This study aims at evaluating if a specific protocol, that can be framed in the innovative concept of Perioperative Goal Directed Therapy (PGDT), based on the evaluation of the Stroke Volume Variation (SVV) - a parameter deriving from the adoption of a minimally invasive advanced hemodynamic monitoring technology with a special sensor called FloTrac® (Edwards) or of a non-invasive monitoring system with the Clearsight® sensor (Edwards) - is able to guarantee a greater precision in the intraoperative management of patients undergoing spinal surgery in prone position.

Inclusion Criteria:

• Age between 18-80 years
• American Society of Anesthesiologists(ASA) Classification I-II-III
• Minor and major spinal surgery
• Prone position
• Sinus rhythm
• Absence of atrial fibrillation. or severe arrhythmia
• Informed consent

Exclusion Criteria:

• Absence of informed consent
• Age less than 18 or older than 80 years
• Cardiac rhythm abnormalities: atrial fibrillation and / or severe arrhythmia

44 Patients undergoing spinal surgery in prone position at "Gaspare Rodolico Presidium" of Policlinico-Vittorio Emanuele University Hospital of Catania, in which a specific protocol, that can be framed in the innovative concept of Perioperative Goal Directed Therapy (PGDT), based on the evaluation of the Stroke Volume Variation (SVV) by using a minimally invasive advanced hemodynamic monitoring technology with a special sensor called FloTrac® (Edwards) o a non-invasive monitoring system with Clearsight® sensor (Edwards), is used.