The Science Journal of the American Association for Respiratory Care
Background: Among the various systems used to deliver inhaled nitric oxide (NO) to ventilated adults, the most accurate and consistent is a pre-ventilator blended system. However, this set-up utilizes large amounts of NO. Instead, continuously injecting NO into the inspiratory limb of the circuit spares NO use but delivers very unstable NO concentrations [NO] when analyzed with a fast response NO analyzer. [NO] could be more stable if appropriate mixing is allowed before reaching the patient. We hypothesized that when NO is injected within the inspiratory limb proximal to the humidifier, [NO] will be more stable. Thus, the purpose of this study was to evaluate the stability of [NO] using both a fast (NOA, Sievers) and slow response NO analyzer (Sensornox, Sensormedics) when NO was continuously injected into the inspiratory limb prior to the humidifier. Method: An NPB 840 ventilator was attached to a 2-chambered test lung configured to separate inspired and expired gases. NO was continuously injected into the circuit, first, at the ventilator output and then just before the humidifier using a high-precision low flow flow-meter. [NO] and nitrogen dioxide (NO2) concentrations were measured from the inspiratory limb just before the Y piece. The ventilator was set to deliver either constant flow/volume breaths or pressure control breaths, with TV's of 0.5L and 1.0L and inspiratory times of 1 and 2 seconds. [NO] was 5, 20 and 40 ppm and the baseline settings of: volume control with Ti 1.0 second and TV of 1.0 L.
Results: [NO] varied both within the breath and when ventilator settings were changed. At 5ppm, total range of [NO] from both injection sites was 2.1-16.2; at 20ppm, 4.6-58.2; and at 40ppm, 14.9-111.5ppm. The mean [NO] from the NOA was similar to that seen on the Sensornox (p=0.428). Conclusion: The humidifier does not act as a sufficient mixing chamber and therefore, NO should not be continuously injected into the inspiratory limb of an adult circuit before the humidifier. Variations are less when injection takes place at the ventilator outlet but remain unacceptable.
Mean NO during volume control and pressure control ventilation using both analyzers when 20 ppm NO was continuously injected into inspiratory limb just before the humidifier.