INACCURACIES OF NITRIC-OXIDE DELIVERY SYSTEMS DURING ADULT MECHANICALVENTILATION

Background: Various systems to administer inhaled nitric oxide (NO) ha ve been used in patients and experimental animals. We used a lung mode l to evaluate five NO delivery systems during mechanical ventilation w ith various ventilatory patterns. Methods: An adult mechanical ventila tor was attached to a test lung configured to separate inspired and ex pired gases. Four injection systems were evaluated with NO injected ei ther into the inspiratory circuit 90 cm proximal to the Y piece or dir ectly at the Y piece and delivered either continuously or only during the inspiratory phase. Alternatively, NO was mixed with air using a bl ender and delivered to the high-pressure air inlet of the ventilator. Nitric oxide concentration was measured from the inspiratory limb of t he ventilator circuit and the tracheal level using rapid and slow-resp onse chemiluminescence analyzers. The ventilator was set for constant- flow volume control ventilation, pressure control ventilation, pressur e support ventilation, or synchronized intermittent mandatory ventilat ion. Tidal volumes of 0.5 l and 1 l were evaluated with inspiratory ti mes of 1 s and 2 s. Results: The system that premixed NO proximal to t he ventilator was the only one that maintained constant NO delivery re gardless of ventilatory pattern. The other systems delivered variable NO concentration during pressure control ventilation and spontaneous b reathing modes. Systems that injected a continuous flow of NO delivere d peak NO concentrations greater than the calculated dose. These varia tions were not apparent when a slow-response chemiluminescence analyze r was used. Conclusions: NO delivery systems that inject NO at a const ant rate, either continuously or during inspiration only, into the ins piratory limb of the ventilator circuit produce highly variable and un predictable NO delivery when inspiratory flow is not constant. Such sy stems may deliver a very high NO concentration to the lungs, which is not accurately reflected by measurements performed with slow-response analyzers.