The Science Journal of the American Association for Respiratory Care

1997 OPEN FORUM Abstracts

Monitoring and Delivering of Inhaled Nitric Oxide

Robert M Kacmarek, PhD, RRT, Sunday, December 7, 1997.

Two approaches have been used to monitor inhaled nitric oxide delivery, electrochemical analysis and chemiluminescent analysis. Electrochemical analyzers operate similar to oxygen analyzers. They have both the same advantages and disadvantages. Overall their accuracy is about ± 2 ppm and they can analyze NO to a minimal level of 2 ppm with acceptable clinical accuracy. However, they are slow responding (30 sec), are affected by water vapor but are small, portable and may to use. Chemiluminiscent analyzers are much more accurate than electrochemical analyzers, they are capable of analysis in the ppb range. However, they are large, costly and more difficult to use. Some chemiluminiscent analyzers can analyze NO in a systems where NO concentration rapidly changes as seem during exhaled gas analysis. However, for this to occur the analyzers 95% response time must be less than about 0.5 sec. Analyzers with longer response times underestimate peak NO concentration and overestimate lowest NO concentration.

Two global experimental approaches have been used to deliver NO. Premixing of NO prior to entry into the mechanical ventilator and injection of NO into the inspiratory limb of the ventilator circuit. Premixing insures a precise and accurate concentration of delivered NO but increases the possibility of NO_{2} formation. NO_{2} formation is based on [NO], [O_{2}] and time the two gases are in control. With ventilators that have a large internal reservoir problems with NO_{2} formation are markedly increased. The Servo 300 ventilator is the only ventilator that mixes NO, O_{2} and air internal to the ventilator and delivers this premixed gas to the patient. Generally at [NO] < =20 ppm NO_{2} formation has not been shown to be a problem with any premixing system. Injection of NO into the inspiratory circuit avoids the formation of NO_{2} but results in very inaccurate and inconsistent delivery of [NO]. Continuous injection being much more problematic than inspiratory phase only injection, however both are problematic. Ideally, inspiratory phase only injections the method of choice if injection is coordinated with the gas delivered by the ventilator. That is as the inspiratory flow changes the amount of NO injected changes. There are three commercially available systems that allow for this proportional delivery of NO available worldwide; the I-NOvent system from Ohmeda, the NODomo system from Drager and the Pulmonox system from Messer Griesheim.

AARC 50th Anniversary, December 6 - 9, 1997, New Orleans, Louisiana.

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