AB BACKGROUND: Collapse of the ipsilateral lung facilitates surgical exposure during thoracic procedures. The use of different gas mixtures during two-lung ventilation (2LV) may improve or impede surgical conditions during subsequent one-lung ventilation (OLV) by increasing or delaying lung collapse. We investigated the effects of three different gas mixtures during 2LV on lung collapse and oxygenation during subsequent OLV: Air/Oxygen (fraction of inspired oxygen [Fio2] = 0.4), Nitrous Oxide/Oxygen (“N2O,” Fio2 = 0.4) and Oxygen (“O2,” Fio2 = 1.0). METHODS: Subjects were randomized into three groups: Air/Oxygen (n = 33), N2O (n = 34) or O2 (n = 33) and received the designated gas mixture during induction and until the start of OLV. Subjects’ lungs in all groups were then ventilated with Fio2 = 1.0 during OLV. The surgeons, who were blinded to the randomization, evaluated the lung deflation using a verbal rating scale at 10 and 20 min after the start of OLV. Serial arterial blood gases were performed before anesthesia induction, during 2LV, and every 5 min, for 30 min, after initiation of OLV. RESULTS: The use of air in the inspired gas mixture during 2LV led to delayed lung deflation during OLV, whereas N2O improved lung collapse. Arterial oxygenation was significantly improved in the O2 group only for the first 10 min of OLV, after which there were no differences in mean Pao2 values among groups. CONCLUSIONS: De-nitrogenation of the lung during 2LV is a useful strategy to improve surgical conditions during OLV. The use of Fio2 1.0 or N2O/O2 (Fio2 0.4) during 2LV did not have an adverse effect on subsequent oxygenation during OLV. (C) 2009 by International Anesthesia Research Society.