Role of End-Tidal CO2 During Passive Leg Raising to Predict Fluid Responsiveness in ICU

This prospective observational study evaluates whether monitoring changes in exhaled carbon dioxide (End-Tidal CO2 or EtCO2) during a "Passive Leg Raising" (PLR) test can reliably predict fluid responsiveness in critically ill patients. In the Intensive Care Unit (ICU), fluid management is a critical balance; while fluid is necessary for tissue perfusion, overload can lead to severe complications. Traditional static measurements (like central venous pressure) are often unreliable for guiding therapy. Dynamic tests like PLR are preferred as they simulate a fluid bolus reversibly by shifting blood from the legs to the heart. Researchers will observe mechanically ventilated patients planned for fluid resuscitation. The study compares the accuracy of non-invasive EtCO2 changes during PLR against a reference standard "Mini Fluid Challenge" (100 mL fluid administration). Fluid responsiveness will be confirmed using echocardiographic measurements (LVOT-VTI) or arterial pressure changes. The goal is to validate EtCO2 as a practical, real-time tool for safe fluid management.

Inclusion Criteria: * Patients aged 18 years or older. * Patients who have been followed in the intensive care unit (ICU) with mechanical ventilation support for at least 24 hours. * Patients for whom a mini fluid challenge (MFC) is clinically planned by the intensive care team. * Patients for whom informed consent has been obtained from first-degree relatives or a legal representative. Exclusion Criteria: * Patients younger than 18 years of age. * Pregnant patients. * Patients without consent from first-degree relatives or a legal representative. * Patients with known deep vein thrombosis (DVT) in the leg veins (risk of embolism during leg raising). * Patients with pathologies that may cause increased intracranial pressure (e.g., intracranial mass, hemorrhage). * Patients with uncontrolled hypertension. * Patients with severe cardiac disease (e.g., right heart failure, pulmonary hypertension, advanced valvular pathology). * Patients with carbon dioxide (CO2) retention. * Patients who have undergone hip surgery or hip replacement surgery where Passive Leg Raising (PLR) cannot be performed. * Patients with spontaneous breathing. * Patients with a probability of high intra-abdominal pressure. * Patients who cannot lie in a supine position. * Patients with current peripheral oxygen saturation (SpO2) \< 88%. * Patients on mechanical ventilation requiring pressure support \> 16 mmHg or PEEP \> 10 mmHg.