Exhaled nitric oxide; relationship to clinicophysiological markers of asthma severity

Bronchial asthma is an airway disorder associated with bronchial hyperresponsiveness, variable airflow obstruction and elevated levels of nitric oxide (NO) in exhaled air. The variables all reflect, in part, the underlying airway inflammation in this disease. To understand their interrelationships we have investigated the relationship between exhaled NO levels and clinicophysiological markers of asthma severity. Twenty-six steroid naive atopic asthmatics participated in the analysis. All were given diary cards and were asked to record their peak expiratory flow (PEF) rates twice daily together with their asthma symptom scores and β-agonist use. Diary cards were collected 2 weeks later and measurements of exhaled NO levels, FEV 1 and histamine bronchial hyperreactivity (PC 20 histamine) were undertaken. Exhaled NO levels were significantly higher in our study population than in normal control subjects and correlated negatively with PC 20 histamine ( r = −0·51; P=0·008) and positively with PEF diurnal variability ( r=0·58; P=0·002), but not with symptom scores, β-agonist use or FEV 1 (%). We conclude that a significant relationship exists between exhaled NO levels and the two characteristic features and markers of asthma severity, namely bronchial hyperreactivity and PEF diurnal variability. The lack of correlation between symptom score and β-agonist use, or FEV 1 (%) predicted and exhaled NO suggests that these measures are reflective of differing aspects of asthma.