Influence of respiratory rate and end-expiratory pressure variation on cyclic alveolar recruitment in an experimental lung injury model

INTRODUCTION: Cyclic alveolar recruitment/derecruitment (R/D) is an important mechanism of ventilator-associated lung injury. In experimental models this process can be measured with high temporal resolution by detection of respiratory-dependent oscillations of the paO(2 )(ΔpaO(2)). A previous study showed that end-expiratory collapse can be prevented by an increased respiratory rate in saline-lavaged rabbits. The current study compares the effects of increased positive end-expiratory pressure (PEEP) versus an individually titrated respiratory rate (RR(ind)) on intra-tidal amplitude of Δ paO(2 )and on average paO(2 )in saline-lavaged pigs. METHODS: Acute lung injury was induced by bronchoalveolar lavage in 16 anaesthetized pigs. R/D was induced and measured by a fast-responding intra-aortic probe measuring paO(2). Ventilatory interventions (RR(ind )(n = 8) versus extrinsic PEEP (n = 8)) were applied for 30 minutes to reduce Δ paO(2). Haemodynamics, spirometry and Δ paO(2 )were monitored and the Ventilation/Perfusion distributions were assessed by multiple inert gas elimination. The main endpoints average and Δ paO(2 )following the interventions were analysed by Mann-Whitney-U-Test and Bonferroni's correction. The secondary parameters were tested in an explorative manner. RESULTS: Both interventions reduced Δ paO(2). In the RR(ind )group, ΔpaO(2 )was significantly smaller (P < 0.001). The average paO(2 )continuously decreased following RR(ind )and was significantly higher in the PEEP group (P < 0.001). A sustained difference of the ventilation/perfusion distribution and shunt fractions confirms these findings. The RR(ind )application required less vasopressor administration. CONCLUSIONS: Different recruitment kinetics were found compared to previous small animal models and these differences were primarily determined by kinetics of end-expiratory collapse. In this porcine model, respiratory rate and increased PEEP were both effective in reducing the amplitude of paO(2 )oscillations. In contrast to a recent study in a small animal model, however, increased respiratory rate did not maintain end-expiratory recruitment and ultimately resulted in reduced average paO(2 )and increased shunt fraction.