Tag Archives: IPF

What does an inverse I:E Ratio during exercise mean?

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Inspiration and expiration usually take different lengths of time, with inspiration almost always being shorter than exhalation. This is due to both to the physiology of breathing and to the pathophysiology of disease processes. During incremental exercise testing there are usually patterns to the way that inspiratory and expiratory times change and these are occasionally diagnostic.

When I started in this field the relationship between inspiratory and expiratory time was usually expressed as the I:E ratio, which was most often written as something like 1:1.2. One of my medical directors pointed out to me that when talking about I:E ratio it was difficult to determine what you meant if you said it was increasing or decreasing. For this reason I started reporting the I:E ratio as the E/I ratio so that instead of 1:1.2 it’s just 1.2.

Somewhere along the way however, for exercise testing at least, the most common way of expressing the I:E ratio seems to have morphed primarily into Ti/TTot (which is the Inspiratory Time/Total Inspiratory and Expiratory Time ratio), less commonly as Ti/Te and almost never as I:E. Even so, I still prefer the E/I ratio approach, partly because I’m used to it but mostly because it emphasizes the expiratory time component. For example:

Ti/TTot: Ti/Te: E/I:
0.50 1.00 1.0
0.48 0.91 1.1
0.45 0.83 1.2
0.43 0.77 1.3
0.42 0.71 1.4
0.40 0.66 1.5
0.38 0.63 1.6
0.37 0.59 1.7
0.36 0.56 1.8
0.34 0.53 1.9
0.33 0.50 2.0

Anyway, at rest most subjects breathe with an E/I ratio somewhere between 1.2 and 1.5 (Ti/TTot 0.45 – 0.40). During exercise the E/I ratio usually decreases more or less steadily and usually reaches 1.0 (Ti/TTot 0.50) at or near peak exercise. When a subject has airway obstruction the E/I ratio often doesn’t decrease and in those with severe airway obstruction it often increases instead. E/I ratios above 2.0 aren’t all that uncommon in subjects with COPD. Occasionally a subject with normal baseline spirometry (i.e. a normal FEV1/FVC ratio) has an elevated and/or increasing E/I ratio throughout testing and this is a clue that they probably have some degree of airway obstruction that’s not otherwise evident, and possibly even EIA if it increases at peak exercise.

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6MWT re-visited, now with the MCID!

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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:

Height_6MWD

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:

Weight_6MWD

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