Ascertainment of Small Airways Dysfunction Using Oscillometry to Better Define Asthma Control and Future Risk: Are We Ready to Implement It in Clinical Practice?

Abstract

The small airways comprise generations 8 to 23 of the bronchial tree, consist of airways with an internal diameter < 2 mm, and are classically difficult to assess and treat in persistent asthma. Small airways dysfunction (SAD) is integral to the asthma management paradigm because it is associated with poorer symptom control and greater levels of type 2 inflammation, and it has been proposed as a potentially treatable asthma trait. Although identification of SAD based on oscillometry has been found to be clinically useful in managing asthma, very few physicians, including specialists, use this technique as part of standard or adjunct evaluation of lung function to diagnose asthma, grade severity of airway obstruction, ascertain disease control or the risk for future exacerbations, or to make management decisions. To rectify the unrecognized value of oscillometry in the asthma community, a consortium of authors who are investigators with knowledge and experience of oscillometry wished to address the most important clinical questions raised by our colleagues who are considering using this technique, including its clinical utility. In this article, we discuss integral concepts, including applicability of oscillometry as a predictive tool for asthma exacerbations and disease control, adequacy of spirometry and oscillometry in assessing SAD, potential limitations of oscillometry, and treatment options for SAD.

Keywords: asthma; oscillometry; small airways dysfunction.

Figure 1

The first component of respiratory impedance is resistance, which is largely dependent on airway caliber. Impedance also consists of reactance, which itself comprises inertance and elastance. Inertance represents measurement of pressure losses mostly resulting from acceleration of the gas column in proximal bronchi, whereas elastance is an assessment of lung stiffness as it can be calculated as the inverse of compliance. (Reprinted, with permission, from Gochicoa-Rangel and Vargas.¹⁰)

Figure 2

Oscillometry determines breathing mechanics by superimposing small external pressure signals generated from the loudspeaker onto the spontaneous breathing of the individual at the mouthpiece. Flow is measured by a heated screen pneumotachograph. (Reprinted, with permission, from Gochicoa-Rangel and Vargas.¹⁰)

Figure 3

Oscillogram depicting resistance and reactance curves. The difference in resistance between 5 Hz (Rrs5) and 20 Hz (Rrs20) corresponds to peripheral airway resistance (Rrs5-20). The resonant frequency (Fres) is the frequency at which reactance equals zero. The area under the curve between reactance at 5 Hz (Xrs5) and Fres corresponds to the reactance area (AX).

Figure 4

Proposed clinical algorithm to guide interpretation of airway oscillometry. AX = area under the reactance curve; Feno = fractional exhaled nitric oxide; ICS = inhaled corticosteroid; PEFR = peak expiratory flow rate; Rrs5 = resistance at 5 Hz; Rrs5-20 = difference in resistance between 5 and 20 Hz; Rrs20 = resistance at 20 Hz; SAD = small airways dysfunction; T2 = type 2 high; Xrs5 = reactance at 5 Hz.