This is a prospective observational study of physiologically-stable, ventilated patients post-cardiac surgery. The primary objective of this study is to evaluate the relationship between the static size of the right IJV and fluid responsiveness in ventilated post-operative cardiac surgical patients. A secondary objective is to evaluate the relationship between dynamic variations in the size of the right IJV related to respiration and leg-raising and fluid responsiveness in the same group. Static and dynamic ultrasound measurements of the right IJV will be performed pre-volume challenge.