There is no a reliable marker of intraoperative fluid excess or overload. The use of lung ultrasound in other settings, such as emergency room and critical care patients, helps us to determine if a patient has a condition of augmented intrathoracic fluid, that could be related to several circumstances, such as fluid overload, but also to heart failure, in example.

Nevertheless, there is no information regarding the basal incidence of this finding, to ascertain if could be eventually used as a potential marker of fluid overload. This protocol looks for the incidence of the finding of B-Lines, which are related to fluid overload, in patients undergoing open abdominal surgery.

The presence of B-lines will be observed in 4 spaces at each evaluation: Anterior-superior, and infero-lateral, by each side.

The times of evaluation will be prior to intubation, after intubation, every one hour of surgery, at the end of the surgery prior to extubation, and after 2 hours in the postoperative care unit.

Other variables addressed will be the vital signs, parameters related to a diminished intravascular status (ie, pulse pressure variability), central venous pressure, volume changes (reposition, bleeding, diuresis), and requirements of blood products.

A correlation with baseline characteristics of patients will be observed; nevertheless, this study is directed as primary endpoint to evaluate the incidence of the B-lines in echography, and the sample size was calculated following this endpoint.

Inclusion Criteria:

• Patients scheduled to open abdominal surgery Older than 18 years.

Exclusion Criteria:

• Previous thoracic surgery, BMI over 30 kg/m2 Baseline respiratory alterations related to preoperative supplementary oxygen requirements Evident malformations of the thoracic wall If the position required for the surgery does not allow the echographic windows required for the protocol

Patients scheduled to open abdominal surgery

Not required; all the data will be managed by the research team