Impulse Oscillometry in the diagnosis of Asthma

OBJECTIVES: The primary objective of this study was to find out reference value for significant reversibility of R5 after bronchodilator by impulse oscillometry with spirometry as the gold standard and the secondary objectives were to grade severity of airway obstruction by impulse oscillometry by correlating with FEV1 by spirometry in patients with a clinical diagnosis of asthma. METHODS: The study was conducted in a South Indian tertiary care hospital. We included a total of 636 patients with a clinical diagnosis of asthma and 177 healthy individuals into the study. All patients and healthy individuals filled up an asthma questionnaire, after which they performed impulse oscillometry and spirometry both before and after bronchodilator. Statistical analysis was performed using Spearman’s correlation coefficients between impulse oscillometry parameters pre bronchodilator R5% predicted, pre bronchodilator R5 kPa/(L/s) and spirometry parameter pre bronchodilator FEV1% predicted and pre bronchodilator FEV1 (liters). Receiver-operator characteristics analysis was done between cases and controls with impulse oscillometry parameters R5 to find out the sensitivity, specificity, area under the curve and cutoff value at the best sensitivity and specificity for the above said impulse oscillometry parameters for significant reversibility, airway obstruction and severe airway obstruction as compared to the above said spirometry parameters. RESULTS: A total of 650 patients with asthma and 188 healthy individuals were screened initially, and of the 650 patients, 14 patients were excluded as they were unable to perform spirometry, and of the 188 healthy individuals, 2 had bronchial asthma, 5 were unable to perform spirometry and 4 did not complete the test. Finally a total of 636 patients with asthma and 177 healthy individuals were included in the study. CONCLUSION: This study to our knowledge is the first Indian study which has evaluated impulse oscillometry parameters by comparing them with spirometry parameters in patients with asthma to find out the significant bronchodilator reversibility of R5%, derive cutoff values of pre bronchodilator R5% predicted for airway obstruction in Indian patients with asthma and to grade airway obstruction in patients with asthma. We suggest that a value of more than 20% change in R5% post bronchodilator may be considered significant reversibility, a value of more than 180% for pre bronchodilator R5% predicted to diagnose airway obstruction and more than 230% for pre bronchodilator R5% predicted to diagnose severe airway obstruction in adult Indian patients with asthma. More studies with larger sample sizes may be required to confirm our findings but we suggest using our cutoff values in Indian patients with asthma till we get better evidence. Impulse oscillometry can be used not only in patients who are not able to perform spirometry but also as a routine practice in patients who have asthma as it is a very convenient and patient friendly test for the patients and requires only passive co operation from the patient.