main pulmonary artery (at the mid-left heart border).
Echocardiography in a patient with an atrial septal defect (ASD) shows the position and the size of the defect and the abnormal flow (shunt) through the interatrial septum. If the defect is moderate to large, echocardiography also shows a dilated right atrium, right ventricle, and pulmonary artery. The finding of dilated right heart chambers proves that the ASD is hemodynamically significant and is in favor of ASD closure.
Cardiac catheterization is not necessary for the diagnosis of an ASD, but it can be necessary in the case of significant pulmonary hypertension to decide if the defect should be closed.

 Natural history of atrial septal defects (ASDs)

Spontaneous closure of an ASD often occurs in patients with secundum ASDs with a diameter < 8mm before the age of 1.5 years, but an ASD >8 mm rarely closes spontaneously.
After the age of 4 years, spontaneous closure is not likely to occur.
Spontaneous closure may occur only in secundum defects. It does not occur in primum or sinus venosus ASDs. If a large ASD is left without closure, pulmonary hypertension and signs of right heart failure can develop in the third or fourth decade of life.

Management of atrial septal defects

 In a patient with an ASD exercise restriction is not required unless the patient has symptoms.
Children with a secundum atrial septal defect are usually observed without intervention for at least the first 3 years of life, due to the possibility of spontaneous closure.
Closure of an ASD in children or adults, is indicated when there is evidence of right ventricular volume overload (a dilated right ventricle) in the presence of an atrial septal defect with size > 5 mm and in the absence of irreversible pulmonary arterial hypertension. In this case measurement of the ratio of pulmonary to systemic flow (Qp/Qs) is not necessary to confirm that the ASD is hemodynamically significant. If the ratio Qp/Qs is used, then the defect is considered hemodynamically significant and closure is decided if Qp/Qs ≥ 1.5:1, in the absence of irreversible pulmonary arterial hypertension. Closure of an ASD can be surgical or with a catheter device. Device closure is the preferred method when feasible, but it is only feasible in patients with a secundun ASD with a diameter ≥5 mm but < 32 mm for Amplatzer device and <18 mm for Helex device, provided there is enough rim ( at least 4 mm) of septal tissue around the defect ( for the appropriate placement of the device). The rim around the ASD is measured with 2-D echocardiography in 4 directions. After device closure of an ASD antiplatelet treatment is given, with aspirin 80-100 mg per day for 6 months. In children, device closure can be performed preferably if they weigh >15 kg.In the rare case of an ASD with pulmonary hypertension, closure of the ASD is indicated if systolic pulmonary arterial pressure is <2/3 of systemic systolic blood pressure and pulmonary vascular resistance <2/3 of systemic vascular resistance. If systolic pulmonary artery pressure and pulmonary vascular resistance exceed the above limits, ASD closure can be decided only when there is a L-R shunt with a ratio of pulmonary to systemic flow Qp / Qs of at least 1.5: 1,  provided that pulmonary hypertension is reversible.

A case (Video) showing the ECG and echocardiographic features of this condition

Ventricular septal defects (VSDs)

There are four anatomic types depending on the location of the defect.
Perimembranous or membranous VSD is the most common type (70%) and it involves the membranous septum, a small part of the septum immediately beneath the aortic valve. These defects usually also have an accompanying defect of the adjacent muscular septum and depending on the location of this defect they are further classified as perimembranous trabecular, perimembranous inlet or perimembranous outlet.
Another type of VSDs are inlet defects, which are located beneath the septal leaflet of the tricuspid valve.
Outlet VSDs, also called conal or supracristal, are located in close proximity to the annulus of the aortic valve and of the pulmonary valve. These two valve annuli form part of the defect's rim. A common complication of this defect involves herniation of the right coronary cusp of the aortic valve through the defect. This can cause a reduction of the shunt, but also it often causes progressive aortic regurgitation. It may also result in a degree of obstruction of the right ventricular outflow tract.
Trabecular (muscular) VSDs may be central or apical, depending on their location in the muscular interventricular septum.

