USMLE Step 3 - Question 29

A 24 year old female with no significant past medical history presents to her primary care physician with exercise intolerance. She has been trying to exercise to lose weight but becomes short of breath after only minimal effort. Her temperature is 37.0, blood pressure 110/70, heart rate 110, respirations 20, and oxygen saturation 94% on room air. Physical examination reveals normal jugular venous pressure, normal breath sounds, a II/VI systolic murmur at the left upper sternal border and a fixed split S2 heart sound. Laboratory studies are normal. ECG reveals normal sinus rhythm, a right bundle branch block, and left axis deviation. Which of the following is her likely diagnosis?

A. Ostium primum atrial setpal defect

B. Ostium secundum atrial septal defect

C. Ventricular septal defect

D. Patent ductus arteriosis

E. Ebstein's anomaly

An atrial septal defect is an abnormal communication between the left and right atrium. This is a congenital defect that initially causes blood to flow from the left atrium  (higher pressure) directly to the right atrium (lower pressure) resulting in a left to right shunt. Depending on these size of the ASD symptoms can vary from no symptoms to severe heart failure symptoms. Over time pulmonary hypertension develops due to the abnormally large amount of blood coursing through the pulmonary vasculature. As the pulmonary pressures increase the right ventricle fails resulting in elevated right atrial pressure and symptoms of right heart failure. When the right atrial pressure exceeds that of the left atrium, the shunt turns from left > right to right > left. This results in severe symptoms of heart failure and is known as Eisenmenger's syndrome. Remember that an ostium primum ASD has a right bundle branch block and left axis deviation on the ECG while an ostium secundum ASD has a right bundle branch block and right axis deviation on the ECG. The physical exam findings of an ASD include a pulmonic valve flow murmur (due to the large amount of blood passing through this valve) and a fixed split S2 heart sound (note this finding is frequently the key to answering the question currently on board exams, so remember if you see fixed split S2 the answer is an ASD). Recall a fixed split S2 is when the S2 is split in both inspiration and expiration (normally the S2 splits only in inspiration aka physiologic split S2).