<qa>  What is the PPV measurement in mechanically ventilated patients?Background: calculation of the pulse pressure variation PPV has repeatedly been shown to be a reliable predictor of fluid responsiveness in mechanically ventilated patients. But PPV measurement has not been yet validated in thoracic surgery during one-lung ventilation. The modifications of intrathoracic pressure induced by one-lung ventilation may altered the capabilities of PPV to predict fluid responsiveness.This study was designed to assess the ability of PPV predict fluid responsiveness during one-lung ventilation in thoracic surgery. 