Expiratory reactance abnormalities in patients with expiratory dynamic airway collapse: a new application of impulse oscillometry

Abstract

Expiratory dynamic airways collapse (EDAC) is a condition that affects the central airways; it is not well characterised physiologically, with relatively few studies. We sought to characterise impulse oscillometry (IOS) features of EDAC in patients with normal spirometry. Expiratory data were hypothesised to be the most revealing. In addition, we compared IOS findings in chronic obstructive pulmonary disease (COPD) patients with and without EDAC. EDAC was identified at bronchoscopy as 75-100% expiratory closure at the carina or bilateral main bronchi. Four patient groups were compared: controls with no EDAC and normal lung function; lone EDAC with normal lung function; COPD-only patients; and COPD patients with EDAC. 38 patients were studied. Mean IOS data z-scores for EDAC compared to controls showed significantly higher reactance (X) values including X at 5 Hz, resonance frequency and area under the reactance curve (A_X). EDAC showed significantly greater expiratory/inspiratory differences in all IOS data compared to controls. Stepwise logistic regression showed that resonant frequency best discriminated between EDAC and normal control, whereas classification and regression tree analysis found A_X ≥3.523 to be highly predictive for EDAC in cases with normal lung function (14 out of 15 cases, and none out of eight controls). These data show a new utility of IOS: detecting EDAC in patients with normal lung function.

FIGURE 1

Bronchoscopic view of example of expiratory dynamic airways collapse case with near total closure of left and right main bronchus at main carina.

FIGURE 2

a) Impulse oscillometry trace in a control patient showing resistance and reactance (three measurements each). The resistance trace is relatively flat, with no significant change by changing frequency of impulse. Here the reactance trace crosses the baseline at 10 Hz, the resonant frequency. b) Expiratory dynamic airways collapse case (normal conventional lung function); higher resistance is seen at lower frequency, and raised resonance frequency (28 Hz). The reactance curve shows a plateau at 10–15 Hz.

FIGURE 3

An example of reactance curves in an expiratory dynamic airways collapse patient in inspiration and expiration showing significant impact of expiration. Area under the reactance curve values <30 Hz.

FIGURE 4

Peak-to-peak reactance at 5 Hz (X₅) values for expiratory dynamic airways collapse patients. The threshold value for expiratory flow limitation (EFL) as defined by Dellacà et al. [24] is 6.0.

FIGURE 5

Tabulated result of the classification and regression tree analysis. EDAC: expiratory dynamic airway collapse.