A 55 year old man who has been on bed rest for the past 10 days, complains of sudden onset breathlessness and chest pain. His chest X-ray is normal. Which of the following is the next investigation of choice for making the diagnosis?

This patient who was bedridden for 10 days is at high risk of developing deep venous thrombosis. Since he has developed chest pain and breathlessness and his chest x ray is appearing normal he has most likely developed pulmonary embolism. The next best investigation for this patient would be ventilation perfusion scan. Cases of high clinical probability combined with a high probability V/Q scan are viually diagnostic of Pulmonary Embolism (Philip A Karla 3rd edition). Lung scanning is a second-line diagnostic test for PE, used mostly for patients who cannot tolerate intravenous contrast. Perfusion scan is done by injecting small paiculate aggregates of albumin labeled with a gamma-emitting radionuclide. Ventilation scans, are obtained using radiolabelled inhaled gas such as xenon or krypton. When the V/P scan shows 2 or more segmental perfusion defects in presence of normal ventilation it is said to e a high probability scan. The diagnosis of PE is about 90% ceain in patients with high-probability scans. Ref: Harrisons Internal Medicine, 18th Edition, Chapter 262