[Effects of temperature and humidity on the stability of nitric oxide, and efficacy of soda lime as a selective absorber of nitrogen dioxide].
Although inhaled nitric oxide (NO) has attracted attention as a pulmonary vasodilator, little heed has been given to its potential toxicity. Nitric oxide is known to be rapidly oxidized to nitrogen dioxide (NO2), which may damage pulmonary tissue. We examined the effects of temperature and humidity on the production of NO2 from NO. We also evaluated the amount of NO2 absorbed by soda lime, which is usually placed in the inspiratory line. For this purpose, we measured changes over time in the concentrations of NO and NO2 in mixtures that included NO, oxygen, and nitrogen in various concentrations, and at different temperatures and humidities. We confirmed that the formation of NO2 from NO follows the equation: -d[NO]/dt = 2 k[NO]2 [O2], where k is the rate constant. We found that k was significantly smaller at 37 degrees C than at 25 degrees C but was not influenced by humidity (0%, 40% or 90%). Although soda lime was very effective in absorbing NO2 from the inspiratory line, NO was simultaneously absorbed at the same molar ratio when the two gases existed together in the line. We thus conclude that inhalation of NO at 37 degrees C is more desirable than inhalation at room temperature, to suppress the production of NO2. When soda lime is used in the inspiratory line, attention should be paid to the reduction in the concentration of NO in the line.