Bactericidal function of alveolar macrophages in mechanically ventilated rabbits

Protective ventilation strategies have been universally embraced because of reduced mortality. We tested the hypothesis that tidal volume (VT) in an in vivo model of mechanical ventilation would modulate bactericidal function of alveolar macrophages (AMs). Adult New Zealand White rabbits were mechanically ventilated for 4 h with a VT of 6 ml/kg (low) or a VT of 12 ml/kg (traditional), with each group receiving 3 cm H2O positive end-expiratory pressure with and without intratracheal lipopolysaccharide (LPS) instillation (20 mg/kg). AMs were isolated from bronchoalveolar lavage fluid taken from the whole left lung and used for bacterial killing assays. There were no significant differences in steady-state levels of nitrite or AM phagocytosis and killing of Klebsiella pneumoniae, although these values trended to be slightly higher in the traditional VT group. However, bronchoalveolar lavage fluid protein concentrations were significantly increased in traditional VT groups receiving LPS compared with animals ventilated with a low VT (1,407.8 +/- 121.4 versus 934.7 +/- 118.2; P < 0.001). Lung wet:dry weight ratio in the traditional VT group was increased when compared with the low VT group without LPS (7.3 +/- 0.4 versus 6.1 +/- 0.3, respectively; P < 0.05). Additionally, IL-8 expression was significantly greater under conditions of LPS treatment and mechanical ventilation at VT of 12 ml/kg. These results suggest that the traditional ventilator approach (12 ml/kg VT) in a model of in vivo mechanical ventilation results in lung pathology without affecting AM antibacterial function.