A couple weeks ago I was asked whether it was safe for a patient with an abdominal aortic aneurysm (AAA) to have pulmonary function testing. My first thought was that it was probably unsafe but after a moment or two of thought I realized that I hadn’t reviewed the subject for a long time. When I checked the 2005 ATS/ERS general testing guidelines (there are no contraindications in the 2005 spirometry guidelines) I found that AAA wasn’t mentioned at all. In fact, the only absolute contraindication mentioned was that patients with a recent myocardial infarction (<1 month) should not be tested. Some relative contraindications were mentioned:
- chest or abdominal pain
- oral or facial pain
- stress incontinence
- dementia or confusional state
and activities that should be avoided prior to testing include:
- smoking within 1 hour of testing
- consuming alcohol within 4 hours of testing
- performing vigorous exercise within 30 minutes of testing
- wearing clothing that restricts the chest or abdomen
- eating a large meal with 2 hours of testing
but these were factors where test results were likely to be suboptimal and not actually contraindications.
This got me curious since I thought that pulmonary function testing was contraindicated for more conditions than just an MI. I reviewed the 1994 and and then the 1987 ATS statements on spirometry but again found no mention of contraindications. Ditto on the 1993 ERS statement on spirometry and lung volumes. Finally, in the 1996 AARC clinical guidelines for spirometry I found a much longer list of contraindications:
- hemoptysis of unknown origin
- recent mycardial infarction
- recent pulmonary embolus
- thoracic, abdominal or cerebral aneuysms
- recent eye surgery
- presence of an acute disease process that might interfere with test performance (e.g. nausea, vomiting)
- recent surgery of thorax or abdomen
So where did the AARC’s list of contraindications come from? And why is there such a discrepancy between the ATS/ERS and the AARC guidelines?
The 2005 ATS/ERS standards for assessing post-bronchodilator changes in FVC and FEV1 have been criticized numerous times. A recent article in the May issue of Chest (Quanjer et al) has taken it to task on two specific points:
- the change in FVC and FEV1 has to be at least 200 ml
- the change is assessed based on the percent change (≥12%) from the baseline value
The article points out that the 200 ml minimum change requires a proportionally larger change for a positive bronchodilator response in the short and the elderly. Additionally, by basing the post-BD change on the baseline value it lowers the threshold (in terms of an absolute change) for a positive bronchodilator response as airway obstruction become more severe. As a way of mitigating these problems the article recommends looking at the post-bronchodilator change as a percent of predicted rather than as a percent of baseline.
The article is notable (and its authors are to be commended) because it studied 31,528 pre- and post-spirometry records from both clinical and epidemiological sources from around the world. For the post-bronchodilator FEV1 and FVC:
- the actual change in L
- the percent change from baseline
- the change in percentage of predicted
- the Z-score
Okay, this may be wrong but at the moment I’m can’t seem to find a reason why it should be. A report like this came across my desk a couple of days ago.
Not particularly unusual and it would usually be interpreted as being within normal limits without a significant post-BD change. If you calculate the FEV1/VC ratio using the pre-BD FEV1 and the post-BD FVC however, it’s 89% of predicted and this indicates mild airway obstruction. But you’re not supposed to use the post-BD FVC this way, are you?
Well, why not?
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:
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.
The current ATS/ERS standards for a positive bronchodilator response are an increase in FEV1 or FVC of ≥ 12% and ≥ 200 ml. These standards are largely based on the ability to detect a change that is far enough above the normal variability in FEV1 and FVC to be considered significant. One problem with this is that the amount of variability that is considered to be “normal” is overly influenced by a relatively small number of subjects that have a high degree of variability.
At least one group of investigators has suggested that a way around this is to subject all of an individual’s pre- and post-bronchodilator spirometry to statistical analysis in order to determine their coefficient of variability. Once this is known, the pre- and post-bronchodilator efforts can be assessed as a group to determine whether whether there has been a statistically significant change. Using this approach they were able to show that a rather large number of subjects that did not meet the ATS/ERS criteria did have a statistically significant improvement in FEV1.
But an increase that is statistically significant or one that is greater than normal variability is not the same thing as clinical significance. Numerous investigators have noted that patient can have a post-bronchodilator clinical improvement as shown by a decrease in dyspnea or an increase in exercise capacity without any notable change in FEV1 or FVC. Clinical significance is hard to measure however, particularly since which criteria should be used to measure it are unclear.
