Evaluation of a new system for ventilatory administration of nitric oxide

A new system for delivery of nitric oxide (NO) to inspiratory gas consisting of two mass flow regulators and a soda-lime absorber for scavenging of nitrogen dioxide (NO2) is described. The system was evaluated using three different techniques for NO analysis (infrared, chemiluminescence and electro-chemical fuel cell technique). The electro-chemical fuel cell was less sensitive to humidity in the sample and is suitable for clinical routine use. The infrared analyser was very sensitive to humidity and the gas sample must be dried by silica gel, which absorbs NO2 and will cause falsely low NO2 values. NO2 was analysed with ultra-violet methodology. NO2 is highly toxic and the highest recommended occupational health and safety level for inhalation is 5 ppm. The highest values of NO2 in our system were detected before the absorber in the inspiratory limb of the breathing system, being 5 ppm at 100% oxygen and 100 ppm NO using "infant" respiratory settings (3 l/min in ventilation, frequency of 30/min). The corresponding value for "adult" respiratory settings (10 l/min in ventilation, frequency of 15/min) was 3.2 ppm. The absorber reduced these levels to well below 1 ppm. When clinically relevant levels of NO were used (20 ppm), no NO2 could be detected after the absorber, irrespective of oxygen concentration in the breathing gas. It was observed that gas cylinders with NO mixed in nitrogen may initially have a high NO2 concentration (around 12 ppm) and should be flushed thoroughly before use.