Influence of ventilatory settings and sampling position on measurements of simulated exhaled nitric oxide levels

Chronic lung disease is a common adverse outcome of prematurely born infants and is associated with an early inflammatory response, which persists over weeks. As a consequence, it is possible that exhaled nitric oxide levels might be raised in affected infants. The majority of such infants will be ventilated in the first weeks of birth and thus it is important to determine the influence of mechanical ventilation on nitric oxide levels. As a consequence, our aim was to determine whether, during mechanical ventilation, simulated exhaled nitric oxide levels were influenced by changes in ventilator settings or the sampling catheter position. A lung model was created consisting of a rubber bag inside a 11 jar. An endotracheal tube (ETT) was fixed securely within the neck of the bag. Nitric oxide was delivered into the bag at a constant rate to simulate production and sampled from within the ETT and the bag. The sampled nitric oxide was analysed using a Sievers chemiluminescence analyser. The ETT was attached to a neonatal ventilator and a nitric oxide scavenger placed in the ventilator's inspiratory limb to ensure nitric oxide free gas was delivered. Comparison of different sampling positions revealed that the highest peak nitric oxide level within the ETT was at the tip. Increasing peak inflating pressure and ventilator rate resulted in a decrease in the peak nitric oxide levels. Increasing the inspired oxygen concentration also was associated with a reduction in the peak nitric oxide levels, the effect being more pronounced when larger volume lung models were examined. The results emphasized that the conditions of measurement must be standardized in infants receiving respiratory support, if exhaled nitric oxide results are to be appropriately interpreted.