Nitrogen dioxide (NO₂) production for different doses of inhaled nitric oxide (NO) during mechanical ventilation with different tidal volumes using two prototypes for the administration of NO

Objectives

To test the amount of NO₂ production during mechanical ventilation with different concentrations of inhaled NO (0.5-90 ppm) and different tidal volumes (V_T 100–1000 ml) using two prototypes for NO inhalation during mechanical ventilation.

Methods

The Servo 300 NO-A prototype (Siemens-Servotek, Solna, Sweden) and the NO-domo prototype combined with an Evita 1 ventilator (Dräger AG, Lübeck, Germany) were tested for controlled mechanical ventilation at an FiO₂ of 1.0 with different tidal volumes (V_T 100, 150, 200, 250, 300, 400, 500,... 1000 ml) using an artificial test lung with a filling volume of 3 1. Different NO concentrations of 0.5, 1,10, 20, 40 and 90 ppm (NOin) were applied. In the Servo prototype, NO is mixed into the inspiratory gas stream inside the ventilator and electrochemical electrodes for NO/NO₂ are mounted in a mixing chamber behind the expiratory valve of the respirator. The NO-domo prototype admixes NO into the inspiratory gas stream immediately behind the respirator's inspiratory outlet and the electrochemical measurement is mounted 30 cm distal from that point in the inspiratory limb. NO₂ concentrations were measured by chemiluminescence in the inspiratory limb of the respiratory tubing immediately before the y-piece (NO₂ insie for the Servo, NO₂ innodo for the NO-domo). Furthermore, for the Servo 300 NO-A an additional chemiluminescence measurement was mounted in the expiratory mixing chamber at the same point where the electrochemical electrodes measure (NO₂ exsie).

Results

The NO₂ values for the different measurements are shown in the figure for the different NO concentrations. Each data point for a given NO concentration reflects a stepwise increasing V_T from 100-1000 ml.

Conclusions

From the presented data we conclude that as long as inhaled NO concentrations < 10 ppm are used, the NO₂ production is below the toxic range even for small tidal volumes. For NO concentrations ≥ 10 ppm the NO₂ production is higher when NO is admixed into the inspiratory gas inside the ventilator (Servo 100 NO-A) compared to mixing NO into the inspiratory limb of the respiratory tubing (NO-domo). One to lower flow rates and therefore increased contact time of NO and O₂, the NO₂ is higher for smaller tidal volumes. For the same reason the expiratory NO₂ measurement results in a clearly higher NO₂ level compared to the inspired concentrations and is therefore strongly misleading. NO concentrations > 20 ppm result in potentially toxic inspiratory NO₂ concentrations and should therefore be carefully monitored.

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