Small Airway Dysfunction by Impulse Oscillometry in Symptomatic Patients with Preserved Pulmonary Function

Background Asthma and chronic obstructive pulmonary disease are characterized by persistent airway inflammation and airflow limitation. Early detection of these diseases in patients with respiratory symptoms and preserved pulmonary function (PPF) defined by spirometry is difficult. Impulse oscillometry (IOS) may have better sensitivity than effort-dependent forced expiratory flow between 25% and 75% (FEF25%-75%) to detect small airway dysfunction (SAD). Objective To identify SAD in patients with respiratory symptoms and PPF using IOS. Methods Medical records of symptomatic patients without acute or known structural lung diseases were evaluated. Patients had bronchodilator testing and IOS in the outpatient clinic between March 1 and July 31, 2017. Correlations between respiratory symptoms, spirometry, and IOS parameters were determined. Results Among 349 patients enrolled to the study, 255 (73.1%) patients met the criteria of PPF. The IOS parameters—difference in resistance at 5 Hz and resistance at 20 Hz , reactance at 5 Hz, resonant frequency (Fres), and area under reactance curve between 5 Hz and resonant frequency—were significantly correlated with FEF25%-75%. The cutoffs for SAD were difference in resistance at 5 Hz and resistance at 20 Hz greater than 0.07 kPa/(L/s), reactance at 5 Hz less than −0.12 kPa/(L/s), Fres greater than 14.14 Hz, and area under reactance curve between 5 Hz and resonant frequency greater than 0.44 kPa/L. Of the IOS parameters, Fres and reactance at 5 Hz had the highest sensitivity and specificity. When compared with FEF25%-75%, Fres had greater sensitivity to detect SAD in patients with PPF. Patients with IOS-defined SAD had a significantly higher incidence of wheeze or sputum production than did those defined by FEF25%-75%. Conclusions Patients with respiratory symptoms and PPF may have SAD, which can be identified with the aid of IOS in addition to spirometry. Asthma and chronic obstructive pulmonary disease are characterized by persistent airway inflammation and airflow limitation. Early detection of these diseases in patients with respiratory symptoms and preserved pulmonary function (PPF) defined by spirometry is difficult. Impulse oscillometry (IOS) may have better sensitivity than effort-dependent forced expiratory flow between 25% and 75% (FEF25%-75%) to detect small airway dysfunction (SAD). To identify SAD in patients with respiratory symptoms and PPF using IOS. Medical records of symptomatic patients without acute or known structural lung diseases were evaluated. Patients had bronchodilator testing and IOS in the outpatient clinic between March 1 and July 31, 2017. Correlations between respiratory symptoms, spirometry, and IOS parameters were determined. Among 349 patients enrolled to the study, 255 (73.1%) patients met the criteria of PPF. The IOS parameters—difference in resistance at 5 Hz and resistance at 20 Hz , reactance at 5 Hz, resonant frequency (Fres), and area under reactance curve between 5 Hz and resonant frequency—were significantly correlated with FEF25%-75%. The cutoffs for SAD were difference in resistance at 5 Hz and resistance at 20 Hz greater than 0.07 kPa/(L/s), reactance at 5 Hz less than −0.12 kPa/(L/s), Fres greater than 14.14 Hz, and area under reactance curve between 5 Hz and resonant frequency greater than 0.44 kPa/L. Of the IOS parameters, Fres and reactance at 5 Hz had the highest sensitivity and specificity. When compared with FEF25%-75%, Fres had greater sensitivity to detect SAD in patients with PPF. Patients with IOS-defined SAD had a significantly higher incidence of wheeze or sputum production than did those defined by FEF25%-75%. Patients with respiratory symptoms and PPF may have SAD, which can be identified with the aid of IOS in addition to spirometry.