Learning Objectives

Adequate exam
There is no single caliber or respirophasic behavior of the IVC that can be considered universally normal in all circumstances. For instance, the IVC seen in Videos 1–4 and Figure 3 was imaged in a healthy, hydrated male experiencing no acute illness. However, notably, this patient's "normal" IVC has a relatively large AP diameter, >2 cm in the ANT IVC LAX view, and shows minimal respirophasic change. This exact same IVC appearance in other circumstances could be considered pathological (e.g., if there is suspicion of any of the following: congestive heart failure, chronic renal disease, pulmonary hypertension, right heart dysfunction, cardiac tamponade, and/or pneumothorax causing high intrathoracic pressure)^13,14,19,20. Similarly, the finding of >50% change in IVC caliber is considered normal in asymptomatic patients¹⁴ but has been associated with hypovolemic shock and with a higher risk of hypotension during the induction of general anesthesia^21,22. Additionally, the relationships between IVC parameters (size and respirophasic change) and intravascular volume status are known to break down in any of the following situations¹⁰: (1) positive pressure ventilation with either small tidal volumes or large PEEP; (2) spontaneous ventilation with either shallow or vital capacity breathing; (3) hyperinflated lung states (e.g., obstructive lung disease); (4)  states of impaired venous return (e.g., pulmonary hypertension, right heart dysfunction, cardiac tamponade, tension pneumothorax); and (5) states of increased intraabdominal pressure .

Since the clinical interpretation of the IVC caliber and respirophasic change is highly context-dependent and this paper is centered on IVC image acquisition, we define an adequate exam as one that permits the visualization of the IVC (Figure 3) and an inadequate exam as one that does not show the IVC or shows it transiently, thus preventing the assessment of the maximal caliber of the vessel, its respirophasic change, or both. As an example of a complete adequate exam, Videos 1–4 each permit IVC visualization and, thus, interpretation.

Inadequate exams
There are two common pitfalls leading to inadequate exams: 1) the abdominal aorta being misidentified as the IVC, and 2) the IVC lateral displacement being mistaken for IVC respirophasic change. In Figure 6 and Video 5, the operator mistakenly obtained a clip of the abdominal aorta in long-axis rather than of the IVC. Since the two vascular structures lie in close proximity to one another and run in parallel²³, the misidentification of one for the other is common.

In non-peer reviewed teaching, an often cited way of identifying the IVC is to visualize the vascular structure draining into the right atrium^24,25. However, the long-axis view of the abdominal aorta often falsely appears to show the cranial portion of the aorta as contiguous with various cardiac chambers, commonly the RA (see Video 5). Without being aware of this pitfall, in the authors' experience, trainees often misidentify the abdominal aorta as the IVC when this criterion is used.

To help distinguish between the two reliably, certain heuristics are helpful. Specifically, the IVC has the following sonographic features: (1) it is located to the right of the midline and is intrahepatic; (2) it is thin-walled; (3) it lacks pulsatility (except in severe tricuspid regurgitation); and (4) it can vary in shape over the course of the respiratory cycle 

Conversely, the abdominal aorta has the following sonographic features: (1) it is located to the left of the midline and is retro-hepatic; (2) it has thick echogenic walls; (3) it is pulsatile (except in cardiac arrest and in the presence of non-pulsatile ventricular assist devices); and (4) it is generally constant in shape throughout the respiratory cycle.

The shape of the pressurized aorta remains generally cylindrical throughout the respiratory cycle, whereas the IVC, which has lower internal pressure, is more easily distorted by external forces. Specifically, changes in intrathoracic pressure are transmitted to the IVC in complex ways, resulting in dynamic changes in the IVC caliber over the course of the respiratory cycle. These changes have been termed IVC respirophasic changes¹⁵.

Depending on the mode of ventilation, the pattern of IVC respirophasic change varies. When a spontaneously breathing patient inspires, the diaphragm contracts and moves caudally, generating negative intrathoracic pressure that promotes venous return to the right heart²⁶. As a result, the IVC collapses in response to this negative inspiratory pressure and expands during expiration (see Video 6).

