A 40-year-old woman has been complaining of a 3-year history of increasing dyspnea and fatigue. She has no other medical illness. Physical examination reveals increased jugular venous pressure (JVP) with prominent c-v wave, and a reduced carotid pulse. Precordial examination reveals a left parasternal lift, loud P2, and right-sided S3 and S4. There are no audible murmurs. CXR reveals clear lung fields and an ECG shows evidence of right ventricular hypertrophy. Pulmonary function tests show a slight restrictive pattern. Primary pulmonary hypertension is suspected. Which of the following is the most appropriate test to confirm the diagnosis?