Atrial Septal Defect

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Diagnosis and reasoning

Exertional dyspnea associated with palpitations is most likely cardiac in origin. Given the young age of the patient and relatively long duration of symptoms, congenital heart disease and rheumatic valvular disease are the likely causes. Examination reveals wide fixed splitting of the second heart sound and an ejection systolic murmur in the pulmonary area. These findings are strongly suggestive of an atrial septal defect (ASD) - she needs further evaluation via an ECG, chest x-ray and echocardiogram. The tall P waves and prolonged PR interval in the ECG are are suggestive of right atrial enlargement, while the right axis deviation and right bundle branch block suggest right ventricular dilation. These findings are supported by the chest x-ray, which shows straightening of the right cardiac border supportive of right heart enlargement. The echocardiogram confirms the clinical diagnosis, revealing a moderate size ostium secondum atrial septal defect (ASD) with significant shunting. Note that coronary angiography is not indicated in this patient. Closure of the ASD is indicated, given the degree of shunting. Antiarrythmic therapy and endocarditis prophylaxis are not indicated. Heart-lung transplantation would be indicated if the Eisenmenger Syndrome develops.


Discussion

ASDs account for about one third of cases of adult congenital heart disease and are two to three times more common in women than men. Most ASDs arise from spontaneous genetic mutations, although they may be inherited. ASDs are classified based on their anatomical location into Ostium Secondum defects (75%), which are found in the region of the fossa ovalis (i.e. centrally); Ostium Primum defects (15%), which are located in the lower part of the atrial septum; and Sinus Venosus defects (10%), which are found in the upper part of the atrial septum. The presence of a defect between the right and left atria results in the shunting of blood from one side to the other. The direction and the magnitude of the shunt depend both on the diameter of ASD as well as the relative compliance (i.e "stiffness") of the right and left ventricles. In general, a small defect (< 0.5 cm) tends to cause minimal shunting, while large defects (> 2 cm) tend to cause significant shunting, with profound hemodynamic compromise. As the right ventricle is more compliant than the left, shunting initially occurs from the left atrium to the right atrium. This results in increased pulmonary blood flow and dilation of the right atrium and ventricle. With the passage of time, the compliance of the right ventricle tends to decline, resulting in a progressive decrease in the degree of left to right shunt and eventual reversal into a right to left shunt. Most patients with ASDs remain asymptomatic until the 3rd to 4th decades of life - even if the defect is quite large. When symptomatic, they typically report fatigue or dyspnea on exertion. In addition, they may also present with complications such as supraventricular arrhythmias, right heart failure, paradoxical embolism, or recurrent pulmonary infections. Typical examination findings include wide fixed splitting of the second heart sound (S2), an ejection systolic murmur in the pulmonary area and a parasternal heave. The wide fixed splitting occurs because the defect equalizes the pressures in the atria, exaggerating the normal physiologic splitting. The murmur results from increased pulmonary blood flow, while the parasternal heave is secondary to right ventricular enlargement. Common ECG findings include tall P waves, a prolonged PR interval secondary to right atrial enlargement, and incomplete right bundle-branch block secondary to right ventricular enlargement. Ostium Secondum defects typically cause right-axis deviation, while Ostium Primum defects usually cause left-axis deviation (due to concomitant left anterior fascicular hemiblock). With progressive dilation of the right atrium, supraventricular tachycardia or atrial fibrillation may occur. Note that these changes may persist even after ASD closure. The chest x-ray may show prominent pulmonary arteries and evidence of right atrial and ventricular dilation. If pulmonary hypertension is present, peripheral pruning may also be noted. TTE will usually allow visualization of ostium primum and ostium secundum ASDs - although sinus venosus defects may not be detected. In addition, dilation of the right atrium and ventricle may also be noted. Color Doppler studies help quantify the direction and degree of the shunt. In certain cases, catheterization may be required to determine the magnitude and direction of shunting, as well as to determine if pulmonary hypertension is present. Unlike Ventricular Septal Defects, ASDs do not close spontaneously. However, neither do all ASDs need endovascular or surgical intervention. The most important factors in determining the management include the degree of shunting (measured by the ratio of pulmonary to systemic flow) and the presence of pulmonary hypertension or Eisenmenger syndrome. In general, small ASDs with minimal left-to-right shunts (characterized by a ratio of pulmonary to systemic flow of less than 1.5) tend to be asymptomatic with no hemodynamic abnormalities and therefore do not require closure. In addition, even large atrial septal defects often remain asymptomatic until the third or fourth decades of life despite substantial left-to-right shunting (characterized by a ratio of pulmonary to systemic flow of 1.5 or more). Over the years, the increased volume of blood flowing through the chambers of the right side of the heart usually causes right ventricular dilatation and failure. Obstructive pulmonary vascular disease (Eisenmenger syndrome) occurs rarely in adults with ADS. An atrial septal defect with a ratio of pulmonary to systemic flow of >= 1.5 should be closed surgically to prevent right ventricular dysfunction. However, Surgical closure is not recommended for patients with irreversible pulmonary vascular disease and pulmonary hypertension. Percutaneous atrial septal closure is an alternative. Other indications for ASD closure include a paradoxical embolism or impending paradoxical embolism (which may be diagnosed by performing contrast echocardiography during and after a Valsalva maneuver). In general patients with sizable shunts often die of right ventricular failure or arrhythmia in their 30s or 40s, if left untreated. Note that prophylaxis against infective endocarditis is not recommended in these patients.


Take home messages

  1. The key clinical findings in patients with an ASD are exertional dyspnea, fixed wide splitting of the S2 and an ejection systolic murmur in the pulmonary area.
  2. The main complications are supraventricular arrhythmias, right heart failure, pulmonary hypertension, Eisenmenger syndrome, paradoxical embolism and recurrent pulmonary infections.
  3. Infective endocarditis prophylaxis is not recommended in these patients.
  4. Indications for intervention include a pulmonary:systemic flow ratio >= 1.5 and a defect > 2 cm in size.

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  1. American Family Physician : Diagnostic Evaluation of Dyspnea (1998)
  2. American Heart Journal : Percutaneous versus surgical closure of secundum atrial septal defect: Comparison of early results and complications (2006).
  3. Circulation : Atrial Septal Defects in the Adult (2006)
  4. Journal of the American College of Cardiology : Surgical Versus Percutaneous Occlusion of Ostium Secundum Atrial Septal Defects - Results and Cost-Effective Considerations in a Low-Income Country (2006)
  5. NEJM : Congenital Heart Disease in Adults (2000)