Peri-operative fluid-therapy is extremely important in thoracic surgery, because excessive administration of fluids during one-lung ventilation is correlated to an increasing risk of postoperative respiratory complications.

Therefore, current guidelines on peri-operative management of patients undergoing thoracic surgery suggest a conservative fluid management strategy, based on intra-operative fluid loss replacement and maintenance of euvolemia.

Nevertheless, intra-operative fluid loss estimation and consequently the correct infusion rate adoption are quite difficult to be addressed in clinical practice, and this often prevents the euvolemia maintenance in the peri-operative period.

This limit claims the necessity to adopt new methods of fluid-therapy administration in thoracic surgery; among these the most promising is the "Goal-Directed Therapy" (GDT). GDT protocols based on Stroke Volume Variation (SVV) or Pulse Pressure Variation (PPV) monitoring have been adopted successfully in major and cardiac surgery but not yet in thoracic surgery.

The aim of this randomized study is to evaluate the effects of a PPV-GDT fluid management protocol versus a conservative "zero-balance" protocol on intrapulmonary gas exchanges, in patients undergoing single-lung ventilation during thoracic surgery.

The intra-operative fluid-therapy (using lactated Ringer) will be based on pulse pressure variation (PPV group) with a target ≤5.8% or on compensation (1:1) of urine output (zero balance group).

In both groups an intraoperative background infusion of lactated Ringer at 1-2 ml/kg/h will be administered.

Inclusion Criteria:

• Adult patients scheduled for Video-assisted thoracic surgery (VATS) lobectomy

Exclusion Criteria:

• Patients who will not sign the informed consent
• Obesity (BMI > 35 kg/m2)
• Cardiovascular disease (heart failure, arrhythmia)
• OSAS requiring or not C-PAP therapy
• Chronic alcoholism
• intraoperative blod loss> 1500 ml
• One-lung ventilation duration< 60 min