Safety aspects of delivery and monitoring of nitric oxide during mechanical ventilation.

In the presence of oxygen NO is oxidised to NO2, which is toxic in higher concentrations. In this technical investigation, we evaluated a dosage system, modified from Stenqvist et al. 1993 (1), regarding NO and NO2 levels. NO was administered before the ventilator and NO2 scavenged using a soda little absorber in the inspiratory limb close to the ventilator. NO/NO2 levels were measured using fuel cell technique. We tested the duration of soda lime scavenging, put in additional soda lime absorbers, used charcoal as absorber and exchanged tubing material. NO was delivered after the ventilator and we studied effect of interruption of ventilation. With concentrations of NO at or below 40 parts per million (ppm) at F1O2 0.9, NO2 levels were 1.2 ppm or lower. Corresponding values for 20 and 10 ppm were 0.4 and 0.2 ppm, respectively. Duration of the soda lime absorber was at least 72 hours. Additional soda lime absorbers did not further reduce NO2 levels. Charcoal absorbers reduced NO2, but also NO by 45% from set value. Tubing materials had no influence on NO and NO2 levels. When administering NO at the Y-piece, levels of NO were increased by 35-60% and NO2 levels by 110-230% compared to set values. Oxidation of NO to NO2 is continuously taking place in the breathing system. Doses of up to 40 ppm NO should be considered safe regarding NO2 levels. Administration of NO at the Y-piece gives high and unpredictable levels of NO2.