The goal of this prospective study is to validate the superior vena cava collapsibility index (SVC-CI) as a predictor of fluid responsiveness during laparotomy and open aortic surgery. The SVC-CI and patients' response to fluid will be assessed based on transesophageal echocardiography.

The study has three arms, in order to validate SVC-CI under the conditions of laparotomy, aortic cross clamping and high PEEP levels. One of the study arms will be an active comparator arm.

The data obtained from this study may help physicians guide intraoperative fluid therapy in a more efficient manner, in order to decrease perioperative mortality.

Patients undergoing high-risk non-cardiac surgery, including acute laparotomies and open aortic surgery, are at high risk of postoperative complications and adverse cardiovascular events. Perioperative fluid therapy is a factor which is strongly associated with postoperative outcomes. To avoid both hypovolemia and excessive fluid administration a strategy of goal-directed fluid therapy (GDFT) has been coined. Traditionally in the setting of operating rooms GDFT relied on stroke-volume variation (SVV) measured by hemodynamic monitors, based on uncalibrated pulse contour cardiac output analysis (uAPCO). However in the described cohort of patients uAPCO methods show a significantly increased measurement error, which is mainly attributed to dynamic changes in peripheral vascular tone. Furthermore, frequent occurrence of arhytmias in those patients limits the use of SVV and all other dynamic indices of fluid responsiveness based on those methods. Superior vena cava collapsibility index (SVC-CI) is a parameter which is devoid of the above mentioned limitations. It relies on respiratory variation of superior vena cava during the respiratory cycle and is therefore reliable in patients with irregular heart rhytm and seems independent from changing afterload conditions.

The main purpose of this study is to validate SVC-CI in patients undergoing controlled mechanical ventilation during laparotomies and during open aortic surgery, with focus on elevated PEEP levels and aortic cross-clamping.

Inclusion Criteria:

• Signed ICF (informed consent form)
• Age > 18 years old Eligibility criteria for each arm (additionally to mandatory criteria)
• Arm 1 - Patients undergoing open aortic surgery
• Arm 2 - Patients undergoing laparotomy
• Arm 3 - Patients undergoing laparotomy with sinus rhytm

Exclusion Criteria:

1. Patients who are deemed to be at risk of harm due to excessive fluid administration:

   • End-stage renal disease (eGFR <15ml/kg/min)
   • Decompensated heart failure
   • Respiratory failure prior to surgery
   • Other cases where the risk of harm from fluid overload is assessed to be significant by the investigator

2. Medical contraindication to the use of transesophageal echocardiography:

   • Active ulcerative gastritis or bleeding from upper gastrointestinal tract
   • Past history of esophageal or gastric surgery
   • Esophageal varices or other esophageal disease (stricture, Barret's disease, perforation in the past, achalasia)

The data will be shared in open access, however no plan has been designed.