The aim of this study is to investigate the hypothesis that the use of the Hypotension Prediction Index algorithm (HPI) can reduce intraoperative hypotension (IOH) in adult patients undergoing spinal surgery in the prone position under general anesthesia, as well as to explore its effect on in-hospital postoperative morbidity and mortality.

The study is a Prospective Randomized clinical trial. Adult patients (>18y) undergoing spinal surgery in the prone position under general anesthesia will be included.

The patients will be randomized using a computer-generated, permuted block randomization with a 1:1 allocation into two groups.

Intervention group: in this group, the HPI algorithm will be used in order to prevent hypotensive episodes Control group: in this group standard anesthetic care will be provided. Hypotensive episodes will be treated with vasoactive agents and fluids.

All patients will receive the same type of anaesthesia (TIVA with propofol) and monitoring including Patient State Index (PSI), non-invasive monitoring of blood pressure, SpO2, continuous electrocardiographic monitoring, ETCO2 and urinary output. Additionally, invasive continuous measurement of the patient's blood pressure will be available via radial artery catheterization. The arterial catheter will be connected to both the standard monitor and the platform which includes the HPI software. Arterial blood gas testing will be performed on an hourly basis.

Intraoperative recordings include PSI values, hemodynamic parameters every 10-15 minutes and urinary output every hour. Hemodynamic parameters will also be collected electronically from the monitor. The total doses of propofol, opioids/sedatives, fluids and vasoactive agents will also be recorded, as well as the estimated blood loss.

In this study, time-weighted average (TWA) in hypotension will be calculated in all patients.[TWA= depth of hypotension x time spent in hypotension / total surgery time].

Postoperatively Blood sampling for hs-TropI will be collected after the surgical procedure and during the following 3 postoperative days. If an increase in the levels hs-Trop I is noted the patient will be inquired for symptoms of myocardial ischemia and a new ECG will be performed. A cardiology consultation will be requested, if necessary.

Creatinine levels and urinary output will be monitored during the first 2 postoperative days. Acute kidney injury will be assessed according to the AKIN classification.

All in-hospital incidents and in-hospital mortality will also be documented.

Inclusion Criteria:

• Adult patients undergoing scheduled spine surgery performed in prone position under general anaesthesia

Exclusion Criteria:

• Heart failure with reduced ejection fraction (LVEF<35%)
• Severe aortic and/or mitral regurgitation
• Persistent atrial fibrillation or other significant cardiac arrhythmias
• Significant preoperative hypotension
• End-stage renal disease on dialysis/RRT

Not aware of the study group