title: Influence of respiratory rate and end-expiratory pressure variation on cyclic alveolar recruitment in an experimental lung injury model.
abstract: BACKGROUND
  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 paO2 (ΔpaO2). 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 (RRind) on intra-tidal amplitude of Δ paO2 and on average paO2 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 paO2. Ventilatory interventions (RRind (n=8) versus extrinsic PEEP (n=8)) were applied for 30 minutes to reduce Δ paO2. Haemodynamics, spirometry and Δ paO2 were monitored and the Ventilation/Perfusion distributions were assessed by multiple inert gas elimination. The main endpoints average and Δ paO2 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 paO2. In the RRind group, ΔpaO2 was significantly smaller (P<0.001). The average paO2 continuously decreased following RRind 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 RRind 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 paO2 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 paO2 and increased shunt fraction.
authors.affiliation: Department of Anaesthesiology, Medical Center of the Johannes Gutenberg-University, Mainz 55131, Germany. hartmane@uni-mainz.de
authors.forename: Erik K
authors.initials: EK
authors.lastname: Hartmann
authors.forename: Stefan
authors.initials: S
authors.lastname: Boehme
authors.forename: Alexander
authors.initials: A
authors.lastname: Bentley
authors.forename: Bastian
authors.initials: B
authors.lastname: Duenges
authors.forename: Klaus U
authors.initials: KU
authors.lastname: Klein
authors.forename: Amelie
authors.initials: A
authors.lastname: Elsaesser
authors.forename: James E
authors.initials: JE
authors.lastname: Baumgardner
authors.forename: Matthias
authors.initials: M
authors.lastname: David
authors.forename: Klaus
authors.initials: K
authors.lastname: Markstaller
mesh_terms: D000818:Animals; D004195:Disease Models, Animal; D055370:Lung Injury / Q000503:physiopathology* / Q000628:therapy; D010865:Pilot Projects; D011175:Positive-Pressure Respiration / Q000379:methods; D011650:Pulmonary Alveoli / Q000502:physiology*; D011897:Random Allocation; D056152:Respiratory Rate / Q000502:physiology*; D013552:Swine; D013997:Time Factors