Positive End-Expiratory Pressure and Variable Ventilation in Lung-Healthy Rats under General Anesthesia

Objectives: Variable ventilation (VV) seems to improve respiratory function in acute lung injury and may be combined with positive end-expiratory pressure (PEEP) in order to protect the lungs even in healthy subjects. We hypothesized that VV in combination with moderate levels of PEEP reduce the deterioration of pulmonary function related to general anesthesia. Hence, we aimed at evaluating the alveolar stability and lung protection of the combination of VV at different PEEP levels. Design: Randomized experimental study. Setting: Animal research facility. Subjects: Forty-nine male Wistar rats (200-270 g). Interventions: Animals were ventilated during 2 hours with protective low tidal volume (VT) in volume control ventilation (VCV) or VV and PEEP adjusted at the level of minimum respiratory system elastance (Ers), obtained during a decremental PEEP trial subsequent to a recruitment maneuver, and 2 cmH(2)O above or below of this level. Measurements and Main Results: Ers, gas exchange and hemodynamic variables were measured. Cytokines were determined in lung homogenate and plasma samples and left lung was used for histologic analysis and diffuse alveolar damage scoring. A progressive time-dependent increase in Ers was observed independent on ventilatory mode or PEEP level. Despite of that, the rate of increase of Ers and lung tissue IL-1 beta concentration were significantly lower in VV than in VCV at the level of the PEEP of minimum Ers. A significant increase in lung tissue cytokines (IL-6, IL-1 beta, CINC-1 and TNFalpha) as well as a ventral to dorsal and cranial to caudal reduction in aeration was observed in all ventilated rats with no significant differences among groups. Conclusions: VV combined with PEEP adjusted at the level of the PEEP of minimal Ers seemed to better prevent anesthesia-induced atelectasis and might improve lung protection throughout general anesthesia.