Fluid Responsiveness Predictability

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.

Fluid administration is the first-line treatment in hypovolemic states in critically ill patients. Prediction of fluid responsiveness is possible with echocardiography by assessing the variation of the sub-aortic velocity-time integral (AoVTI) during a passive leg raising test (PLR) or Mini-fluid challenge. However, VTI-Ao measurement is not feasible in all patients due to poor echogenicity. Validation of new fluid-responsiveness indices may facilitate the evaluation in this patient population. Among the available indices, variation of the sub-pulmonary VTI is a potential criterion.

Personalzing intraoperative anesthetic fluid management may help in preventing fluid accumulation and related complications. Fluids are gine as boluses in operating room (the so-called FC). The response to the FC is due to several physiological conditions related to the "preload dependency" (i.e. the intrinsic ability of the heart of increasing the stroke volume - SV - in response to fluid administration). The minimal volume required to appropriately "challenge" the cardiovascular system is 4 ml/kg of fluid, but higher volumes (up to 6 ml/kg may be needed). Predicting the response to FC administration may be possible by applying a physiological test (called functional hemodynamic test), such as the end-expiratory occlusion test, consisting in interrupping the mechanical ventilation and hence promoting venous return and consequente SV changes. The percentage of SV increase associated to EEOT may predict fluid responsiveness to the FC (patients responders will increase SV to a bigger extent, as compared to non-responders)