A 37-year-old woman is evaluated for a 6-month history of progressive shortness of breath. Although she remains physically active, she becomes dyspneic when walking up multiple flights of stairs or running to catch a bus. Medical history is significant for a diagnosis of a pulmonary embolism 2 years ago, which was associated with oral contraceptive use. She was initially treated with low-molecular-weight heparin followed by therapeutic warfarin for 3 months. She is a nonsmoker. Medical history is otherwise unremarkable, and she takes no medications.
On physical examination, she is afebrile, blood pressure is 128/76 mm Hg at rest, pulse rate is 72/min, and respiration rate is 15/min. Oxygen saturation is 98% breathing ambient air. Pulmonary examination reveals clear lungs. Cardiac examination is significant for a fixed, split S2, a holosystolic murmur at the left sternal border that increases on inspiration, and a heave. Trace lower extremity bilateral edema is present. The remainder of the examination is noncontributory.
Walking up stairs at the office at a moderate pace, she becomes short of breath after two flights of stairs, oxygen saturation decreases to 92%, and pulse rate increases to 145/min.
A chest radiograph is normal, showing no parenchymal abnormalities. Transthoracic echocardiography shows right atrial and ventricular dilation and moderate tricuspid regurgitation but no other valvular abnormalities.
Which of the following is the most appropriate diagnostic test to perform next?