Fractional exhaled nitric oxide — F ENO — methodology and application in asthma epidemiology in Northern Europe

In Northern Europe, the prevalence of asthma differs between countries, but an objective clinical comparison of bronchial inflammation is missing. Therefore, we aimed to study the fractional exhaled nitric oxide FENO, a biomarker of airway inflammation, in population samples of the general populations of Sweden, Finland and Estonia. We further aimed to analyse methodological and physiological features of FENO acquisition, such as mouthwashes and dependency of expiratory flow. We performed clinical interviews (n = 2658), FENO (n = 1498) and skin prick tests (SPT) in a random population from Sweden (Stockholm and Örebro), Finland (Helsinki), and Estonia (Narva and Saaremaa), during 1997–2003. To analyse the methodology of FENO, we performed two pilot clinical studies. Firstly, we measured FENO with an expiratory flow rate of 50 mL/s in a small random sample (12 asthmatic or healthy) and acquired a baseline, then repeated measurements (for 20 min) after a mouthwash with tap water or carbonated water. Additionally, we obtained FENO from 30 volunteers for multiple expiratory flow rates of 50, 30, 100 and 300 mL/s, after different mouthwash settings. With this dataset, we analysed the influence of mouthwashes in multiple-flow FENO and developed a conversion model, with a further cross-validation in five populations: healthy adults, healthy children, and patients with chronic obstructive pulmonary disease (COPD), asthma and alveolitis. In the pilot studies, the tap water mouthwash reduced FENO for only 2 min. The mouthwash with carbonated water lowered FENO more notably, for 12 min. Compared to the tap water mouthwash, the carbonated water mouthwash reduced FENO at all expiratory flows — 50, 30, 100 and 300 mL/s. The carbonated water mouthwash also lowered the maximum airway NO flux (_JawNO), but not the alveolar NO concentration (CANO) or capacity of the airways for NO diffusion (DawNO). We developed a non-linear model to perform FENO estimations obtained at different flows. The cross-validation resulted in a low deviation between estimated ^FENO (from 100 mL/s to 50 mL/s) and measured FENO (at 50 mL/s) in children (0.27 ppb), the mixed adult population (0.28 ppb), and in healthy adults (0.44 ppb). The deviation was higher in patients with COPD (1.16 ppb), alveolitis (1.47 ppb), and asthma (1.68 ppb). We applied the non-linear model to standardise the FENO values at 50 mL/s in the epidemiological study. In the population study, the median (interquartile range) of FENO (ppb) was 15.5 (9.3) in Sweden, in Finland 15.4 (13.6), and in Estonia 12.5 (9.6). We found the lowest FENO in Estonian centres —Saaremaa 13.1 (9.5) and Narva 11.8 (8.6). Asthma was associated with FENO≥25 ppb, odds ratio (OR) 3.91 (95% confidence interval: 2.29–6.32) and adjusted for skin prick test (SPT), iv Paul G. Lassmann-Klee smoking, sex and study centre. Atopy increased the likelihood of asthma, OR 3.19 (2.02–5.11). Having asthma was more likely in Stockholm OR 5.54 (2.18–14.79), Örebro OR 3.38 (1.59– 8.09), Helsinki OR 2.40 (1.04–6.02), and Narva OR 2.45 (1.05–6.19), compared to Saaremaa. We conclude that a carbonated water mouthwash reduces oral NO contamination for 12 min, and is more pronounced than tap water, with multiple flows, without affecting CANO or DawNO. We established a model for converting FENO between multiple expiratory flows, with further tentative use of predicting extended flow parameters, _JawNO and CANO. We confirmed the higher prevalence of allergic airway inflammation and asthma in Sweden and Finland, compared to Estonia. An increased FENO and atopy were independently associated with a higher risk of asthma. Our epidemiological findings support the west–east disparity of allergic diseases