Intraoperative hypotension can impact patient outcome. Vasopressors are usually used to correct hypotension and ensure adequate organ perfusion.

The investigators have recently developed an automated system (closed-loop system) to titrate vasopressor agents in surgical and intensive care patients.

The purpose of this study is to compare two strategies to correct hypotension based on an individual definition of hypotension (therefore, the target MAP used to define hypotension will differ for each patient (individualized approach):

1. Control group = standard practice ( manually adjusted norepinephrine infusion to correct hypotension and keep MAP within 90% of patient's baseline MAP
2. Intervention group = closed-loop (automated) vasopressor administration system will deliver norepinephrine using feedback from standard operating room hemodynamic monitor (EV1000 Monitor-Flotrac, Edwards Lifesciences, IRVINE, USA) to correct hypotension and keep MAP within 90% of patient's baseline MAP

In order to prevent the known postoperative complications of intraoperative hypotension, vasopressor agents are occasionally used to ensure adequate perfusion. These vasopressors are usually administered as manually adjusted infusions, but this practice requires considerable time and attention.

To overcome this issue, the investigators have developed a closed-loop vasopressor (CLV) controller to potentially correct hypotension more efficiently. After completing extensive in-silico, in-vivo studies and a pilot human trial in a small cohort of 20 patients, the investigators aimed to conduct now a randomized control trial comparing manual vasopressor adjustment versus closed-loop vasopressor adjustment in high risk patients undergoing major abdominal surgeries in order to correct hypotension during surgery

The primary outcome will be the incidence of hypotension (defined as a reduction of > 10% from patient's MAP target, or a allowed tolerance of 10% reduction from patient's baseline MAP). This has been chosen based on the recent study of Emmanuel Futier and colleagues (Effect of Individualized vs Standard Blood Pressure Management Strategies on Postoperative Organ Dysfunction Among High-Risk Patients Undergoing Major Surgery: A Randomized Clinical Trial. JAMA. 2017 Oct 10;318(14):1346-1357), even if it was originally done with systolic blood pressure

Participants in both groups will receive standard patient care in that in no way will their anesthetic or surgical procedure will be altered as part of the study, with the exception of vasopressor administration.

Fluids will be standardized in both groups and will be given as a continuous baseline infusion of 3 ml/kg/h (balanced crystalloid solution) and additional fluid boluses (mini fluid challenges of 100 ml) as a goal directed fluid therapy strategy to maintain stroke volume variation < 13%. The only difference is the way norepinephrine is delivered to the patient (manual versus closed-loop assisted)

Inclusion Criteria:

• High-risk adult patients undergoing major abdominal surgeries requiring an advanced cardiac output monitoring device (EV1000-Edwards LifeSciences, Irvine, USA) and a tight blood pressure control

Exclusion Criteria:

• Atrial Fibrillation
• Severe Arythmia