PRESSURE LIMITED VENTILATION WITH PERMISSIVE HYPERCAPNIA AND MINIMUM PEEP IN SALINE LAVAGED RABBITS ALLOWS PROGRESSIVE IMPROVEMENT IN OXYGENATION, BUT DOES NOT AVOID VENTILATOR INDUCED LUNG INJURY

Objective. To determine whether pressure-limited intermittent mandator y ventilation with permissive hypercapnia and positive end-expiratory pressure (PEEP) titrated to arterial oxygen tension (PaO2) prevents or reduces acute lung injury, compared to conventional ventilation, in s aline-lavaged rabbits. Design. Prospective randomised trial. Setting. University animal laboratory. Subjects. 18 New Zealand White rabbits.I nterventions. Following five sequential saline lung lavages, anaesthet ised rabbits were randomly allocated in pairs to receive either of two ventilation protocols using intermittent mandatory ventilation. The s tudy group had peak inspiratory pressure limited to 15 cm H2O and arte rial partial pressure of carbon dioxide (PaCO2) was allowed to rise, T he control group received 12 ml/kg tidal volume with rate adjusted for normocarbia. PEEP and fractional inspired oxygen (FIO2) were adjusted to maintain PaO2 between 8 and 13.3 kPa (60 and 100 mm Hg) using a pr edetermined protocol. At 10 h or following death, lung lavage was repe ated and lung histology evaluated. Measurements and main results. The mean increase in lavage cell counts and protein concentration and hyal ine membrane scores were not significantly different between the group s, Oxygenation progressively improved more in the study group (p = 0.0 1 vs control for PaO2/FIO2 ratio and alveolar-arterial oxygen tension gradient (AaDO(2))). PEEP was similar and the mean airway pressure hig her in the control group,suggesting that this probably resulted from l ess ventilator-induced injury in the study group. Four deaths occurred in the control group (three due to pneumothorax and one to hypoxaemia ) and none in the study group (p = 0.08). Conclusions: This ventilator y protocol may have failed to prevent lung overdistension or it may ha ve provided insufficient PEEP to prevent injury in this model; PEEP gr eater than the lower inflection point of the pressure-volume curve has been shown to prevent injury almost entirely.