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).

Clinical Indications

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.

Pathophysiology

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.

Performing the Scan

Positioning and probe selection. Place the patient in the supine position. The degree of elevation of the head of the bed has not been shown to make a significant difference in measurements (see References section). A low-frequency probe (3.5-5 MHz), such as a phased array or curvilinear probe, should be selected.

Landmarks. Two approaches may be used. The first is to obtain a subxiphoid view of the heart by placing the probe on the patient’s abdomen just below the xiphoid bone with the marker facing to the right of the patient. Once an appropriate subxiphoid view of the heart is obtained, the probe is rotated 90 degrees until the marker is pointing toward the head of the patient.

At this point, the IVC should be visualized in the longitudinal plane as it enters the right atrium.

The second approach is to scan using the liver as an acoustic window by placing the probe in the right anterior midaxillary line, similar to the placement for evaluating Morison’s pouch in the focused assessment with sonography for trauma (FAST) examination. The marker should be pointing to the head of the patient. By scanning more anteriorly and cephalad than the Morison’s pouch view, the IVC can be visualized running longitudinally adjacent to the liver and crossing the diaphragm. Following the vessel along until it enters the right atrium allows confirmation that the IVC is being visualized and not the aorta running parallel to it.

Measurements. The diameter of the IVC for calculation of the caval index should be measured 2 cm from where it enters the right atrium (Figures 1, 2, and 3).

An alternative way to visualize respiratory variation is to use M-mode, with the beam overlying the IVC 2 cm from the right atrium.

The inspiratory and expiratory diameter can then be measured on the M-mode image, at the smallest and largest locations, respectively (Figures 4 and 5).

Findings

• Volume depletion. In these patients, the diameter of the IVC will be decreased and the percentage collapse will be greater than 50%. With complete collapse, the IVC may become difficult to visualize (Figure 6).
• Volume overload. Patients with increased intravascular volume will have a large IVC diameter and minimal collapse on inspiration (Figure 7). In severe cases, there may not be any notable respiratory variation seen in M-mode.

References

• ACEP Policy Statement on Emergency Ultrasound Guidelines. Ann. Emerg. Med. 2009;53:550-70.
• Blehar DJ, Dickman E, Gaspari R. Identification of congestive heart failure via respiratory variation of inferior vena cava diameter. Am. J. Emerg. Med. 2009;27:71-5.
• Chen L, Santucci KA, Kim Y. Use of ultrasound measurement of the inferior vena cava diameter as an objective tool in the assessment of children with clinical dehydration. Acad. Emerg. Med. 2007:14:841-5.
• Feissel M, Michard F, Faller JP, et al. The respiratory variation in inferior vena cava diameter as a guide to fluid therapy. Intensive Care Med. 2004;30:1834-7.
• Fields JM, Lee PA, Jenq KY, et al. The interrater reliability of inferior vena cava ultrasound by bedside clinician sonographers in emergency department patients. Acad. Emerg. Med. 2011;18:98-101.
• Kircher BJ, Himelman RB, Schiller NB. Noninvasive estimation of right atrial pressure from the inspiratory collapse of the inferior vena cava. Am J. Cardiol. 1990;66:493-6.
• Kosiak W, Swieton D, Piskunowicz M. Sonographic inferior vena cava/aorta diameter index, a new approach to the body fluid status assessment in children and young adults in emergency ultrasound preliminary study. Acad. J. Emerg. Med. 2008;26:320-5.
• Nagdev AD, Merchant RC, Tirado-Gonzalez A, et al. Emergency department bedside ultrasonographic measurement of the caval index for noninvasive determination of low central venous pressure. Ann. Emerg. Med. 2010;55:290-5.
• Randazzo MR, Snoey ER, Levitt MA, et al. Accuracy of emergency physician assessment of left ventricular ejection fraction and central venous pressure using echocardiography. Acad. Emerg. Med. 2003;10:973-7.