The effect of inhaled nitric oxide on pulmonary ventilation-perfusion matching following smoke inhalation injury

BACKGROUND: We previously reported that inhaled nitric oxide (NO) improved pulmonary function following smoke inhalation. This study evaluates the physiologic mechanism by which inhaled NO improves pulmonary function in an ovine model. METHODS: Forty-eight hours following wood smoke exposure to produce a moderate inhalation injury, 12 animals were anesthetized and mechanically ventilated (FIO2, 0.40; tidal volume, 15 mL/kg; PEEP, 5 cm H2O) for 3 hours. For the first and third hours, each animal was ventilated without NO: for the second hour, all animals were ventilated with 40 ppm NO. Cardiopulmonary variables and blood gases were measured every 30 minutes. The multiple inert gas elimination technique (MIGET) was performed during the latter 30 minutes of each hour. The data were analyzed by ANOVA. RESULTS: Pulmonary arterial hypertension and hypoxemia following smoke inhalation were significantly attenuated by inhaled NO compared with the values without NO (p < 0.05, ANOVA). Smoke inhalation resulted in a significant increase in blood flow distribution to low VA/Q areas (VA/Q < 0.10) with increased VA/Q dispersion. These changes were only partially attenuated by the use of inhaled NO. The SF6 (sulfur hexafluoride) retention ratio was also decreased by inhaled NO. Peak inspiratory pressures and pulmonary resistance values were not affected by inhaled NO. CONCLUSIONS: Inhaled NO moderately improved VA/Q mismatching following smoke inhalation by causing selective pulmonary vasodilation of ventilated areas in the absence of bronchodilation. This modest effect appears to be limited by the severe inflammatory changes that occur as a consequence of smoke exposure.