Unravelling orthopnea in COPD: assessment of supine respiratory mechanics by forced oscillometry

The pathophysiology of orthopnea in COPD remains poorly understood and the effects of the supine position on respiratory mechanics have only been sparsely investigated. We hypothesized that supine-induced expiratory flow limitation during tidal breathing (EFL tb ), along with the consequent increase in respiratory system impedance in the supine position, contributes to the development of orthopnea in patients with COPD. We analysed and compared spirometric, particularly inspiratory capacity (IC), and forced oscillometry parameters including resistance (R 5 , R 5 -R 20 ) and reactance parameters (AX, X 5 and ΔX 5 , a surrogate of EFL tb ) in 46 COPD patients and 22 healthy controls in sitting and supine positions. A simple questionnaire and a visual analogue scale were used to assess dyspnea and orthopnea in all participants. Univariable and multivariable linear regression models were used to analyse the relationship between pulmonary function parameters and changes in dyspnea during postural transition. In COPD patients, IC decreased significantly in the supine position, suggesting the presence of dynamic hyperinflation, whereas it increased significantly in control subjects. The prevalence of EFL tb, assessed by ΔX 5 , increased significantly from the sitting to the supine position in COPD patients, accompanied by significant increases in resistance and reactance parameters. In control subjects, resistance parameters and X 5 also increased in the supine position, although to a lesser extent than in COPD patients. Positional changes in dyspnea were significantly associated with changes in reactance parameters, particularly ∆X 5 and X 5 . In COPD, the transition to the supine position is associated with a decrease in IC and the development of EFL tb , resulting in an increased respiratory system reactance and elastic load. The increase in respiratory system reactance was significantly associated with orthopnea.