How is an atrial septal defect diagnosed?
Most individuals with a significant ASD are diagnosed in utero or in early childhood with the use of ultrasonography or auscultation of the heart sounds during physical examination.
Diagnosis in adults: Many individuals with an ASD will have undergone surgical repair during childhood. The development of signs and symptoms due to an ASD are related to the size of the intracardiac shunt and the age of the individual. Individuals with a larger shunt tend to present with symptoms at a younger age. Adults with an uncorrected ASD will present with symptoms of dyspnea on exertion (shortness of breath with minimal exercise), congestive heart failure, or cerebrovascular accident (stroke). They may be noted on routine testing to have an abnormal chest x-ray or an abnormal EKG and may have atrial fibrillation.
Physical exam: The physical findings in adult with an ASD include those related directly to the intracardiac shunt, and those that are secondary to the right heart failure that may be present in these individuals. Upon auscultation of the heart, there may be an ejection systolic murmur that is attributed to the pulmonic valve. This is due to the increased flow of blood throug the pulmonic valve rather than any structural abnormality of the valve leaflets. In normal individuals, there is respiratory variations in the splitting of the second heart sound (S2). During respiratory inspiration, the negative intrathoracic pressure causes increased blood return into the right side of the heart. The increased blood volume in the right ventricle causes the pulmonic valve to stay open longer during ventricular systole. This causes a normal delay in the P2 component of S2. During expiration, the positive intrathoracic pressure causes decreased blood return to the right side of the heart. The reduced volume in the right ventricle allows the pulmonic valve to close earlier at the end of ventricular systole, causing P2 to occur earlier. In individuals with an ASD, there is a fixed splitting of S2. This is pathognemonic of ASD, as no other cardiac defect causes a fixed splitting of S2. The reason why there is a fixed splitting of the second heart sound is that the extra blood return during inspiration gets equalized between the left and right atrium due to the communication that exists between the atria in individuals with ASD.
Echocardiography: On trans-thoracic echocardiography, an atrial septal defect may be seen on color flow imaging asa jet of blood from the left atrium to the right atrium. If agitated saline is injected into a peripheral vein during echocardiography, small air bubbles can be seen on echocardiographic imaging. It may be possible to see bubbles travel across an ASD either at rest or during a cough. Because better visualization of the atria is achieved on a trans-esophageal echocardiogram, this test may be performed in individuals with a suspected ASD which is not visualized on trans-thoracic imaging. If the individual has adequate echocardiographic windows, it is possible to use the echocardiogram to measure the cardiac output of the left ventricle and the right ventricle independantly. In this way, it is possible to estimate the shunt fraction using echocardiograpy.
Electrocardiogram: The ECG findings in atrial septal defect vary with the type of defect the individual has. All individuals with atrial septal defects have a prolonged PR interval (a first degree heart block). The prolongation of the PR interval is probably due to the enlargement of the atria that is common in ASDs and the increased distance due to the defect itself. Both of these can cause an increased distance of internodal conduction from the SA node to the AV node. In addition to the PR prolongation, individuals with a primum ASD have a left axis deviation of the QRS complex while those with a secundum ASD have a right axis deviation of the QRS complex. Individuals with a sinus venosus ASD exhibit a left axis deviation of the P wave (not the QRS complex).