Long-term survival is certainly clinically significant and a recent article in Chest (Ward et al) has linked the increase in post-bronchodilator FEV1 to this fact. What these investigators have been able to show was that individuals with a post-bronchodilator increase in FEV1 that was 8% of predicted or greater showed a significantly better long-term survival than individuals with a smaller increase.
When assessing the response to bronchodilators the change in FEV1 is used far more frequently than any other spirometry result. Other values such as inspiratory capacity (IC) and peak inspiratory flow (PIF) have also been proposed as indicators, but the ATS/ERS standards includes changes in FVC as well as changes in FEV1 and this is often overlooked. Specifically they:
“…recommend using the per cent change from baseline and absolute changes in FEV1 and/or FVC in an individual subject to identify a positive bronchodilator response. Values >12% and 200 mL compared with baseline during a single testing session suggest a ‘‘significant’’ bronchodilatation.”
I don’t have any particular disagreement with this since post-BD increases in FVC are probably similar in nature to the post-BD changes in IC seen in some individuals with COPD. So when spirometry results like this:
comes across my desk, I’m inclined to consider that the results show a positive bronchodilator response. Post-BD increases in FVC are not usually quite as large as 40% however, so I took a closer look at this particular test. When I did what I saw was that the post-BD test length was significantly longer than the pre-BD test length.
The current ATS/ERS guidelines require that an individual have a post-bronchodilator increase in FEV1 or FVC of at least 12% and 200 ml in order for it to be considered a significant response. Numerous studies have shown however, that many patients that don’t meet these criteria, particularly those with COPD, do have a clinically significant improvement with bronchodilators.
The September 2014 issue of Chest had a point-counterpoint set of editorials on this standard and on assessing the response to bronchodilator in general. Both sides had a number of interesting things to say but to a large extent one side was talking apples (physiology) and the other side oranges (statistics). I actually think that both sides feel there are significant problems with the ATS/ERS standards, they just differ in what they think is wrong and in the best way to fix it.
One statistical argument was that the ATS/ERS guideline is a one-size-fits-all solution that is designed more to detect asthmatic-type responses than the more subtle changes that can occur in individuals with COPD that although small, are clinically relevant. I am inclined to agree with this but as much as I and others think that the current ATS/ERS standard likely needs revision the difficulty with this is that spirometry is a “noisy” measurement with a lot of variability.
Spirometry variability and the limits to its accuracy comes from issues to familiar to all of us:
- the patient’s ability to understand and perform spirometry
- the technician’s ability to encourage and guide a patient through a spirometer maneuver
- the spirometer’s ability to measure exhaled flow and volume accurately
“Noise” is one of the reasons why it can be difficult to determine statistically and clinically significant changes. It’s also why the threshold for significant post-bronchodilator change is set as high as it is and may also be the reason why it shouldn’t be lowered.
Even though there may be reasons why the ATS/ERS guidelines for post-bronchodilator improvements in FEV1 and FVC shouldn’t be revised at the very least they need to become more inclusive and consider more factors. There is too much emphasis placed on the changes in FEV1 and FVC as the sole indication for a response to bronchodilator particularly since there are other ways in which a response can be measured.
The ATS/ERS recommendation for assessing the response to bronchodilator is based solely on changes in FEV1 or FVC. An FEV1 that does not improve significantly following bronchodilator inhalation is considered to be one of the hallmarks of COPD. Many individuals with COPD however, can have symptomatic relief and an improvement in their exercise capacity without a significant post-bronchodilator increase in FEV1. This means that FEV1 may not be the only criteria for assessing bronchodilator response.
One of the hallmarks of COPD is expiratory flow limitation. This can cause hyperinflation and is often reflected in an elevated FRC. It is also an important factor is exercise limitation. When ventilation increases during exercise in an individual with COPD, expiratory flow limitation causes the tidal volume and FRC to shift towards higher lung volumes. FRC is difficult to measure during exercise so this usually observed by measuring Inspiratory Capacity (IC).
COPD patients who don’t show a significant change in their FEV1 can respond to bronchodilators by becoming less expiratory flow-limited and when this happens their FRC decreases. Bronchodilator response in these individuals can therefore be assessed by measuring pre- and post-bronchodilator FRC or IC. At present there appears to be a consensus that an increase in IC or decrease in FRC of at least 0.30 liters or 12% should be considered to be a significant response.