Which of the following decreases as body mass index increases? 1. Airway resistance 2. Functional residual capacity 3. Forced vital capacity 4. Forced expiratory volume in 1 second

a. 2 only
b. 1, 2, and 3 only
c. 3 and 4 only
d. 2, 3, and 4 only

ANS: D
Lung volumes progressively decrease as BMI increases. Total lung capacity and residual volume decrease linearly as BMI increases, while functional residual capacity is exponentially reduced at higher BMI values, especially in patients with a central fat distribution (high waist circumfer-ence). Furthermore, forced vital capacity, forced expiratory volume in 1 second, maximum vol-untary ventilation, and forced mid-expiratory flow are significantly reduced in obese subjects, with airway closure occurring during tidal breathing in the extreme range of obesity.