Intuitively, the opposite is true for mechanically ventilated patients. With mechanical ventilation, positive pressure is generated down the bronchioalveolar trees, thus expanding the lungs and creating positive intrathoracic pressure²⁶. This positive pressure impedes venous return and distends the IVC during inspiration. Subsequently, the pressure release during expiration allows a proportional decrease in the caliber of the IVC.

The presence of respirophasic change can be a marker of both normal and abnormal physiology, depending on the context^{18,21,22,27,28,29,30,31}. In either case, to detect respirophasic change, the maximal dimension of the IVC must remain in the 2-dimensional plane of the ultrasound beam throughout a clip. However, the IVC and aorta can move laterally during the respiratory cycle, regardless of the mode of ventilation¹⁵. In the long-axis views of either structure, this lateral movement may falsely appear to be a respirophasic change. Differentiating this pseudo-collapsibility from true collapsibility is best performed by supplementing long-axis views with short-axis views, in which the lateral displacement can be viewed directly, while simultaneously assessing for true compression or expansion during respiration.

An example of IVC lateral displacement is shown in Video 7. In this video, the IVC's seeming collapsibility is due to its movement relative to the ultrasound transducer. This relative movement would prevent a clinician from assessing the true respirophasic change in IVC size. Therefore, the clip shown is inadequate for IVC assessment.

Figure 1: Anterior IVC short-axis view. To obtain the anterior IVC short-axis view, the probe is placed just caudal to the xiphoid process in the coronal plane, with the indicator mark pointing toward the patient's left. Please click here to view a larger version of this figure.

Figure 2: Anterior IVC long-axis view. To obtain the anterior IVC long-axis view, first the anterior IVC short-axis view is obtained. Then, the IVC is centered, and the probe is rotated 90° counterclockwise so that the probe's indicator mark faces cranially and the probe is aligned with the long axis of the patient's body. Please click here to view a larger version of this figure.

Figure 3: Anterior IVC long-axis view AP measurement. Still image of the anterior IVC long-axis view showing where the standardized measurement of the antero-posterior diameter of the vessel should be made (i.e., 1-2 cm caudal to the hepatic vein confluence, where the hepatic veins empty into the IVC). Please click here to view a larger version of this figure.

Figure 4: Right lateral IVC long-axis view. To obtain the right lateral IVC long-axis view, the ultrasound probe is placed just anterior to the mid-axillary line along the left flank, with the ultrasound beam in the coronal plane and the indicator mark pointing cranially. Please click here to view a larger version of this figure.

Figure 5: Right lateral IVC short-axis view. To obtain the right lateral IVC short-axis view, first the right lateral IVC long-axis view is obtained. Then, the IVC is centered, and the probe is rotated 90° clockwise so that the probe's indicator mark faces anteriorly, perpendicular to the long axis of the patient's body. Please click here to view a larger version of this figure.

Figure 6: Anterior abdominal aorta long-axis view: This is a labeled still image of Video 5. This view was obtained by searching for the anterior IVC long-axis view whilst angling the ultrasound beam slightly toward the patient's left. In this image, the aorta appears to be contiguous with the right atrium (RA), a frequent finding that undermines the utility of looking for drainage into the RA as a way of distinguishing between the IVC and the abdominal aorta. Please click here to view a larger version of this figure.

Video 1: Anterior IVC short-axis view. Video and accompanying still image showing the typical sonographic appearance of the anterior IVC short-axis view. In this view, the intrahepatic nature of the inferior vena cava (IVC) can be easily appreciated. In this view, the IVC is surrounded by liver anteriorly and posteriorly. In contrast, under normal circumstances, the abdominal aorta (AO) lies posterior to the liver. Further, the anterior IVC short-axis view typically allows the visualization of the spine, which is located deeper than both the IVC and abdominal aorta. The cartoon schematic seen at the beginning and end of this clip was reprinted with permission from www.countbackwardsfrom10.com. Please click here to download this Video.

