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Exhaled nitric oxide (FENO) is established as a surrogate of airway inflammation.1 Based on the two-compartment model of nitric oxide production in the lungs, the contribution of the alveolar compartment to exhaled nitric oxide (CANO) can be calculated.2 CANO is raised in chronic obstructive pulmonary disease and severe asthma, even when treated with inhaled corticosteroid.2 Forced manoeuvres and bronchial challenge are known to reduce FENO measurements;1 however, changes in CANO after challenge have not been reported.
Forty-eight patients with mild to moderate asthma performed fractionated exhaled nitric oxide before methacholine challenge and again after the methacholine concentration provoking a fall in forced expiratory volume in 1 s (FEV1) of 20% or more (PC20) or 8 mg/ml had been reached. Participants had a physician diagnosis of persistent asthma and were receiving treatment with ⩽1000 μg/day beclometasone or equivalent. Spirometry was performed using a SuperSpiro spirometer (Micro Medical, Chatham, Kent, UK). Exhaled nitric oxide was performed on a NIOX chemiluminescence analyser (Aerocrine AB, Solna, Sweden) at three flow rates (50, 100 and 200 ml). A linear regression equation was applied to derive values for FENO, CANO and bronchial flux (JNO).3 Nitric oxide values were logarithmically transformed to achieve Gaussian distribution prior to analysis. Mean pre- and post-challenge values were analysed with a paired t test. Analyses were performed using SPSS version 15.0 (Chicago, Illinois, USA).
The mean age of the patients was 38.5 years. Thirty-eight patients had a methacholine PC20 <8 mg/ml. The mean fall in FEV1 was 21%. Geometric mean pre-challenge FENO was 20.4 ppb compared with 16.9 ppb post-challenge, a difference of 17% (95% CI 13% to 21%, p<0.001; fig 1). Geometric mean CANO was 2.9 ppb pre-challenge and 1.9 ppb post-challenge, a difference of 31% (95% CI 17% to 43%, p<0.001). Differences in NO at flow rates of 50, 100 and 200 ml were 15% (95% CI 10% to 19%), 11% (95% CI 6% to 16%) and 17% (95% CI 11% to 22%), respectively (p<0.001). Baseline values for FENO and CANO showed no correlation with methacholine PC20, baseline FEV1 or final percentage fall in FEV1. The percentage change in CANO following challenge showed a positive correlation with the baseline value (r = 0.59, p<0.001).
To our knowledge, this is the first study to report the effects of methacholine challenge on CANO. We have shown that methacholine challenge significantly reduces CANO, and this effect is relatively more marked than for FENO. The effect on FENO is known, and is thought to be due to washout of nitric oxide from the airways. There was a proportionally greater suppression of FENO at 200 ml (17%) than at 50 ml (15%) and 100 ml (11%). This has a more significant effect on the slope of the regression line and hence the CANO is relatively more suppressed than FENO. This is an important consideration for planning and interpreting study visits in clinical trials.
Competing interests: None.