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 lime 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 effects 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 pprn 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 pprn NO should be considered safe regarding NO₂ levels. Administration of NO at the Y-piece gives high and unpredictable levels of NO₂.