The investigators believe that the concept of FTC as a marker of preload can be combined conveniently with PWD of the carotid artery and a passive leg raising maneuver (PLR) to estimate fluid responsiveness in critically ill patients. The method is very attractive due to the ease of access to the carotid artery, reproducibility, low cost, and since the FTC is a measurement of time (not velocity), the angle of insonation should be inconsequential, making the exam technically easier to perform compared to carotid artery VTI. This can be compared to a 10% increase in SVI following a PLR demonstrated by the Flotrac/Vigileo being considered the "gold standard". While the absolute values of cardiac output obtained with the Flotrac/Vigileo when compared with the PAC are debatable, the ability of the device to track changes in cardiac output/stroke volume in response to changes in preload and PLR have been shown to be accurate.16-19 A meta-analysis published by Cavallaro and colleagues showed that PLR induced changes in cardiac output were able to predict fluid responsiveness with a sensitivity and specificity of 89.4% and 91.4% with a pooled area under the ROC value of 0.95 regardless of ventilation mode, underlying cardiac rhythm, and technique of measurement. Thus, an increase in SVI > 10% with PLR detected by a Flotrac/Vigileo monitor without the need for a fluid bolus, should be sufficient to determine whether PLR induced changes in carotid FTC are able to detect fluid responsiveness.