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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.

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