Effectiveness of Goal-Directed Fluid Therapy Guided by Cardiac Index Versus Central Venous Pressure in Major Abdominal Surgery Patients in the ICU.

Inclusion Criteria: * Adult patients aged 18 to 65 years. * Patients with a Body Mass Index (BMI) between 18 and 30 kg/m². * Patients undergoing elective major abdominal surgery with a planned postoperative admission to the ICU. * Patients with an American Society of Anesthesiologists (ASA) physical status classification of I to III. Exclusion Criteria: * Patients with pre-operative cardiovascular disorders (Coronary Artery Disease, or heart failure NYHA Class III and IV). * Patients with severe renal impairment (Chronic Kidney Disease Stage V) with or without hemodialysis and fluid restrictions. * Non-sinus cardiac rhythm. * Pregnancy. * Patients with right heart failure (patients with signs of congestion and symptoms of right heart failure accompanied by evidence of reduced right ventricular function, indicated by TAPSE \< 17 mm). * Patients with pulmonary hypertension (patients with echocardiographic suspicion of tricuspid regurgitation, tricuspid regurgitation velocity \> 3.4 m/s, or mean pulmonary artery pressure \> 20 mmHg from right heart catheterization). * Patients with Acute Respiratory Distress Syndrome (ARDS) or patients requiring specialized ventilation modes. * Patients who refuse to participate in the study.