Dyspnea, right-axis deviation, and abnormal P waves in a young woman

A 22-year-old woman is seen for progressive dyspnea on exertion experienced for the past six months. She has no dyspnea at rest, orthopnea, cough, or other symptoms. There is no history of heart murmurs, rheumatic fever, or pulmonary disease. Physical examination shows a healthy-appearing young woman with no cyanosis or clubbing. The jugular venous pulse shows a prominent A wave, with normal mean venous pressure. The lungs are clear to examination. The cardiac examination is normal except for a loud second heart sound that is narrowly split in expiration and a little more widely split in inspiration. The chest roentgenogram shows slight cardiac enlargement of nonspecific contour, with a considerably enlarged main pulmonary artery segment and moderately prominent central pulmonary artery branches. The peripheral lung fields are unremarkable. The electrocardiogram is shown.