The purpose of the study is to evaluate the influence of different durations of the end-expiratory occlusion test to predict fluid responsiveness in mechanically ventilated patients in the operating room.

The reduction of postoperative complications is an essential aspect of care in perioperative medicine.

Many studies have shown that individualized hemodynamic therapy, by optimizing fluid administration and cardiac output, can reduce the risk of postoperative morbidity and mortality.

A fluid challenge, consisting in the administration of a small bolus of intravenous volume of fluid over a short period of time, is one of the commonest therapeutic hemodynamic intervention to test the preload reserve and the cardiac response to an increase in intravascular volume of a patient (fluid responsiveness). However, an increase in stroke volume (or cardiac output) is only achieved in 30 to 50% of patients after a fluid challenge test. Several dynamic tests have been described to predict the response to fluid challenge, however their limits are numerous.

The end-expiratory occlusion (EEO) test, consisting in a brief interruption of mechanical ventilation at end-expiration, by preventing the variation in intra-thoracic pressure, allows an increase in venous return, cardiac preload and stroke volume. Thus, an increase in stroke volume during an EEO can predict fluid responsiveness, simulating a fluid challenge though avoiding the administration of possibly unnecessary fluids.

The EEO test has already been successfully evaluated in several studies in ICU patients. Only few studies were conducted in the operating room, with conflicting findings: one study, consisting of an EEO test of 30 seconds found that changes in stroke volume during an EEO can predict fluid responsiveness, whereas another one, consisting of an EEO test of 15 seconds did not.

The purpose of the study is to evaluate the influence of the duration of EEO test to predict fluid responsiveness in mechanically ventilated patients in operating room.

Inclusion Criteria:

• Adults (age >18 years)
• Scheduled to undergo planned surgery under general anesthesia
• Surgery requiring invasive arterial and stroke volume monitoring
• Clinical indication to perform a fluid challenge
• Patients able to give informed consent
• Affiliated to a social security scheme

Exclusion Criteria:

• Patient refusal to participate or inability to provide informed consent
• Protected major
• History of lobectomy or pneumectomy
• Patient with reduced left (ejection fraction < 45%) or right ventricular systolic function
• Arrythmia
• Severe valvulopathy
• Body Mass Index <15 or > 40kg/m2
• Contraindication to insertion of invasive arterial line into radial artery
• Emergency surgery
• Pregnancy

A randomization sequence by permuted blocks (random block sizes) with an allocation of 1:1 will be generated by a computer program (Stata software, version 15, StataCorp, College Station, US) The group assignment will be concealed until arrival in the operating room so that only the anesthesiologist in charge of the patient will aware to the randomization group