Comparison of PPV and LVOT VTI During Passive Leg Raising to Predict Fluid Responsiveness

This study is designed as a prospective observational trial. No additional intervention, procedure, or treatment beyond standard clinical practice will be performed. All data will be obtained during routine clinical management in our intensive care unit. In mechanically ventilated patients with suspected fluid deficit, the passive leg raising (PLR) maneuver is applied as part of standard practice, during which changes in pulse pressure variation (PPV) are recorded by invasive arterial monitoring. In this observational study, transthoracic echocardiographic (TTE) measurements of LVOT VTI will be performed by an experienced operator in addition to routine practice. Using pulse wave Doppler (PW), LVOT VTI will be recorded, and stroke volume (SV) and cardiac output (CO) will be calculated by multiplying the LVOT VTI value by the LVOT diameter. Consequently, changes in PPV and LVOT VTI-based CO during PLR will be compared. Following TTE measurements of LVOT VTI and LVOT diameter in both the semi-recumbent position (45°) and immediately after PLR, SV and CO will be calculated. Based on these measurements, patients will be allocated into two groups: * Fluid responsive: Patients showing ≥10% increase in LVOT VTI-based CO following PLR * Non-fluid responsive: Patients showing \<10% change in LVOT VTI-based CO following PLR After grouping patients accordingly, PPV changes during PLR will be compared between the two groups. This will allow evaluation of the predictive performance of PPV variation relative to LVOT VTI-based CO change in determining fluid responsiveness. All measurements will be performed by an experienced clinician without disrupting standard patient care. No additional fluid loading, invasive procedures, or therapeutic interventions will be undertaken throughout the study. The primary aim is to compare the diagnostic performance of PPV variation and LVOT VTI-based cardiac output changes following a PLR maneuver in predicting fluid responsiveness. The study seeks to determine the diagnostic value of both methods to support more individualised fluid management. The recorded data will include: * Pre- and post-PLR LVOT VTI, stroke volume, cardiac output, and PPV values * Routine monitoring parameters (systolic/diastolic blood pressure, mean arterial pressure, heart rate, and SpO₂) * Demographic characteristics (age, sex, comorbidities) retrieved through the hospital database Informed consent will be obtained from all participating patients or their legally authorized representatives.