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

The goal of this clinical trial is to learn if a new medical device called ResQ works to predict fluid needs in adults undergoing surgery under general anesthesia. The main questions it aims to answer are: * Can the Acoustic Variability Index (AVI) measured by ResQ predict if a participant needs more fluids as accurately as the standard arterial blood pressure-based method (PPV)? * Is the ResQ device safe to use during surgery? Researchers will compare the new method (AVI) to the standard method (PPV) to see if ResQ provides reliable information for managing patient fluids. Participants will: * Undergo their planned surgery as scheduled. * Have a soft probe placed in the esophagus to monitor heart and lung sounds. * Receive a set amount of intravenous (IV) fluid as part of their regular surgery care. * Have their heart function checked before and after the fluids are given.

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