Objective To investigate the 50% effective concentration (EC₅₀) and 95% effective concentration (EC₉₅) of nitrous oxide (N₂O) fraction in oxygen (O₂) for fast lung collapse during one-lung ventilation in video-assisted thoracoscopic surgery (VATS).

Methods Thirty-eight consecutive patients undergoing VATS with American Society of Anesthesiologists (ASA) physical status Ⅰ to Ⅱ were selected. All patients underwent lung sequestration by left double-lumen endotracheal. The lung collapse score of each patient was evaluated by the same surgeon 5 minutes after skin incision. The first patient received 30% N₂O in O₂, and subsequent concentrations were determined by the lung collapse score of the previous patient using Dixon up-and-down method. The testing interval was set at 10%, and the lowest concentration was 10%. The EC50 and EC95 for fast lung collapse of N₂O fraction in O₂ was analyzed by probabilistic regression analysis.

Results All patients received 50% N₂O in O₂ showed successfully fast lung collapse. The EC50 and EC95 of N₂O fraction in O₂ for fast lung collapse was 27.7% (95% confidence interval 19.9%~35.7%) and 48.7% (95% confidence interval 39.0%~96.3%), respectively.

Conclusions In patients with a left double-lumen endotracheal tube undergoing VATS, the EC50 and EC95 of N₂O fraction in O₂ for fast lung collapse were 27.7% and 48.7%, respectively.