•Impulse oscillometry (IOS) is the most commonly used forced oscillation technique.
•Relatively little is known about its application in COPD.
•Lung resistance (R) and reactance (X) reflect airway geometry and compliance.
•IOS indices relate to disease severity, bronchodilatation and bronchoconstriction.
•Trials are required for the predictive value of IOS in relation to COPD exacerbations.
Impulse oscillometry (IOS) is the most commonly used type of forced oscillation technique in clinical practice, although relatively little is known about its application in COPD. Resistance at 20 Hz (R20) is unrelated to COPD severity and does not improve with bronchodilatation or bronchoconstriction, inferring a lack of large airway involvement in COPD. Peripheral airway resistance expressed as frequency dependent heterogeneity between 5 Hz and 20 Hz (R5-R20), and peripheral airway compliance as area under the reactance curve (AX), are both closely related to COPD severity and exacerbations. Both R5-R20 and AX markedly improve in response to long acting bronchodilators, while AX appears to be more sensitive than R5-R20 in response to bronchoconstriction.
Future studies may be directed to assess if IOS in combination with spirometry is more sensitive at predicting future exacerbations. Perhaps AX might also be useful as a screening tool in early stage disease or to monitor long term decline in COPD.
COPD, Resistance, Reactance, Oscillometry, IOS, FOT