Association between respiratory impedance measured by forced oscillation technique and exacerbations in patients with COPD

Abstract

Background: It is well known that increased airflow limitation as measured by spirometry is associated with the risk of exacerbation in patients with COPD. The forced oscillation technique (FOT) is a noninvasive method used to assess respiratory impedance (resistance and reactance) with minimal patient cooperation required. The clinical utility of the FOT in assessing the risk of exacerbations of COPD is yet to be determined. We examined the relationship between respiratory impedance as measured by FOT and exacerbations in patients with COPD.

Materials and methods: Among 310 patients with COPD (Global Initiative for Chronic Obstructive Lung Disease stages I-IV) who presented at the outpatient clinic of the Showa University Hospital from September 2014 through January 2015, 119 were collected and assigned into 2 groups according to their history of exacerbation: exacerbators and nonexacerbators. Respiratory resistance components and respiratory reactance components, as measured by FOT, were compared between the two groups.

Results: Exacerbators were significantly older and had a higher white blood cell count than nonexacerbators. Resistance at 20 Hz, reactance at 5 Hz (X5), resonant frequency (Fres), and area of low reactance (ALX) differed significantly between the two groups. In addition, among patients with stage II COPD, there were significant differences in X5, Fres, and ALX between the two groups despite no significant differences in respiratory function as assessed by spirometry. Finally, receiver operating characteristic curve analysis revealed that the reactance components rather than the resistance components were associated with the risk of exacerbation.

Conclusion: There were significant differences in respiratory impedance between exacerbators and nonexacerbators in patients with moderate COPD. FOT is a promising tool for assessing future exacerbations in patients with COPD.

Keywords: forced oscillation technique; respiratory system resistance and reactance; spirometry.

Figure 1

Flow chart of patient enrolment.

Figure 2

ROC curve of spirometry and FOT parameters for identification of exacerbations in 119 patients with COPD. Notes: ROC curve of % predicted FVC (A), % predicted FEV₁ (B), FEV₁/FVC (C), R5 (D), R20 (E), R5–R20 (F), X5 (G), Fres (H), and ALX (I) with the presence of exacerbation are shown. High AUC of all parameters of FOT to identify exacerbations were observed. Abbreviations: ALX, reactance area; AUC, area under the curve; FEV₁, forced expiratory volume in 1 second; FOT, forced oscillation technique; Fres, resonant frequency; FVC, forced vital capacity; ROC, Receiver operating characteristic; R5, resistance at 5 Hz; R20, resistance at 20 Hz; R5–R20, differences between R5 and R20; ΔX5, difference between inspiratory and expiratory respiratory system reactance; X5, reactance at 5 Hz.