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?
We recently performed a 6-minute walk test with helium-oxygen (heliox) for a patient of one of the physicians that specializes in airway stenting. His reasons for the test weren’t particularly clear (and he hasn’t bothered to try to clarify them with me) but most probably it has to do with differentiating between central and peripheral airway obstruction. Interestingly, he predicted the patient would have a significant improvement in 6-minute walk distance and instead there was little difference between the heliox 6MWT and one performed with 3 LPM supplemental O2.
|80% Helium – 20% O2, by mask
|3 LPM O2, by nasal cannula
Helium is an inert, insoluble, low mass gas and both its therapeutic use and its use in physiological measurements has to do with it’s low density (and the fact that it’s highly insoluble, but that’s for purposes different than those discussed here).
|Air (78% N2, 21% O2)
|Heliox (80% He, 20% O2)
A typical way to assess its effect is by comparing air and heliox flow-volume loops:
Interestingly, despite an apparent increase in flow rates there is usually no significant difference in FEV1 (one study showed a range of +2% to +7% in a group of over 1500 subjects). The most common heliox FVL measurements are the change in expiratory flow at 50% of the FVC (ΔMEF@50%) and the Volume of Isoflow (which is the point at which the air and heliox expiratory flows become equivalent). Many of the earlier studies with heliox also measured ∆MEF@75% and ∆FEF25-75, and a tiny handful of studies (particularly given the technical difficulties) have measured ∆RAW and ∆sGAW.
I often find topics for this blog in a sideways fashion. Recently while searching for something else I ran across an article about the minimum clinically important difference (MCID) of the Residual Volume (RV) in patients with emphysema. I’ve come across the MCID concept before but I had never really followed up on it. This time I started researching MCID and immediately ran across a number of articles about the MCID of the 6-minute walk test (6MWT). This got me to review the articles I have on hand and I found that since I last wrote about the 6MWT I’ve accumulated quite a few new (or at least new to me) reference equations as well as a number of articles about performance issues. Given all this how could I not re-visit the 6MWT?
In addition to the 6 reference equations I had previously I’ve found another 13 female and 14 male reference equations for the 6MWT (total 19 female, 20 male) which is an opportunity to re-visit the selection process. This immediately raises the question about what factors should be used to calculate the predicted 6-minute walk distance (6MWD). Because the 6MWT is essentially an exercise test age has an obvious effect on exercise capacity so it is no surprise that with the exception of one set all of the reference equations consider age to be a factor. It should be noted however, that many of the reference equations are intended to be only applied over a limited range of ages and this may limit their utility.
Given the fact that stride length and therefore walking speed are directly related to height it is somewhat surprising to find that only twelve of the male and eleven of the female reference equations consider height to be a factor. When height is a factor, the predicted 6MWD is usually affected something like this:
Weight also affects exercise capacity but an interesting question is whether the observed 6MWD should be compared to a predicted 6MWD based on a “normal” weight or whether the 6MWD should be adjusted to the individual’s actual weight and assessed accordingly. To some extent this is already an issue in current PFT predicted equations. For example, weight is not a factor in any of the FVC or TLC reference equations and when lung volumes are decreased in the presence of obesity they are considered to be abnormal. On the other hand, the reference equations I use for maximum oxygen consumption during a CPET include weight as a factor and for a number of reasons this is likely the correct approach. For this last reason I would think that weight should be a factor and ten of the reference equation sets consider weight (or BMI) to be a factor. When weight is a factor, the predicted 6MWD is usually affected like this:
The distance attained during a 6-minute walk (6MWD) has been used extensively to assess the functional capacity of patients with a variety of diseases and conditions. It is relatively easy to perform and requires a minimum of equipment. Changes in 6MWD before and after rehabilitation, surgery, or medications are often used to signal the success or failure of these therapies. For the last dozen years every drug and device research study my lab has been involved with has used the 6MWD as one of their outcomes.
The 6-minute walk distance has been noted to depend on the age, gender, height and weight of the individual. There are, however, relatively few studies to choose from when it comes to selecting normal values for the 6MWD and each of these studies differs not only in the degree of importance it assigns to these variables but in the predicted 6MWD. Despite the clinical significance of changes in the 6-minute walk distance it is far from clear what a normal 6-minute walk distance actually is.