The ratio of the forced expiratory volume in the first second of exhalation (FEV1) to the total forced vital capacity (FVC) is reduced in an asthmatic patient. What is the normal ratio of FEV1/FVC in an adult male?

The volume of air expired in the first second of expiration (the FEV1, or forced expiratory volume in 1 second), especially when expressed as a ratio with the total amount of air expired during the FVC, is a good index of expiratory airways resistance. In normal young subjects, the FEV1/FVC is greater than 0.80; that is, at least 80% of the FVC is expired in the first second. An FEV1/FVC of 75% would be more likely in an older person. Patient with airway obstruction caused by asthma will have an FEV1/FVC far below 0.80. Ref: Levitzky M.G. (2013). Chapter 2. Mechanics of Breathing. In M.G. Levitzky (Ed), Pulmonary Physiology, 8e.