During stepwise decrease of PEEP, Vrec at any given PEEP was higher than during the corresponding inflation with PEEP, especially during run 1 (Fig 5, B). Although part of this difference could be explained by other mechanisms (surface tension phenomena, tissue hysteresis, etc), recruitment of atelectatic lung units was probably paramount because with small tidal volumes, the other mechanisms play a small role, as indicated by a small difference between the static inflation and deflation V-P curves of the respiratory system (ie, static hysteresis) in the absence of atelectasis.
It has been shown that a minimum critical opening transpulmonary pressure (Pl) is needed before any gas could enter gas-free lungs. In humans, this minimum critical opening pressure is about 20 cm HoO. Since atelectasis occurs preferentially in the dependent lung zones, where the end-expiratory values of Pl at ZEEP are around zero in the supine position, the end-inspiratory Pst,rs required to reopen the atelectatic alveoli should exceed 20 cm H20, considering that Pst,rs includes a component due to the chest wall. In our patients, the initial values of end-inspiratory Pst,rs at ZEEP and PEEP of 5 cm H20 amounted to 17.0±1.3 cm H20 and 20.9±1.2 cm H20, respectively. Thus, even if we assume that our patients had normal chest wall compliance, their values of static end-inspiratory Pl were necessarily lower than the critical opening pressure needed for recruitment.