57 y/o man has acute SOB, Tachypnea, and tachycardia while hospitalized for right hip replacement. On examination, HR 120/min, BP 130/75mmHg, RR 25/min and oxygen sat 90% on room air. He has no jugular venous distension, and the lung are clear to auscultation. There is a loud P2 sound. Lower extremities show no swelling or tenderness. Which of the following is the most appropriate diagnostic test in this patient?