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

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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