NITRIC-OXIDE ADMINISTRATION AFTER THE VENTILATOR - EVALUATION OF MIXING CONDITIONS

Background: Because of the potential toxicity of nitric oxide (NO) and its oxidising product nitrogen dioxide (NO2), any system for the deli very of inhaled NO must aim at stable and predictable levels of NO and as low concentrations as possible of NO2. Methods: In a laboratory se t-up, we have evaluated mixing conditions in a system where NO is adde d after the ventilator with continuous flow. Mixing was studied by usi ng carbon dioxide (CO2) as a tracer gas since capnography has a short response time (360 ms) in comparison with measurements of NO with elec trochemical fuel cells (response time of 18 s). CO2 (in volumes corres ponding to an ideal mixture of 1, 3 and 6%) was fed, after the ventila tor, either into plain breathing tubing, into one or two soda lime abs orbers, or into an empty and a soda lime-filled canister, at different ventilatory rates and different I:E ratios. Samples were drawn from t he inspiratory limb close to the Y-piece. NO was added in the same way and in the same volume as the highest concentration of CO2. Results: CO2 added to plain tubing resulted in peak levels up to five times the set levels, while addition to a mixing box with an empty and a soda l ime-filled canister resulted in even mixing with gas concentrations cl ose to the ideal. When NO was fed into plain tubing, low levels were m easured at the Y-piece, indicating poor mixing. Gas supply to a mixing chamber resulted in even concentrations. Conclusion: Even and predict able levels of NO can be obtained with continuous flow of NO to the in spiratory limb, after the ventilator, if a mixing chamber is used. To obtain adequate mixing, the volume of the mixing box should be greater than the tidal volume. (C) Acta Anaesthesiologica Scandinavica 41 (19 97).