In clinical practice, children with asthmatic-related symptoms such as wheezing, cough, and chest tightness should first undergo pulmonary ventilation function examination. When forced expiratory volume in one second (FEV₁) <70%, which indicates obvious airflow restriction or airway obstruction, and bronchodilation test should be performed. However, some children with normal FEV₁ but decreased small airway function may have mild airflow restriction or airway obstruction. In addition, the bronchial stimulation test is complicated, expensive, risky, and more primary hospitals do not have the equipment and personnel conditions to carry out, but the bronchial diastole test for these children can also be positive, which has certain significance for the diagnosis of asthma.

To investigate the necessity of bronchial diastole test in children with normal FEV₁, forced vital capacity (FVC) and FEV₁/FVC but decreased small airway function.

A case-control study was conducted on 180 children with asthma aged 5 to 14 years who had symptoms such as wheezing, coughing and chest tightness and visited the outpatient department of the Third Affiliated Hospital of Zhengzhou University from May 29, 2019 to September 20, 2024. All subjects underwent routine pulmonary ventilation function examination and bronchodilation test, and the results of routine pulmonary ventilation function examination showed that FEV₁, FVC and FEV₁/FVC were normal, but the small airway function was decreased. Subjects were divided into positive group (n=33) and negative group (n=147) with FEV₁ improvement rate≥12% as the positive criterion of bronchodilation test, and the basic data and pulmonary ventilation function indexes of the two groups were compared. The correlation between small airway function index and bronchodilation test was analyzed by Pearson. Receiver operating characteristic (ROC) curve was used to analyze the efficacy of small airway function indicators in predicting positive results of bronchodilation tests.

The forced expiratory flow 50% of vital capacity as a percentage of the predicted value (FEF₅₀%pred), forced expiratory flow 75% of vital capacity as a percentage of the predicted value (FEF₇₅%pred) and maximum mid-expiratory flow as a percentage of the predicted value (MMEF%pred) in positive group were significantly lower than those in negative group (P<0.05). FEF₅₀%pred, FEF₇₅%pred and MMEF%pred were correlated with the results of bronchodilation test (P<0.001). The area under curve (AUC) of FEF₅₀%pred, FEF₇₅%pred and MMEF%pred in predicting positive results of bronchial diastolic tests were 0.733 (95%CI=0.644-0.822) 、0.827 (95%CI=0.756-0.898) 、0.849 (95%CI=0.789-0.909), respectively, and the cut-off values were 66.85%, 50.00% and 63.35%, respectively. The AUC of FEF₅₀%pred combined with FEF₇₅%pred, FEF₅₀%pred combined with MMEF%pred, FEF₇₅%pred combined with MMEF%pred and the combination of the three indexes were 0.859, 0.855, 0.865, 0.865, respectively. The AUC value of positive bronchial diastole test predicted by the combination of the three factors was greater than that predicted by FEF₅₀%pred and FEF₇₅%pred alone (Z=-2.801, -1.994; P=0.005, 0.046).

For children with asthma-related symptoms such as wheezing, chest tightness, and cough, small airway function indicators should be paid attention to even if FEV₁, FVC, and FEV₁/FVC are normal. When FEF₅₀%pred≤66.85% or FEF₇₅%pred≤50.00% or MMEF%pred≤63.35%, the positive result of bronchial diastole test can be predicted to a certain extent, and bronchial diastole test should be actively performed to assist the diagnosis of asthma.