Low-volume ventilation causes peripheral airway injury and increased airway resistance in normal rabbits

Lung mechanics and morphometry of 10 normal open-chest rabbits ( group A), mechanically ventilated (MV) with physiological tidal volumes (8–12 ml/kg), at zero end-expiratory pressure (ZEEP), for 3–4 h, were compared with those of five rabbits ( group B) after 3–4 h of MV with a positive end-expiratory pressure (PEEP) of 2.3 cmH2O. Relative to initial MV on PEEP, MV on ZEEP caused a progressive increase in quasi-static elastance (+36%) and airway (Rint; +71%) and viscoelastic resistance (+29%), with no change in the viscoelastic time constant. After restoration of PEEP, quasi-static elastance and viscoelastic resistance returned to control levels, whereas Rint remained elevated (+22%). On PEEP, MV had no effect on lung mechanics. Gas exchange on PEEP was equally preserved in groups A and B, and the lung wet-to-dry ratios were normal. Both groups had normal alveolar morphology, whereas only group A had injured respiratory and membranous bronchioles. In conclusion, prolonged MV on ZEEP induces histological evidence of peripheral airway injury with a concurrent increase in Rint, which persists after restoration of normal end-expiratory volumes. This is probably due to cyclic opening and closing of peripheral airways on ZEEP.