Mechanism of ozone-induced tachypneic response to hypoxia and hypercapnia in conscious dogs

In 7 studies on 3 dogs exercising on a treadmill (1.6 km/hr), the authors studied the effect of ozone on ventilatory responses to hypercapnia and to hypoxia. After ozone exposure (0.67 ± 0.02 ppm by vol; 2 hr), the responses of minute volume of ventilation (VE) to progressive hypercapnia and hypoxia were not changed, but the breathing pattern in response to these stimuli changed. The authors analyzed the breathing pattern by plotting the relationship between VE and tidal volume (VT). During progressive hypercapnia, the slope of VE-VT relationship increased from a control value of 36.1 ± 1.6 (mean ± SE) to 93.5 ± 8.9 min^-1 after ozone (n = 7, P < 0.005); during hypoxia, the slope increased from a control value of 46.1 ± 8.6 to 142.7 ± 18.3 min^-1 after ozone (n = 6, P < 0.005). The ozone-induced tachypneic responses to hypercapnia and hypoxia were not affected by inhalation of atropine sulfate or isoproterenol aerosols, but were completely abolished by bilateral vagal blockade. These findings indicate an effect of ozone on the vagal receptors located in the airways and lungs that causes reflex tachypnea during hypercapnia and hypoxia.