Abstract: Objective　To explore the right-to-left transpulmonary shunt flow in thoracic surgery.Methods　The oxygen tensions of alveolar gas and arterial blood were measured with blood gas analysis settings in 26 patients scheduled for thoracic surgery during the procedure. The alveolar-arterial oxygen tension difference (A-aDO2) and right-to-left shunt fraction (Qs/Qt) was calculated before and 10 min after lateral decubitus positioning, 15 min and 60 min after thoracotomy.Results　There were no significant changes in A-aDO2 and Qs/Qt after the patients were positioned from supine to lateral decubitus. Both A-aDO2 and Qs/Qt increased after the chest was opened, which could be aggregated as the procedure prolonged. The Qs/Qt reached (12.16±5.57) % at 60 min of thoracotomy. However, no hypoxemia occurred during the operation.Conclusion　Shunt fraction of transpulmonary flow was significantly increased by thoracotomy and deteriorated as the open chest prolonged.