A 55-year old man who has been on bed rest for the past 10 days, presents with sudden onset of breathlessness and chest pain. The chest x-ray is normal. Which of the following represents the next step in investigation?

Ventilation-perfusion scan has a sensitivity and specificity of 90% if results of the scan correlate with clinical risk factor assessment. Lung scanning is the second-line diagnostic test for Pulmonary Embolism, used mostly in patients who cannot tolerate intravenous contrast. The perfusion scan defect is indicated by absent or decreased blood flow. A high-probability scan indicates two or more segmental perfusion defects in the presence of normal ventilation. Echocardiography: Is not a reliable diagnostic imaging tool for acute PE because most patients with PE have normal echocardiograms. It is used to detect conditions that may mimic PE. Pulmonary Angiography: Chest CT with contrast has viually replaced invasive pulmonary angiography as a diagnostic test. Ref: Current Diagnosis and Treatment: Surgery, 13th Edition, Chapter 35