Comparison of Acoustic Variability Index (AVI) and Pulse Pressure Variation (PPV) for Predicting Fluid Responsiveness in Mechanically Ventilated Patients

Inclusion Criteria: To participate in this study, individuals must meet all of the following criteria: * Adults aged 19 years or older. * If female, not pregnant or breastfeeding. * Scheduled for a planned surgery of major organs in the abdominal, retroperitoneal, or pelvic cavities (e.g., hepatobiliary and pancreatic, small intestine, large intestine, kidney, uterus) under general anesthesia with mechanical ventilation * Classified as American Society of Anesthesiologists (ASA) Class I, II, or III. (Note: Excluded regardless of class if any exclusion criteria are met) * Scheduled to receive mechanical ventilation in a volume-controlled or pressure-controlled mode in the operating room or intensive care unit, with an anticipated tidal volume of 6-8 mL/kg (Ideal Body Weight), respiratory rate of 8-14 breaths/min, and PEEP of 0-5 cmH2O * Scheduled for hemodynamic monitoring to measure the comparison indices during surgery * Able to understand the study's purpose and procedures and communicate clearly with the investigator and research staff * Voluntarily signed the written informed consent form before participating in the study Exclusion Criteria: Individuals meeting any of the following criteria will be excluded from participation: * Undergoing emergency or unplanned surgery * Anticipated to have difficulty with the application of general anesthesia or mechanical ventilation * Anticipated to experience massive bleeding of 1 L or more during surgery. * Undergoing a reoperation due to the same underlying disease * History of severe esophageal damage (e.g., esophageal varices or stricture) that makes inserting the probe difficult * Inability to have an arterial catheter inserted, making the measurement of Stroke Volume Variation (SVV) and Pulse Pressure Variation (PPV) difficult * Condition where a fluid challenge is contraindicated or considered high risk (e.g., clinical signs of fluid overload, pulmonary congestion, or pulmonary edema; acute exacerbation of heart failure or renal failure; or a hemodynamically unstable state) * Body Mass Index (BMI) greater than 30 kg/m² * Uncontrolled coagulopathy prior to surgery (e.g., Platelet count \< 100,000/μL or INR \> 2) * Arrhythmias, such as atrial fibrillation/flutter or premature atrial/ventricular contractions (\>2 per 10 seconds), that make it difficult for the variability index to operate accurately * Other cardiovascular history that makes accurate cardiac output measurement impossible (e.g., severe valvular disease, history of intracardiac shunt surgery, or structural heart surgery) * Pregnant or lactating women * Any other condition deemed unsuitable for study participation by the investigator