Impact of exacerbations on respiratory system impedance measured by a forced oscillation technique in COPD: a prospective observational study

BACKGROUND: Forced oscillation technique (FOT) has been reported to be useful in the evaluation and management of obstructive lung disease, including COPD. To date, no data are available concerning long-term changes in respiratory system impedance measured by FOT. Additionally, although exacerbations have been reported to be associated with excessive lung function decline in COPD, the impact of exacerbations on the results of FOT has not been demonstrated. The aim of this study was to investigate the longitudinal changes in respiratory system impedance and the influence of exacerbations thereon. METHODS: Between March 2011 and March 2012, outpatients who attended Kobe City Medical Center West Hospital with a diagnosis of COPD were assessed for eligibility. Baseline patient characteristics (age, sex, body mass index, smoking history, current smoking status, COPD stage), lung function (post-bronchodilator forced expiratory volume in 1 second [FEV(1)]), blood tests (neutrophils and eosinophils), FOT, and COPD assessment test results were collected at enrollment. Lung function and FOT were examined every 6 months until March 2016. Annual changes in FEV(1) and FOT parameters were obtained from the slope of the linear regression curve. The patients were divided into 2 groups based on exacerbation history. RESULTS: Fifty-one of 58 patients with COPD were enrolled in this study. The median follow-up period was 57 (52–59) months. Twenty-five (49%) patients experienced exacerbations. A significant annual decline in FEV(1) and respiratory system impedance were shown. Additionally, annual changes in FEV(1), respiratory system resistance at 5 Hz, respiratory system reactance at 5 Hz, and resonant frequency were greater in patients with exacerbations than in those without exacerbations. CONCLUSION: Exacerbations of COPD lead not only to a decline in lung function but also to an increase in respiratory system impedance.