Fig. 2

Intra-breath changes of impedance values of two representative patients with OHS. The first patient (Fig. 2a) exhibits minimal intra-breath changes in Rrs and Xrs in the sitting position, while in the supine position, a marked looping develops in the Xrs vs V plot. During expiration, Xrs departs gradually from the inspiratory path with an increasing negative slope, thus creating a clockwise loop with a significant area (AXV). The corresponding Xrs vs V’ diagram reveals a gradual decrease in Xrs with increasing expiratory V’, followed by a further fall in both Xrs and expiratory V’ in the second half of expiration where Xrs approaches its lowest point with decreased V’. This characteristic pattern is considered the manifestation of tEFL. Inspiratory effort leads to a sharp increase in Xrs and, eventually, the formation of a loop. The value of the area (AXV’) is negative by definition due to the counter-clockwise change in Xrs. In connection with tEFL, a characteristic phenomenon appears in the Rrs vs. V diagram. When inspiratory activity starts, although still at a net expiratory flow, tEFL resolves, flow is abruptly restored, causing a transient increase in Rrs, likely via upper airway nonlinearities. In another subject (Fig. 2b), tEFL was already present in the sitting position with both decreased Xrs and V’ at late expiration. The change in position augmented this looping pattern of tEFL

Zrs = respiratory impedance; Rrs = respiratory resistance; Xrs = respiratory reactance;