Offline measurements of IJV size will be obtained from unmodified stored video loops by a single observer blinded to patient identity, hemodynamic, and fluid response data. Fluid responsiveness will be assessed by examining the change in cardiac index after volume challenge with a fluid responsive state defined as an increase of ≥ 12 % of cardiac index after volume challenge. The relationship between volume responsiveness and IJ ultrasound measures will be evaluated continuously and dichotomously.