COMMENTARY

Does Isolated Small Airway Dysfunction Matter? We Still Don't Know

Aaron B. Holley, MD

Disclosures

May 27, 2016

Small Airways Are Tough to Assess

Assessing the small (< 2 mm in diameter) airways is a challenge. Standard spirometry is no good; it's just not sensitive enough. Some physicians still advocate for using the forced expiratory flow (FEF) 25-75% from standard spirometry output. Unfortunately, because it includes flow rates over time, as opposed to a single measurement at one point in time, establishing a precise definition for normal versus abnormal is difficult. In short, the test is sensitive but not specific.[1] Forced expiratory volume in the first 3 seconds (FEV3) seems to be a reasonable option, but it hasn't caught on and there are few studies correlating this parameter with clinical or objective outcomes. Impulse oscillometry lacks universally accepted reference ranges,[2,3] body plethysmography is cumbersome and labor intensive, and mosaicism on high-resolution CT can be seen in asymptomatic patients with otherwise normal lung function.[4]

Does Small Airway Dysfunction Impede Exercise Capacity?

A recent study published in the Annals of the American Thoracic Society[5] reported that measures for small airway disease are independently associated with increased respiratory work while running on a treadmill. Small airway disease was assessed by flow changes while breathing an 80% helium/20% oxygen gas and FEF25-75%. In linear regression modeling, small airway dysfunction was the only variable significantly associated with ventilatory equivalents for carbon monoxide.

Unfortunately, in this author's opinion, this study doesn't help assess the impact of isolated small airway dysfunction, defined as normal baseline spirometry with evidence of small airway abnormalities. Though the investigators ran a regression analysis to try to isolate the effect, all of the variables they entered in their model (body mass index, FEV1 percent predicted, and diffusing capacity of the lungs for carbon monoxide) were significantly associated with small airway dysfunction. Entering variables that are correlated into the same model causes "overmodeling" and inaccurate results. Small airway dysfunction was also entered as a dichotomous variable while all other covariates were linear. Had they entered FEF25-75% or response to heliox as linear variables, it's unclear whether they would have performed as well. In short, I detect an effort to ask more from the data than the data are able to provide.

All Signs Point to Maybe

If anything, this study confirms what we already know. Past data from COPD patients has shown that even mild deficits on spirometry cause increased respiratory work during exercise.[6,7] I suspect that isolated small airway dysfunction does lead to respiratory impairment. As part of our profession, we pulmonologists need to continue working on ways to diagnose it and quantify its effect.

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