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