Video 2: Anterior IVC long-axis view. Video and accompanying still image showing the typical sonographic appearance of the anterior IVC long-axis view. In this view, the IVC is seen in its long-axis cross-section as a rectangular structure within the liver extending from the diaphragm cranially to the caudal portion of the screen. Other structures often seen in this view include the spine and a portion of the supradiaphragmatic space. The cartoon schematic seen at the beginning and end of this clip was reprinted with permission from www.countbackwardsfrom10.com. Please click here to download this Video.

Video 3: Right lateral IVC long-axis view. Video and accompanying still image showing the typical sonographic appearance of the right lateral IVC long-axis view. In this view, the IVC is seen in its long-axis cross-section as a rectangular structure within the liver extending from the diaphragm cranially to the caudal portion of the screen. Other structures often seen in this view include the abdominal aorta (seen in long-axis in this view) and the diaphragm. Notably, in most patients, the IVC lateral-to-medial (L/M) diameter is on average about 4 mm greater than the antero-posterior (A/P) IVC diameter³². However, despite this discrepancy in the absolute size, the respirophasic change is similar in both directions for a given IVC. Accordingly, there is evidence that the two views can be used interchangeably for some purposes³². The cartoon schematic seen at the beginning and end of this clip was reprinted with permission from www.countbackwardsfrom10.com. Please click here to download this Video.;

Video 4: Right lateral IVC short-axis view. Video and accompanying still image showing the typical sonographic appearance of the right lateral IVC short-axis view. The superficial portion of this view contains structures in the right flank, such as the liver. The deep portion of this view contains structures located near the midline of the body, such as the spine, IVC, and abdominal aorta (AO). Both the IVC and aorta are seen in this view in their short-axis cross-sections (i.e., as relatively round structures). The cartoon schematic seen at the beginning and end of this clip was reprinted with permission from www.countbackwardsfrom10.com. Please click here to download this Video.

Video 5: Anterior aorta long-axis view. Video and accompanying still image showing the abdominal aorta (AO) in long-axis view. This view was obtained by searching for the anterior IVC long-axis view whilst angling the ultrasound beam slightly toward the patient's left. In this clip, the aorta appears to be contiguous with the right atrium (RA), a frequent finding that undermines the utility of looking for drainage into the RA as a way of distinguishing between the IVC and abdominal aorta. Please click here to download this Video.

Video 6: Anterior IVC long-axis view respirophasic change. This video clip shows an anterior long-axis view of the IVC in a spontaneously breathing patient. Normally, as seen here, a large negative pressure breath or sniff lowers the intrathoracic pressure significantly, creating a large enough gradient for venous return to increase from the abdomen into the thorax and, thus, causing an increase of >50% in the IVC's antero-posterior dimension. Please click here to download this Video.

Video 7: Anterior IVC long-axis view pseudo-collapsibility. This video clip shows an anterior long-axis view of the IVC in a spontaneously breathing patient. However, the IVC is seen moving in and out of the plane of the ultrasound beam, as evidenced by the disappearance and reappearance of the hepatic veins draining into the IVC, which have a fixed position relative to the IVC itself. Cases like this of lateral IVC displacement are, in our experience, commonly misinterpreted by trainees as IVC collapsibility, which results in the potential for treatment error. To minimize the chances of committing this error, we recommend always complementing long-axis views of the IVC with supplementary short-axis views. Please click here to download this Video.

Video 8: Anterior IVC in long-axis view that is narrow and collapsible. This video clip shows an anterior long-axis view of the IVC in a spontaneously breathing patient, with findings suggestive of grossly low right atrial pressure: an IVC anterior-posterior dimension <1 cm and >50% collapse of the IVC diameter with respiration. IVC parameters this extreme are typically a sign of intravascular hypovolemia and, in the setting of hypotension, can be used as a justification for administering a fluid challenge. Please click here to download this Video.

Video 9: Anterior IVC in long-axis view that is distended. This video clip shows an anterior long-axis view of the IVC in a spontaneously breathing patient, with findings suggestive of grossly elevated right atrial pressure: an IVC anterior-posterior dimension of ~2.5 cm and essentially no respirophasic change. IVC parameters this extreme are typically a sign of intravascular normovolemia to hypervolemia. In cases of hypotension, these IVC findings suggest that something other than hypovolemia is likely primarily driving the hypotension. Please click here to download this Video.