The 2008 ACEP Policy Statement on Emergency Ultrasound Guidelines includes the evaluation of intravascular volume status and estimation of central venous pressure (CVP) based on sonographic examination of the inferior vena cava (IVC).
The primary utility of bedside ultrasound of the IVC is to aid in assessment of the intravascular volume status of the patient. This may be of particular utility in cases of undifferentiated hypotension or other scenarios of abnormal volume states, such as sepsis, dehydration, hemorrhage, or heart failure.
Changes in volume status will be reflected in sonographic evaluation of the IVC, where increased or decreased collapsibility of the vessel will help guide clinical management of the patient. The combination of the absolute diameter of the IVC and the degree of collapse with respiration may give an estimate of CVP and substitute for more invasive measurements.
The IVC is a thin-walled compliant vessel that adjusts to the body’s volume status by changing its diameter depending on the total body fluid volume. The vessel contracts and expands with each respiration. Negative pressure created by the inspiration of the patient increases venous return to the heart, briefly collapsing the IVC. Exhalation decreases venous return and the IVC returns to its baseline diameter.
In states of low intravascular volume, the percentage collapse of the vessel will be proportionally higher than in intravascular volume overload states. This is quantified by the calculation of the caval index: IVC expiratory diameter – IVC inspiratory diameter, divided by IVC expiratory diameter × 100 = caval index (%).
The caval index is written as a percentage, where a number close to 100% is indicative of almost complete collapse (and therefore volume depletion), while a number close to 0% suggest minimal collapse (i.e., likely volume overload).
Studies (see References section) have correlated the absolute IVC diameter and caval index with CVP (see table above).
Certain exceptions must be noted, such as the plethoric IVC that may be found in cardiac tamponade, where the patient may be normo- or even hypovolemic despite a suggestion of volume overload by the ultrasound images. As such, findings should always be interpreted within their clinical context and/or in conjunction with a cardiac evaluation.