Background: The increased tidal volume (VT) applied tothe ventilated lung during one-lung ventilation (OLV) enhancescyclic alveolar recruitment and mechanical stress. It isunknown whether alveolar recruitment maneuvers (ARMs)and reduced VT may influence tidal recruitment and lungdensity. Therefore, the effects of ARM and OLV with differentVT on pulmonary gas/tissue distribution are examined.Methods: Eight anesthetized piglets were mechanically ventilated(VT 10 ml/kg). A defined ARM was applied to thewhole lung (40 cm H2O for 10 s). Spiral computed tomographiclung scans were acquired before and after ARM.Thereafter, the lungs were separated with an endobronchialblocker. The pigs were randomized to receive OLV in thedependent lung with aVT of either 5 or 10 ml/kg. Computedtomography was repeated during and after OLV. The voxelswere categorized by density intervals (i.e., atelectasis, poorlyaerated, normally aerated, or overaerated). Tidal recruitmentwas defined as the addition of gas to collapsed lung regions.Results: The dependent lung contained atelectatic (5610 ml),poorly aerated (18310 ml), and normally aerated (18729 ml)regions before ARM. After ARM, lung volume and aeration increased(42635 vs. 52669 ml). Respiratory compliance enhanced,and tidal recruitment decreased(95%vs.79%of the wholeend-expiratory lung volume).OLVwith10ml/kgfurther increasedaeration (atelectasis, 152 ml; poorly aerated, 9424 ml; normallyaerated, 580 98 ml) and tidal recruitment (81% of thedependent lung). OLV with 5 ml/kg did not affect tidal recruitmentor lung density distribution. (Data are given as meanSD.)Conclusions: The ARM improves aeration and respiratorymechanics. In contrast to OLV with high VT, OLV withreduced VT does not reinforce tidal recruitment, indicatingdecreased mechanical stress.