Formation of nitrogen dioxide from nitric oxide and their measurement in clinically relevant circumstances

Therapy with inhaled nitric oxide in oxygen requires adequate monitoring of nitric oxide and nitrogen dioxide. The characteristics of chemiluminescence and electrochemical measurement techniques were determined by analysis of continuously flowing gas mixtures and comparisons with traceable gas standards. Gas mixtures were also diluted with mass flow controllers and in addition created in ventilator breathing systems. Factors influencing the formation of nitrogen dioxide were defined. Both techniques accurately measured nitric oxide (10-80 parts per million, ppm) and nitrogen dioxide (0.5-5 ppm) in normoxic and hyperoxic (90% oxygen) gas in the studied ranges. Nitrogen dioxide in hyperoxic gas had three origins: (1) from the premixing point of nitric oxide in nitrogen, (2) as a result of the mixing process, and (3) from post-mixing and time-dependent continuous formation of nitrogen dioxide in oxygen. We conclude that adequate monitoring is possible and that factors affecting nitrogen dioxide generation can be defined.