Utility of Forced Oscillation Technique in detecting obstruction in asthma and the significance of ‘Isolated Increase in R5-R20’

Objective

The objective of this retrospective observational study was to investigate the role of the forced oscillometry technique in detecting airway obstruction in asthma and to compare the sensitivity of forced oscillometry to that of spirometry in detecting airway obstruction.

Method

The retrospective observational study was conducted in the Pulmonary Medicine Department in a secondary hospital in South India. Six-month data of the diagnosed cases of asthma from 1st October 2024 to 31st March 2025 were collected and analyzed to find out the efficiency of the oscillometry technique in detecting airway obstruction in asthma when compared with spirometry. The diagnosis of asthma was made by the treating pulmonologist based on the history, examination, and investigations. The cohort consisted of patients above eighteen years of age in whom both oscillomtery and spirometry was done.

Results

The total number of patients in the cohort was 182. The mean age was 48.5 years. Spirometry was able to demonstrate obstruction in 36.8% of the diagnosed cases of asthma, whereas forced oscillometry was able to detect obstruction in 69.7% of the cases (p < 0.0001, 95% CI 20%–46%, RR-1.89). When both spirometry and oscillometry were combined, the sensitivity of detecting an underlying airway obstruction increased to 80.8% (n = 147). Isolated increase in R5-R20 (II R5-R20) was seen in 36.81% of the cohort. This phenomenon was seen in 52.5% of the cases where oscillometry was successful over spirometry.

Conclusion

Oscillometry is more sensitive than spirometry in detecting airway obstruction in asthmatics. It is better to apply both for evaluation, as that will increase sensitivity. ‘Isolated increase in R5-R20’ (II R5-R20) is defined as an increase in R5-R20 with a normal R5. It is found to be the most sensitive parameter in detecting airway disease in asthmatics. It could be detecting the earliest change in airway resistance and hence can be used to identify airway obstruction at a much earlier stage.