I had finished reviewing a pre- and post-BD spirometry report yesterday and was about to toss it on my out pile when I noticed something a bit odd about the post-BD results. I pulled it back and spent some time trying to decide if the interpretation needed to be changed but after a lot of internal debate I finally let it go as it was. I’ve continued to think about it however, and although I’m not sure that was the right decision I still haven’t come up with a clear answer.
Here’s what I saw:
| Observed: | %Predicted: | Post-BD: | %Predicted: | %Change: | |
| FVC: | 3.70 | 97% | 3.91 | 103% | +6% |
| FEV1: | 2.82 | 94% | 2.79 | 93% | -1% |
| FEV1/FVC: | 76 | 95% | 71 | 89% | -6% |
| PEF: | 6.62 | 94% | 7.19 | 102% | +9% |
| Exp. Time: | 10.92 | 11.15 |
The reported pre-BD and post-BD results were from good quality tests and met the criteria for repeatability. My problem is that the baseline results were normal but if I had seen the post-BD results by themselves I would have considered them to show mild airway obstruction.
So what’s going on here? The results from baseline spirometry aren’t anywhere close to the borderline for airway obstruction, no matter which guidelines you use. There was a small post-BD increase in FVC (+6%) but since the ATS/ERS standards for a significant post-bronchodilator change in FEV1 or FVC is ≥12% and ≥200 ml it isn’t significant. The post-BD decrease in FEV1 was also small (-1%) and well within normal test-to-test variability, but when you put the small increase in FVC together with the small decrease in FEV1 suddenly the FEV1/FVC ratio below the LLN.
Does that mean these results show airway obstruction? One argument in favor of this is that the GOLD standards for spirometry only consider the post-BD results. The basic point of this as I see it is that since COPD is expected to have little or no bronchodilator response, this allows asthma to be ruled out as the cause of obstruction. But obstruction was only in the post-BD results, not the baseline results, so I’m not sure this applies.
The ATS/ERS interpretation algorithm also only looks at the baseline results when assessing results for obstruction and only assesses post-BD results for significant improvement.
Could the decrease in the FEV1/FVC ratio be a side-effect of albuterol? We usually use albuterol as our bronchodilator and some individuals are sensitive to it and bronchoconstrict instead of bronchodilating (read the product insert and you’ll find that this is one of the possible side-effects). This occurs in probably less than 1% of the population but I see it a couple times a year. In this case though, even though there was a small post-BD decrease in FEV1 there was a larger increase in FVC without any significant change in expiratory time, and this means there was probably more bronchodilation than bronchoconstriction.
One final point is that post-BD changes are only assessed using the FEV1 and FVC, not the FEV1/FVC ratio. If the pre-BD and post-BD results were reversed, even though the individual would have gone from mild airway obstruction to normal, the bronchodilator response still wouldn’t have been called a significant.
This situation doesn’t seem to be covered by any of the guidelines. On the one hand, if the pre-BD and post-BD results were reversed I’d have no hesitation about saying the results showed mild airway obstruction. On the other hand, the baseline spirometry was normal, the decrease in FEV1 was well within normal test-to-test variability and the increase in FVC was not significant. In the end I had to let it go as normal spirometry mostly because I couldn’t find any way to legitimately use the post-BD FEV1/FVC ratio. That doesn’t mean I haven’t second-guessed that decision a half dozen times since then, but since I keep second-guessing the second-guessing, I’ve let it stand.
Even though I couldn’t come up with any clarity for this situation what this does show is the effect that FVC can have on the FEV1/FVC ratio. When individuals respond to a bronchodilator it’s usually the FEV1 that increases and any increases in FVC are relatively small. Post-BD increases in FVC without any increase in FEV1 are relatively rare and when seen it’s almost always in individuals who’s baseline shows severe airway obstruction. When individuals have post-BD bronchoconstriction it’s usually fairly clear and usually both the FVC and FEV1 are affected. In this case however, the baseline spirometry was normal and the post-BD changes were just ambiguous enough that I don’t think there ever could be a clear answer.
PFT Blog by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License