Background   Asthma is a chronic inflammatory disease of the airway, and the inflammation involves the proximal airway and peripheral small airway. More than 50% of patients with asthma have small airway dysfunction, and evaluating small airway dysfunction is crucial in the early diagnosis, disease control and risk management of asthma deterioration. Concentration of alveolar nitric oxide (CaNO) is an indicator of small airway inflammation in the peripheral airway/alveolar area.

  Objective   To investigate the diagnostic value of concentration of alveolar nitric oxide (CaNO) in small airway dysfunction in asthmatic patients and its correlation with small airway function indicator.

  Methods   A total of 55 asthma patients admitted to the Department of Respiratory Medicine of the First Medical Center of Chinese PLA General Hospital from March to November in 2019 were included. According to whether the patients had small airway dysfunction, they were divided into small airway dysfunction group (n=25) and non-small airway dysfunction group (n=30). Lung function, CaNO level and FeNO level were compared between the two groups. The diagnostic value of CaNO and FeNO in small airway dysfunction in asthmatic patients was evaluated by the area under receiver operator characteristic curve (ROC). Pearson correlation test was used to analyze the correlation between CaNO and lung function indicator of small airway and FeNO.

  Results   There were 25 cases (15 males and 10 females) aged (44.84±14.34) years in the small airway dysfunction group and 30 cases (16 males and 14 females) aged (38.60±10.68) years in the non-small airway dysfunction group, and no significant differences in gender, age, body mass index and smoking history were detected between the two groups (all P＞0.05). CaNO and FeNO in the small airway dysfunction group were higher than those in the non-small airway dysfunction group (P<0.05, respectively). The lung function indexes, including FEV1%pred, FEV1/FVC%, FEF50%pred, FEF75%pred and MMEF%pred in the small airway dysfunction group were significantly lower than those in the non-small airway dysfunction group (all P<0.05). When CaNO, FeNO used alone or in combination to diagnose small airway dysfunction, the ROC-AUC (95% CI) was 0.707 (0.413-0.984), 0.674 (0.364-0.971) and 0.787 (0.619-0.928), respectively. CaNO was negatively correlated with FEF50%pred, FEF75%pred, MMEF%pred, and positively correlated with FeNO (P<0.05).

  Conclusion  CaNO has diagnostic value for small airway dysfunction, and the optimal cut-off value is 5.70 × 10⁹ mol/L. However, the diagnostic performance is not satisfactory, so it still needs further exploration for its value in detecting small airway dysfunction.