Ultrasound evaluation of dynamic change in inferior vena cava (IVC) diameter has become useful for the diagnosis and treatment of acutely ill patients with hypotension, dehydration, and dyspnea. In the early 2000s, goal-directed therapy became the standard of care for the management of patients with septic shock. Central venous pressure (CVP) was specifically recognized as the best hemodynamic parameter to determine volume responsiveness (1, 2, 3, 4, 5). Central venous access required for CVP monitoring carries risk for complications such as arterial puncture, venous thrombosis, and infection. Not only is the procedure to obtain central venous access time consuming, but placement is often delayed until after intravenous (IV) fluid boluses fail or after elevated lactic acid is reported. Practical limitations exist as well, ranging from expensive and specialized equipment to the availability of trained personnel to monitor CVP continuously (6,7). A survey conducted in 2004 highlighted these limitations after reporting that only 7% of academic emergency departments (ED) had initiated protocols for invasive hemodynamic monitoring (8). Interest in dynamic measurement of the IVC grew as investigators began to search for a widely available, rapidly employed, cost-effective, and noninvasive means of quantifying CVP.

The IVC is a distensible conduit for return of blood from the inferior systemic circuit to the right heart. Because the IVC is so distensible, the diameter is dependent on an assortment of variables, including circulating blood volume, right heart pressure, intrathoracic pressure, and abdominal pressure. Since the 1970s, sonographic description of IVC caliber variation with respiratory cycling has been widely reported (9, 10, 11, 12, 13, 14). In 1990, Kircher et al. reported a noninvasive estimation of right atrial pressure by measuring IVC collapse (caval index) with end inspiration. The caval index opened the door for numerous studies correlating CVP with IVC diameter and degree of IVC respiratory collapse (Fig. 6.1) (15, 16, 17, 18, 19, 20, 21, 22, 23). Today, bedside ultrasound assessment of the IVC provides the emergency physician with a quick, inexpensive, noninvasive, valuable hemodynamic parameter that serves to narrow the differential for undifferentiated hypotension and guide initial resuscitation efforts.

There are three principal clinical scenarios in which bedside ultrasound evaluation of IVC collapse is beneficial: (1) Adjunct to the clinical assessment of the undifferentiated hypotensive patient, (2) Adjunct to volume resuscitation of the volume-depleted patient, (3) Adjunct to the clinical assessment of acute dyspnea.

To image the IVC, place the patient in a supine position. A curvilinear 3 to 5 MHz probe or phased array 2 to 5 MHz probe provides adequate penetration to interrogate the IVC. The scan should begin in a midline sagittal orientation just below the xyphoid process with the probe indicator pointing to the patient’s head. The sonographer then slides the probe 1 to 2 cm to the patient’s right to bring the IVC into the field of view (Fig. 6.2). Slight angling of the probe to aim under the costal margin will reveal the entry point of the IVC
into the right atrium. Ideally, the hepatic vein inlet should be visualized entering the IVC about 1 cm caudal to the right atrium (Fig. 6.3). The hepatic vein inlet is a key point of reference because measurements of the caval index are typically conducted 1 cm caudal to this location. In volume-depleted patients the hepatic veins may not be visible secondary to complete collapse of the vessel. If this is the case, the sonographer should conduct caval index measurements 2 to 3 cm caudal to the right atrium (Fig. 6.4).

In instances of extreme volume depletion the hepatic segment of the IVC may be completely collapsed and not be easily visualized in a longitudinal axis (Fig. 6.5). In instances where the hepatic segment of the IVC cannot be identified in the longitudinal axis the sonographer can switch to a subxyphoid view of the heart to identify the right atrium (Fig. 6.6). After identifying the right atrium the probe is slid to the patient’s right, bringing the right atrium center on the ultrasound screen. The probe is then angled nearly perpendicular to the skin surface to identify the IVC (Fig. 6.7), followed by a 90-degree clockwise rotation of the probe to turn the probe indicator to the patient’s head and obtain a long-axis view of the IVC (Fig. 6.8). It is worth mentioning however, that if the hepatic segment is not visible secondary to collapse, then clinical concern for low volume status is answered. If clinically indicated, the patient should be bolused with IV fluids followed by repeat ultrasound of the IVC with the patient remaining in the supine position to evaluate for change in diameter.

Various locations for sampling the IVC diameter have been reported, ranging from just cephalad to the hepatic vein inlet to the inlet of the left renal vein (21,23,31,35, 36, 37, 38, 39, 40, 41, 42, 43, 44). Locations between the hepatic vein inlet and the right atrium are not ideal because the muscular attachment of the diaphragm may result in decreased compliance (44). The most frequent location for sampling is 1 cm distal to the hepatic vein inlet or 2 to 3 cm from the right atrium if the hepatic vein inlet is not visualized. However, Lichtenstein reports a saber profile or bulge of the IVC, caused by the incoming hepatic vein giving concern for unreliable diameter measurements (36). In addition, the collapsibility of the hepatic segment may be influenced by the surrounding parenchyma, with liver fibrosis or cirrhosis causing impaired diminution (35). In instances where the hepatic segment appears to be an unreliable location for diameter sampling, a feasible alternative is the inlet of the left renal vein to the IVC (Fig. 6.9) (45).