"Ideal PEEP" is superior to high dose partial liquid ventilation with low PEEP in experimental acute lung injury.

Objective: To determine the effects of a high dose partial liquid ventilation (PLV) approximating the amount of the functional residual capacity (FRC) with low levels of positive end-expiratory pressure (PEEP) compared to a lung-protective strategy with volume-controlled mechanical ventilation (vcMV) with a PEEP level above the lower inflection point (LIP) on pulmonary gas exchange, haemodynamics, respiratory mechanics and lung injury in an experimental model of acute lung injury (ALI). Design: Prospective, randomised, controlled study. Methods: Twenty-four anaesthetised, tracheotomised and mechanically ventilated (FIO2 1.0) pigs underwent induction of ALI by repeated saline wash-out of surfactant. Animals were randomly assigned to receive either PLV (PLV, n=8) with 30 ml/kg of perfluorocarbons (PF 5080, 3 M, Germany) and a PEEP level of 5 cmH2O, to receive vcMV with a PEEP level of 1 cmH2O above the LIP ( ideal PEEP, n=8), or to receive vcMV with a PEEP level of 5 cmH2O (Controls, n=8). Measurements and results: Measurements of pulmonary gas exchange, respiratory mechanics and haemodynamics were performed hourly for a 6 h period. In the ideal PEEP group, intra-pulmonary shunt (Qs/Qt) decreased from 55±5% after induction of ALI to 10±3% (p<0.05 versus Controls and versus PLV) and PaO2 increased from 52±4 to 566±19 mmHg after 6 h of treatment (p<0.05 versus Controls and versus PLV). In the PLV group, Qs/Qt decreased from 50±5% after induction of ALI to 24±3% (p<0.05 versus Controls) and PaO2 increased from 59±5 to 306±35 mmHg after 6 h of treatment (p<0.05 versus Controls). In the PLV group and in Controls, mean pulmonary artery pressure (MPAP) was significantly increased from 27±2 to 38±2 mmHg and from 29±1 to 40±1 mmHg, respectively, 6 h after induction of ALI (p<0.05 versus ideal PEEP), while in the ideal PEEP group, MPAP was maintained between 26±1 and 31±2 mmHg for 6 h after ALI. Cardiac output (CO) decreased significantly in the ideal PEEP group compared to Controls (p<0.05), while CO did not change in the PLV group and in Controls. The compliance of the respiratory system (CRS) increased in the ideal PEEP group after induction of ALI from 11±2 to 22±5 ml/mbar (p<0.05 versus Controls and versus PLV) and in the PLV group from 10±2 to 13±3 ml/mbar after 6 h of treatment (p<0.05 versus Controls). On histological examination, the highest total injury scores were found in animals of the PLV group (p<0.05 versus Controls and versus ideal PEEP), while the lowest total lung injury score was found in the dependent lung regions of the ideal PEEP group (p<0.05 versus Controls). Conclusion: In this porcine model of ALI, vcMV with a PEEP level of 1 cmH2O above the LIP was superior to high dose PLV with a PEEP of 5 cmH2O in improving gas exchange and lung mechanics. In terms of lung damage, the treatment in the ideal PEEP group resulted in the lowest total lung injury scores.