A 60-year old male has a long standing history of breathlessness. He has been a heavy smoker since the age of 20 years. Physical examination reveals an anxious male who is emaciated, puffing for breath but is not cyanosed. The chest is barrels shaped. The investigation of choice in this patient be

The patient is having chronic obstructive pulmonary disease -emphysematous type.High resolution CT scan is the current definitive test for establishing the presence or absence of emphysema in living subjects.It allows the detection,characterisation(centriacinar/panacinar) and quantification of emphysema and is more sensitive than a chest X ray for detecting bullae. Ref:Harrison's medicine-18th edition,page no:2157;Davidson's medicine-22nd edition,page no:675.