Positive -versus negative- pressure ventilation in healthy rats

Background Observations in healthy rats and mice (Moriondo, A. et al. J Appl Physiol 2007; 103: 747-756; Vaneker, M. et al. Anesthesiology 2007; 107: 419-426) suggest that conventional positive-pressure ventilation with normal tidal volumes worsens lung mechanics and induces cytokine release. Aims To assess whether in normal rats, different kinds of mechanical ventilation produce adverse effects as compared to spontaneous ventilation. Methods Anesthetized, tracheotomized rats were randomly assigned to 3 groups of 6 animals each: spontaneous breathing, positive-pressure ventilation, and negative-pressure ventilation group. Duration of ventilation (4 h) and mean tidal volume (6.8 ml/kg) were the same for all groups. Dynamic lung elastance (E) and resistance (R) were assessed throughout the test period. The left lung was used for the measure of the wet-to-dry (W/D) ratio and collection of bronchoalveolar lavage fluid (BALF), while the right lung was fixed for histologic assessment (mean linear intercept, bronchiolar injury score, lesioned bronchiolar-alveolar attachments). Cytokines (TNF- IL-1, IL-6, MIP-2, and IL-10) were measured in BALF and serum. Six additional animals, killed immediately, served as control. Results During the test period, E moderately increased (+16%), independent of the type of ventilation, while R remained unchanged. W/D ratio and cytokine levels in serum and BALF did not differ among groups. Relative to control, only serum levels of IL-6 and IL-10 were increased. Histologic indexes were similar to control in all groups. Conclusion No deleterious effect of mechanical ventilation was found in comparison to spontaneous breathing, independent of the modality with which mechanical ventilation was performed