Difficulties in the accurate assessment of intravascular volume in critically ill patients are frequently encountered. In addition to clinical evaluation, bedside echocardiographic measurements of fluid responsiveness can be technically difficult, especially in critically ill mechanically ventilated patients. The carotid artery is an easily accessible structure that is amenable to bedside ultrasonography performed by Intensivists. The investigators hypothesize that measurement of the carotid artery Corrected Flow Time (FTC) in response to a passive leg raise (PLR), which simulates a fluid bolus, can be used to predict fluid responsiveness.