Exhaled nitric oxide (FE_NO) 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 (CA_NO) can be calculated.2 CA_NO 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 FE_NO measurements;1 however, changes in CA_NO 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 FE_NO, CA_NO and bronchial flux (J_NO).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 FE_NO 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 CA_NO 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 FE_NO and CA_NO showed no correlation with methacholine PC₂₀, baseline FEV₁ or final percentage fall in FEV₁. The percentage change in CA_NO following challenge showed a positive correlation with the baseline value (r = 0.59, p<0.001).

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Figure 1

Scatter plots for effect of methacholine challenge on exhaled nitric oxide (FE_NO) and the contribution of the alveolar compartment to exhaled nitric oxide (CA_NO). Individual data points are shown with geometric means and 95% confidence intervals.

To our knowledge, this is the first study to report the effects of methacholine challenge on CA_NO. We have shown that methacholine challenge significantly reduces CA_NO, and this effect is relatively more marked than for FE_NO. The effect on FE_NO is known, and is thought to be due to washout of nitric oxide from the airways. There was a proportionally greater suppression of FE_NO 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 CA_NO is relatively more suppressed than FE_NO. This is an important consideration for planning and interpreting study visits in clinical trials.