Pathophysiology of ventricular septal defects (VSDs)

The main pathophysiologic feature of a VSD is a left to right (L-R) shunt. The magnitude of the shunt depends on the size of the defect and the pressure difference between the two ventricles, which in turn depends on the pulmonary arterial pressure and thus, on the level of the pulmonary arterial resistance. The larger the size of the defect and the lower the pulmonary vascular resistance, the larger the L-R shunt. 
A small defect does not cause any ventricular dilation or hypertrophy, a moderate-sized defect usually can cause left ventricular hypertrophy or dilation and a large defect can cause hypertrophy or dilation of both ventricles. Moreover, a small VSD, also called a restrictive VSD, has a large resistance and results in a significant pressure gradient between the two ventricles and a small shunt with Qp/Qs<1.5 :1 and pulmonary systolic pressure/systemic systolic pressure<0.3. 
On the contrary, a large nonrestrictive defect will cause a large shunt with Qp/Qs >2.2 and a small pressure gradient between the two ventricles. Pulmonary arterial systolic pressure/systemic systolic arterial pressure will be > 0.6. With a VSD of a moderate size these values will be intermediate, with a Qp/Qs between 1.5 and 2.2.
A large VSD if left untreated will cause over the years a progressive rise of the pulmonary vascular resistance, due to progressive obstructive changes of the pulmonary arterioles.This leads to an elevated pulmonary arterial and right ventricular systolic pressure resulting in a reduction in the magnitude of the L-R shunt. This condition has also effects on the ventricles with the left ventricle decreasing in dimensions, whereas the size of the right ventricle increases. When the obstructive changes in the pulmonary vasculature become serious and irreversible, bidirectional shunt develops across the VSD (or even net right to left R-L shunt) and this leads to the development of cyanosis because unoxygenated venous blood from the right ventricle enters the left ventricle and the systemic circulation.

Symptoms and signs of a VSD

In infants and children, a small restrictive defect does not cause any symptoms and is diagnosed because of the auscultation of a murmur. A large non-restrictive VSD can cause dyspnea, failure to thrive and signs of congestive heart failure early, even at the age of 2-3 months, or later. In adults a small restrictive defect will be asymptomatic (and the only sign will be the murmur), a defect of a moderate size may cause dyspnea on exertion and palpitations due to the development of atrial fibrillation, whereas a large non-restrictive VSD can cause symptoms and signs of right heart failure and central cyanosis (Eisenmenger syndrome). Apart from cyanosis patients with Eisenmenger syndrome often have edema (due to the right-sided heart failure) and clubbing.
The murmur of a VSD is pansystolic (holosystolic, i.e. it is present during the whole duration of systole), grade 2/6-5/6, best heard at the lower left sternal border. The murmur can be accompanied by a palpable systolic thrill (also at the lower sternal border). The murmur occasionally can be early systolic in patients with small muscular VSDs.
Murmurs of increased flow may be audible if the shunt is moderate to large, such as a systolic murmur due to increased flow through the pulmonary valve, or an apical diastolic rumble due to increased flow through the mitral valve (The L-R shunt causes not only increased flow through the pulmonary valve and the pulmonary circulation , but also an increased flow through the mitral valve which also leads to volume overload of the left ventricle. This happens because as a result of the increased flow through the pulmonary circulation, there is also an increased pulmonary venous return of blood to the left atrium).
If pulmonary hypertension develops, the pulmonic component (P2) of the second heart sound becomes loud (generally a loud P2 is a sign indicative of pulmonary hypertension, regardless of the etiology). With the development of pulmonary hypertension the systolic murmur of a VSD becomes less prominent and if severe pulmonary hypertension develops the murmur usually disappears. This happens because in the presence of pulmonary hypertension there is a smaller pressure difference between the left and right ventricle resulting in a reduction in the flow of blood through the defect from the left into the right ventricle.

The ECG and the chest X-ray in patients with a VSD

In patients with a small restrictive VSD both the ECG and the chest X-ray are usually normal. In patients with a defect of moderate size the ECG will usually show features of left ventricular and left atrial hypertrophy and enlargement respectively and in a large non-restrictive defect left atrial enlargement with combined ECG features of left and right ventricular hypertrophy. In the case of pulmonary vascular obstructive disease with pulmonary hypertension the ECG usually shows features of isolated right ventricular hypertrophy.
The chest X-ray in the case of a VSD with a moderate or large shunt will show enlargement of the cardiac silhouette (due to dilation of the left atrium and ventricle and occasionally also the right ventricle). A moderate to large shunt will also result in increased pulmonary vascular markings (to a degree that is in direct proportion to the magnitude of the L-R shunt). If pulmonary vascular obstructive disease with significant pulmonary arterial hypertension has developed, the main pulmonary artery and its main branches at the hila of the lungs are dilated, while there are markedly reduced pulmonary vascular markings at the lung fields (this is a typical chest X-ray pattern of pulmonary arterial hypertension of any cause).

