Presented at the Neonatal Society 2001 Autumn Meeting.
Williams O, Greenough A, Wong M-L, Hannam S, Rafferty GF, Milner AD
Department of Child Health, Guy’s, King’s and St Thomas’ Medical School, London, UK
Introduction: Exhaled nitric oxide (NO) levels are raised in children and adults with inflammatory respiratory disorders (1) and reduced in such patients when corticosteroids are administered (2). Nitric oxide products have been found to be elevated in the bronchial alveolar lavage fluid of prematurely born infants developing CLD (3). Serial measurement of exhaled NO levels might then indicate CLD development and response to treatment, but only if accurate results could be obtained in the clinical setting.
Aims: To determine if changes in ventilator settings influenced exhaled NO levels and whether upper airway NO production was influenced by gestational age or post-conceptional age.
Methods: The influence of ventilator settings was assessed using a lung model into which NO was delivered to stimulate NO production. The lung model was connected to the ventilator by an endotracheal tube (ETT) from which NO was sampled. Serial measurements (days, 1, 3, 5, 7, 14, 21 and 28) of NO were made in 16 infants (gestational age 25 to 32 weeks). Lower airway NO was measured from the ETT in ventilated infants and in non-ventilated infants a sampling catheter was placed within a face mask with a 2 litre/min flow of NO free air in order to generate a positive end expiratory pressure. Upper airway NO levels were measured with a small sampling tube placed in one nostril.
Results: In the lung model, increasing peak inflating pressure, ventilator rate and inspired oxygen concentration resulted in a decrease in NO levels. The effect of the inspired oxygen concentration was more pronounced when larger volume lung models were examined. Measurements in infants showed there was upper airway production of NO even on day one, and in the most prematurely born infants. In all infants, upper airway NO levels were higher than lower airway levels (for example, on day one median upper airway levels were 23.64, range 5.67 – 112 ppb and lower airway levels were a median of 4.31, range 1.68 – 9 ppb) (p<0.01). No significant relationships were found between NO levels and either gestational age or post-conceptional age.
Conclusions: Conditions of measurement must be standardised in infants receiving respiratory support if exhaled NO levels are to be appropriately interpreted and – in all infants, precautions must be taken to avoid contamination from the upper airway. The source and function of the exhaled NO in the upper airway of preterm infants remains uncertain.
References
1. Alving K, Weizberg, Lundberg JM. Increased amount of nitric oxide in exhaled air of asthmatics. Eur Respir J 1993;6:1368-1370
2. Baraldi E, Azzolin N, Zanconato S, Dario C, Zacchello F. Corticosteroids decrease exhaled nitric oxide in children with acute asthma. J Paediatr 1997;131,3:381-385
3. Vyas JR, Currie AE, Struker DEG, Field DJ, Kotecha S. Concentration of nitric oxide products in bronchio alveolar fluid obtained from infants who develop chronic lung disease of prematurity. Arch Dis Child 1999;81:F217-F220