The ATS/ERS guidelines state that a significant response to a bronchodilator consists of a ≥0.20 L and ≥12% increase in FEV1 and/or FVC.  These criteria has been debated on numerous points and both a lower threshold and the inclusion of other test results have been proposed more than once.  Although these proposed changes likely have some merit, they are not currently recommended.

The post-bronchodilator changes in volume and percent for FEV1 and FVC are easily calculated but the finding of a significant increase may be erroneous depending on the quality of the pre- and post-bronchodilator FEV1 and FVC results.  Strictly speaking, post-bronchodilator testing should not be performed when the baseline spirometry has notably poor quality or lacks reproducibility.  A patient cough, however, can prevent spirometry being performed with adequate quality, and cough can be a symptom of asthma and other airway diseases.  For this reason there can be clinical value in performing post-bronchodilator testing even when the baseline spirometry quality is poor, but it may not be possible to interpret the response definitively.

Given the nature of reversible airway obstruction, post-bronchodilator increases in FEV1 are seen more frequently than increases in FVC.  Obviously however, when the baseline FEV1 is underestimated due to a cough or pause that is not present in the post-bronchodilator test then the probability that the response is truly significant decreases. Similarly, a post-bronchodilator increase in FEV1 that is due in part to excessive back-extrapolation also reduces the probability the response is truly significant.

A significant increase in FVC when it is accompanied by a increase in expiratory time is a somewhat equivocal signal for a positive bronchodilator response.  In these circumstances the apparent increase in FVC may be solely due to the increased expiratory time but numerous researchers have noted that an increased expiratory time can itself be considered a positive bronchodilator response.  For this reason when the baseline FVC is likely underestimated due to a short expiratory time then a significant increase in post-bronchodilator FVC is probably not an indication of a significant bronchodilator reponse.  Conversely when the baseline FVC has adequate test quality then a post-bronchodilator increase in FVC, even when accompanied by an increase in expiratory time, can be taken as a positive bronchodilator response.

It’s possible that the lack of a significant post-bronchodilator increase in FEV1 and FVC could be due to poor post-bronchodilator test quality.  In this situation it’s not possible to assign any level of probability that this is the case and the only certainty that a subject has or does not have a significant bronchodilator response can only from repeat repeat testing. 

 

 

PFT Interpretation by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License