Pulmonary function tests in the detection of small airway obstruction in a canine model of bronchiolitis obliterans.

We compared parameters of maximal expiratory flow with other tests used in the detection of small airways obstruction (SAD) in a canine model of bronchiolitis obliterans. Bronchiolitis was produced by instilling a 1% solution of nitric acid into the airways of 8 dogs (Group N). In 5 control dogs, a normal saline solution was instilled (Group C). Plethysmographic lung volumes, oscillatory airway resistance (RL), and tests for SAD were examined after bronchiolitis was produced. To evaluate peripheral airway function, the single-breath nitrogen washout curve was used to obtain the slope of phase III and to estimate the lung volume at which a terminal increase in N2 concentration was observed (closing capacity). Maximal expiratory flow-volume curves while the dogs breathed air and 80:20 helium:oxygen (HeO2) were performed to obtain the air flow rate at 50% vital capacity (V50), the corresponding HeO2 flow rate (delta Vmax), and the lung volume at which air and HeO2 flow rates were equal (Viso). After injury, the histologic aspects of the lung in Group N showed acute and chronic inflammation of the small airways. The RL did not change in Group N, despite a relative increase in peripheral airway resistance, which when measured increased about 4 times. Compared with Group C, significant increases in closing capacity and Viso and significant decreases in V50 were observed in Group N. Although predicted to decrease in SAD, delta Vmax did not change. We conclude that delta Vmax is relatively insensitive to SAD. Possible mechanisms resulting in reduced V50 but maintained delta Vmax in this model were further examined in terms of the concepts outlined by the wave-speed theory of flow limitation.