Monday, March 26, 2012

What is Atrial Septal Defect (ASD)?


Atrial Septal Defect is the most common shunt lesion in adults. It often remains undetected until adulthood because patients are often asymptomatic. The condition remains unnoticed in infancy and childhood till pulmonary hypertension is induced causing late cyanotic heart disease and right-sided heart failure. A large ASD may be associated with growth failure in infants and children. It is more common in women thin in men.

There are three types of Atrial Septal Defect

Ostium Secundum ASD (75%)
It is the most common type (70% of patients) and results from defect in the region of the fossa ovalis. In this type there is no involvement of AV valves.

Ostium Primum ASD (15%)
It results from absence of septum primum at the lower portion of the atrial septum. It is more common in Down’s syndrome. It is more common in Down’s syndrome. In this type there is also involvement of AV valves.

Sinus Venosus Defect (10%)
It is in the upper part of the septum near the entrance of the inferior vena cava and coronary sinus therefore producing communication between coronary sinus and left atrium.

Pathophysiology
Blood flows from high pressured left atrium to low pressured right atrium (left to right shunt) through ASD. Pulmonary flow is increased and right ventricle is dilated and hypertrophied owing to the volume overload. With time, pulmonary vascular resistance increases resulting in pulmonary hypertension. This pulmonary hypertension in turn causes right heart failure or reversal of the left to right shunt (i.e now blood flows from right atrium to left atrium through this Atrial Septal Defect). Direct mixing of this deoxygenated blood produces cyanosis.
Above the age of 30 years there may be an increase in pulmonary vascular resistance, which gives rise to pulmonary hypertension. Atrial fibrillation is common at this stage.

Clinical Features
Symptoms
  • Asymptomatic for may years if there is small or moderate defect and no pulmonary hypertension. Normal or minimally diminished exercise tolerance may develop.
  • Exertional dyspnea after age 30
  • Palpitations due to atrial arrhythmia
  • Patients are more prone to develop recurrent pulmonary infections.
  • Right ventricular failure with large defect and prolonged pulmonary hypertension may develop later in life.

On CVS Examination
Systolic ejection murmur over the pulmonary area that does not vary with respiration.

Investigations
X-ray Chest
  • Prominent pulmonary artery
  • Pulmonary plethora (plethora means fullness)
  • Right ventricular hyperthrophy

ECG: Shows incomplete right bundle branch block (RBBB) and right axis deviation.

Echocardiography: It can identify the type of defect, estimates pulmonary artery pressure, demonstrate right ventricular hypertrophy and pulmonary arterial dilation. It also demonstrates paradoxical motion of the interventricular septum. Doppler echocardiography is also required. Transesohageal echocardiography can detect small atrial septal defect that is missed on routine transchoracic 2-D echocardiography.

Cardiac Catheterization
It is performed unless the patient has large defect on echocardiography.
  • It is the definitive diagnostic procedure
  • It quantifies the shunt 
  • It measures pulmonary vascular resistance.

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