A Study Comparing Carotid Doppler Ultrasound and Echocardiography to Assess Fluid Responsiveness in Septic Patients on Ventilator

The goal of this clinical trial is to determine whether common carotid artery blood flow measured by bedside Doppler ultrasound can accurately identify fluid responsiveness in mechanically ventilated adults with sepsis. The study will compare this method with transthoracic echocardiography-derived cardiac output, which is a commonly used non-invasive technique for assessing hemodynamic status in critically ill patients. The study also aims to evaluate whether carotid artery ultrasound can provide a faster and more practical bedside assessment during fluid resuscitation in the intensive care unit. In patients with sepsis, giving too little intravenous fluid may worsen organ perfusion and shock, while excessive fluid administration may lead to complications such as pulmonary edema, worsening oxygenation, prolonged mechanical ventilation, and longer intensive care unit stay. Because of this, clinicians need reliable methods to identify which patients are likely to benefit from fluid administration. This concept is known as fluid responsiveness. The main questions this study aims to answer are: * Can common carotid artery blood flow measured by ultrasound predict fluid responsiveness as accurately as transthoracic echocardiography-derived cardiac output? * How closely do changes in carotid artery blood flow correspond to changes in cardiac output during passive leg raising? * Can carotid ultrasound provide a quicker and simpler bedside method for assessing fluid responsiveness in critically ill patients receiving mechanical ventilation? Researchers will compare two ultrasound-based methods during a passive leg raising test. Passive leg raising is a temporary bedside maneuver used to simulate a fluid challenge without immediately giving intravenous fluids. During this maneuver, the patient's legs are elevated while the upper body is lowered, allowing blood from the lower limbs to temporarily return to the central circulation. This increases blood flow to the heart and may increase cardiac output in patients who are likely to respond to fluid administration. The study team will first record baseline measurements of common carotid artery blood flow and transthoracic echocardiographic cardiac output while the patient is in a semi-recumbent position. The passive leg raising maneuver will then be performed, and both measurements will be repeated to evaluate changes in blood flow and cardiac output. If the predefined criteria for fluid responsiveness are met, the participant will receive intravenous crystalloid fluid according to the study protocol. Participants will: * Undergo bedside Doppler ultrasound measurement of common carotid artery blood flow * Undergo transthoracic echocardiography to measure cardiac output * Have ultrasound measurements recorded before and after passive leg raising * Receive intravenous fluid administration if fluid responsiveness criteria are met * Continue routine monitoring of blood pressure, heart rate, oxygenation, urine output, and other intensive care parameters during the study period The findings from this study may help determine whether carotid artery ultrasound can be used as a practical bedside tool for fluid assessment in mechanically ventilated patients with sepsis, particularly in settings where rapid and non-invasive monitoring is needed.