Central venous pressure (CVP) is a key physiologic estimate of preload, which in turn helps define the vascular fluid status. It is a particularly important parameter to measure in critically ill and injured patients who may require resuscitation. Unfortunately, measurement of the CVP requires invasive central venous catheters which can be difficult or time-consuming to insert. A non-invasive means of inferring the CVP would provide clinicians with an alternative. Preliminary data suggests that the diameter of the vena cava may reflect the CVP. Ultrasonography can provide reliable measures of internal body structures including the vena cava, and therefore may be useful in this regard. Collecting simultaneous values of vena cava diameter and CVPs will allow for the identification and quantification of correlation.