Exhaled nitrogen oxides and carbon monoxide in asthma and cystic fibrosis : markers of inflammation?

Abstract

Background: Exhaled markers of airway inflammation have attracted much interest as potential tools for monitoring the bronchial inflammation of respiratory disease. Exhaled nitric oxide (NO), its metabolites nitrite and nitrate in breath condensate (EBC), and exhaled carbon monoxide (CO), have been suggested for this purpose. However, their site of origin has not been fully investigated and the enterosalivary circuit of nitrate, with its possible off springs of nitrite and NO, could constitute a confounding factor. Exhaled NO is increased in asthma, but its levels are unexpectedly low in cystic fibrosis (CF). Exhaled CO and EBC nitrite are instead elevated in both these conditions, where the latter could reflect an increased NO activity also in CF.

Aims: The aim of study I was to compare the profiles and possible origins of exhaled NO and CO in children and adults with asthma and CF by the introduction of highly specific infrared technique and controlled flow rate for CO measurements. In study II we wanted to investigate the possible influence from salivary formation of NO on measurements in exhaled air. Study III was designed to evaluate the postulated link between nitrite and nitrate in EBC and exhaled NO in children with allergic asthma, also in relation to other disease markers. In study IV, these EBC metabolites were examined in relation to the salivary contents of the same in subjects with CF, to evaluate a possible influence from oral bacteria, which may reduce nitrate to nitrite.

Methods: In study I, 56 children and adults with asthma, 16 with allergic rhinitis, 9 CF patients and 30 age-matched controls performed exhaled CO and NO measurements with two different flow rates. Study II was performed on ten healthy adults who ingested 240 mg of nitrate on empty stomach for consecutive measurements of exhaled and nasal NO and salivary nitrate and nitrite, followed by a series of mouthwash experiments. In study III, 27 children with allergic asthma and 21 age-matched controls were examined with exhaled NO, EBC nitrite and nitrate, blood eosinophils, spirometry and methacholine challenge. Whereas EBC and salivary nitrite and nitrate, together with exhaled NO, were studied before and after mouthwash with the anti-bacterial solution of chlorhexidine in15 CF patients and 15 controls in study IV.

Results: Exhaled CO, was in contrast to NO, not elevated in asthma and allergic rhinitis, and both markers were negative in CF. The change of exhalation flow rate did, furthermore, not affect the levels of CO but gave a proportional change of NO. The intake of nitrate resulted in a 150% increase of exhaled NO after 2 h, whereas nasal NO was unaffected. This increase was largely abolished by chlorhexidine mouthwash, which also decreased baseline NO levels with 30%. EBC nitrite, but not nitrate, was significantly elevated in the children with allergic asthma, but no correlation was found to increased levels of NO or other disease markers. EBC nitrite was also significantly higher in the CF patients, as was salivary nitrite, but these levels were almost eradicated by chlorhexidine, which in addition reduced exhaled NO more in CF than in controls.

Conclusions: The flow independence of exhaled CO proves that it has its origin in the alveoli and is therefore not a suitable marker for bronchial inflammation. There is a substantial salivary contribution to exhaled NO from the non-enzymatic reduction of nitrite, which can be greatly increased by the intake of nitrate-rich foods. There is also a most prominent salivary contribution to EBC nitrite in CF, and probably even in asthma, which indicates an altered activity of oral bacteria in these conditions, rather than increased NO metabolism, as an explanation for their higher levels of EBC nitrite.