In a prospective study of critically-ill patients with undifferentiated shock (n=77), a meta-analysis of ten studies in anesthesia and critical care (n=424), a pilot study of CABG patients (n=23) and 2 studies in healthy volunteers with moderate-to-severe central blood volume loss (n=25), carotid artery corrected flow time was observed to track changing stroke volume (i.e., fluid responsiveness) with excellent accuracy1-6. For comparison, mean arterial pressure (MAP) is less than 50% sensitive for detecting fluid responsiveness7,8.