Fluctuations of inspiratory concentration of nitric oxide (NO) during mechanical ventilation

Objectives

To test inspiratory NO concentrations along the inspiratory limb of the respiratory circuit during mechanical ventilation with different inhaled NO concentrations.

Methods

A Servo 300 NO-A prototype (Siemens-Servotek, Solna, Sweden) was used for controlled mechanical ventilation of an artificial test lung with a filling volume of 1 l at an FiO₂ of 1.0. Different NO concentrations of 0.1, 1, 10 and 100 ppm (NOin) were applied. In the Servo 300 NO-A a digital controlled valve is used allowing the exact flow-proportional admixture of NO into the inspiratory gas stream. NO is delivered from an NO tank with a fixed concentration dissolved in N₂. NO was measured by chemiluminescence (CLD 700 AL, Eco Physics, Duernten, Switzerland) at four different positions (Pos 1–4) along the inspiratory limb of standard respiratory tubing including an active heating system (Concha Therm III with Aerodyne humidification column. Kendall, Neustadt, Germany). Pos 1: immediately behind the respirator's inspiratory outlet: Pos 2: immediately behind the heating column: Pos 3: in the middle between the heating column and the y-piece: Pos 4: immediately before the y-piece.

Results

The results for the NO measurements at the different positions are shown in the figure (left hand side) for the different inhaled NO concentrations. Values are shown as a percentage of NOin. NO₂ concentrations in ppm are shown in the figure (right hand side) for the different NOin.

Conclusions

These data suggest that NO concentrations are fluctuating also when NO inhalation is accomplished with a device to administer NO flow proportional only during inspiration by means of a digital controlled valve. Interestingly, at the beginning of the inspiratory circuit NO concentrations are highest and decrease immediately behind the active heating system. This phenomenon is also obvious for NO₂ when the inspiratory NO concentrations are ≥ 10 ppm, so that a relevant oxidation to NO₂ takes place. The more distal along the inspiratory limb NO₂ is measured, the more it inereases again, probably due to further oxidation from NO and O₂. It might well be that the sharp decline in NO and NO₂ is due to a reaction of those compounds with the water in the active heating system. For the clinical use, this phenomenon should he cautiously observed during the inhalation of NO and the point for monitoring the inspiratory gas mixture should be as distal as possible along the inspiratory limb.

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