Echocardiography in a patient with a ventricular septal defect

Transthoracic echocardiography can identify the presence of a VSD, from the turbulent jet of flow across the interventricular septum, and it can also demonstrate the location and size of the defect and it can provide information about its hemodynamic significance. An increased left ventricular and left atrial size (and also a Qp/Qs>1.5) indicates the presence of a significant (moderate or large) L-R shunt.
 With the exception of the trabecular VSDs, a useful marker for the identification of the type of the VSD is its location relative to the valves. A membranous defect is near the aortic valve, an infudibular defect near both the pulmonic and the aortic valve and an inlet defect near the tricuspid valve.

From the peak gradient across a VSD measured with continuous wave Doppler, by subtracting this gradient from the systolic arterial pressure, the right ventricular systolic pressure (RVSP) can be calculated. RVSP is equal to PASP (pulmonary artery systolic pressure) if there is no coexisting stenosis of the right ventricular outflow tract or the pulmonary valve.
Cardiac catheterization

Cardiac catheterization is not needed for the diagnosis of a VSD, but only if there is pulmonary hypertension to calculate the pulmonary vascular resistance and in some cases where echocardiography does not fully clarify the anatomic diagnosis, or if there is a significant possibility of coronary artery disease. In a VSD with a L-R shunt cardiac catheterization will reveal an elevated oxygen saturation in the pulmonary artery, and this is also an indication that there is no severe pulmonary hypertension. In case that severe pulmonary hypertension has developed this will result in a significant reduction in the L-R shunt and so, much less oxygenated blood from the left ventricle will enter the right ventricle and the pulmonary artery. Therefore in this case the oxygen saturation in the pulmonary arterial blood will be low. In these cases simultaneous comparison of the pulmonary arterial and systemic blood pressures is mandatory, as well as calculation of the pulmonaty vascular resistance and assessment of its responce to vasodilators. This assessment is necessary to demonstrate if pulmonary arterial hypertension is reversible or irrreversible.

The natural history of ventricular septal defects (VSds)

Membranous and muscular VSDs can decrease in size with time, or close spontaneously. Until the age of 6 months spontaneous closure occurs in about 30-40% of these defects.On the contrary, inlet or infudibular defects do not decrease in size and thus they also do not demonstrate spontaneous closure.
In infants with large VSDs congestive heart failure can occur but usually after the first 6-8 weeks of life, because at that time the pulmonary vascular resistance has fallen enough to allow a large L-R shunt which can cause severe volume overload of the left heart chambers due to the increased pulmonary venous return.
Large VSDs can cause a slowly progressive elevation of the pulmonary arterial pressure. This is the result of progressive obstructive changes in the pulmonary arterioles as a reaction of the pulmonary arterial circulation to the chronically increased blood flow. Although pulmonary vascular changes may begin early, even at an age of 6-12 months, severe pulmonary arterial hypertension leading to a R-L shunt and the appearance of cyanosis usually does not occur before the second decade of life.
Another complication that can develop in some infants with a large VSD is infudibular stenosis, i.e. stenosis of the right ventricular outflow tract just below the pulmonary valve. This reduces the L-R shunt and in some cases it may even cause the development of a R-L shunt with cyanosis, a condition similar to the tetralogy of Fallot.

Treatment of the patient with a VSD

In infants with large defects usually medical treatment is given initially (furosemide, ACE inhibitor) to control heart failure, while waiting for the gradual spontaneous decrease in the size of the defect, but if this does not happen or if there is inadequate control of the manifestations of congestive heart failure then closure of the defect is decided.
In children and adults indications for VSD closure are the following: A VSD of a hemodynamically significant size, i.e a defect causing symptoms or dilation of the left ventricle, or a gradual worsening in left ventricular function, or Qp/Qs> 1.5:1 or an elevation in pulmonary arterial pressure > 50 mmHg but in the last case with a pulmonary vascular resistance <7 Wood Units. If the pulmonary vascular resistance > 7 Woods then the defect is closed if there is a shunt with a Qp/Qs> 1.5 : 1 or reactivity of the pulmonary circulation to vasodilators has been demonstrated (a fall in pulmonary vascular resistance and pulmonary arterial pressure with vasodilators). If there is irreversible pulmonary arterial hypertension and Eisenmenger syndrome then closure of any pathologic communication between the left and right circulation (such as a VSD or an ASD) is contraindicated.
Another indication of closure of a VSD is related to the development of aortic regurgitation as a complication of an outlet VSD, which can be progressive. There is an indication for closure if such a VSD causes more than mild AR.

LINK: Cardiology book- Table of contents