Respiratory distress syndrome aspects of inhaled nitric oxide, surfactant and nasal CPAP

Abstract

Respiratory distress syndrome (RDS) still represents one of the main problems in the treatment of premature infants. Despite the use of surfactant replacement therapy RDS adds to the need for endotracheal intubation and mechanical ventilation (M). Enhancing the efficacy of CPAP treatment could reduce the use of MV thereby possibly minimising complications such as bronchopulmonary dysplasia (BPD).

The overall aim was to find methods to reduce the severity of lung damage in newborns by augmenting the efficacy of early treatment by combining nasal CPAP and exogenous surfactant with inhaled nitric oxide (iNO).

We devised an iNO delivery system by modifying a commercially available nCPAP system and measured the N02 formation at different stock gas concentrations. We used this system to record the acute effects on oxygenation, respiratory rate and C02 levels in 15 RDS infants during a short term exposure to 10 ppm iNO under nCPAP treatment in a cross-over study.

The modified system had a low rate of N02 formation. Stock gas cylinder concentration of 50 ppm NO had several advantages allowing simplification of administration and monitoring in comparison with 1000 ppm. Applying the system to premature newborns with moderate RDS, results in significantly improved oxygenation especially in the more premature patients. Respiratory rate and C02 levels remain unaffected in spontaneously breathing patients during a 30-min exposure.

A secondary aim was to investigate different possible adverse effects of iNO to patients and staff. We measured the occupational exposure levels encountered working close to a baby on iNO-CPAP and described the expected addition of NO and N02 to the air in an ICU room. We also investigated the levels produced in case of a sudden release from an NO gas cylinder in an ICU room. The system contributes only to a small extent to ICU room levels of NO. As a point source, brief peaks of more concentrated but still low NO levels were seen. The total release of a 20 litre cylinder 1000 ppm NO resulted in room levels of 30 ppm NO and 0.8 ppm N02.

Our proposal to use early NO in illnesses characterized by surfactant dysfunction clearly calls for studies of larger groups of patients. It was therefore important to investigate to what extent NO or concurrently produced N02 might damage surfactant and at what dose such effects would occur. Sedated spontaneously breathing piglets were given a high NO dose of 100 ppm for 4h or 10 ppm N02. The effect of only sedation on surfactant function was studied separately. The exposure to 100 ppm NO in air for 4 hours resulted in slightly impaired surfactant function with higher surface tension of lung lavage fluid compared to control. This was not due to concomitantly formed N02 since piglets exposed to only 10 ppm N02 had normal surfactant function.

We also exposed sedated piglets under assisted spontaneous breathing to 40 ppm iNO for 24 and 48 h which resulted in no surfactant abnormalities and preserved features in lung tissue histology. Just as clinical use of exogenous surfactant has moved in the direction of prophylaxis from rescue, after extensive studies, clinical use of iNO may face a similar development with an increased emphasis on lung development and anti-inflammatory action rather than acute improvements in oxygenation and pulmonary vascular tone.