Image Acquisition Method for the Sonographic Assessment of the Inferior Vena Cava

Summary

Point-of-care ultrasound evaluation of the inferior vena cava (IVC) is commonly utilized to identify, among other things, the volume status. Imaging should be performed systematically to ensure repeatability. This manuscript reviews the methods and pitfalls of sonographic IVC examination.

Abstract

Over the past several decades, clinicians have incorporated several applications of diagnostic point-of-care ultrasound (POCUS) into medical decision-making. Among the applications of POCUS, imaging the inferior vena cava (IVC) is practiced by a wide variety of specialties, such as nephrology, emergency medicine, internal medicine, critical care, anesthesiology, pulmonology, and cardiology. Although each specialty uses IVC data in slightly different ways, most medical specialties, at minimum, attempt to use IVC data to make predictions about intravascular volume status. While the relationship between IVC sonographic data and intravascular volume status is complex and highly context-dependent, all clinicians should collect the sonographic data in standardized ways to ensure repeatability. This paper describes standardized IVC image acquisition including patient positioning, transducer selection, probe placement, image optimization, and the pitfalls and limitations of IVC sonographic imaging. This paper also describes the commonly performed anterior IVC long-axis view and three other views of the IVC that can each provide helpful diagnostic information when the anterior long-axis view is difficult to obtain or interpret.

Introduction

Over the last several decades, the accessibility of point-of-care ultrasound (POCUS) has increased dramatically. Providers across medical disciplines can now integrate POCUS into their bedside exams and more readily identify important contributors to patients' conditions¹. For example, in acute care settings, one of the most important areas of focus is the assessment and management of volume status². Inadequate fluid resuscitation can result in tissue hypoperfusion, end-organ dysfunction, and severe acid-base abnormalities. However, overzealous fluid administration is associated with worsened mortality³. The determination of volume status has primarily been accomplished using the combination of physical exam findings and dynamic hemodynamic measures, including pulse pressure variation, central venous pressure, and/or fluid challenges via either passive leg-raise testing or intravenous fluid boluses⁴. With the growing availability of POCUS devices, some providers are seeking to use ultrasound imaging to supplement these measures⁵. The sonographic assessment of the anterior-to-posterior dimension of the IVC and the respirophasic change in that dimension can assist in the assessment of right atrial pressure and, possibly, intravascular volume status^6,7,8,9.

Notably, however, the relationship between IVC parameters (i.e., size and respirophasic change) and volume responsiveness is distorted in many common situations, including but not limited to, the following: (1) passively ventilated patients receiving either high positive end-expiratory pressure (PEEP) or low tidal volumes; (2) spontaneously breathing patients making either small or large respiratory effort; (3) lung hyperinflation; (4) conditions impairing venous return (e.g., right ventricular dysfunction, tension pneumothorax, cardiac tamponade, etc.); and (5) increased abdominal compart pressure¹⁰.

While the utility of IVC sonography as a standalone measure for assessing the intravascular volume status is debated^5,10,11,12, there is no debate about the fact that its use as a diagnostic tool requires imaging in standardized ways and the ability to utilize alternative views when a single vantage point proves to be inadequate². Toward this end, this manuscript defines the four sonographic views of the IVC, illustrates common sonographic pitfalls and how to avoid them, and provides examples of both typical and extreme IVC sonographic states. There are four views in which the IVC can be adequately visualized by transabdominal sonography: anterior short-axis, anterior long-axis, right lateral long-axis, and right lateral short-axis. The protocol below describes a standardized method of image acquisition.

Protocol

All procedures performed in the studies involving human participants were conducted in accordance with the ethical standards of the Duke University Health System Institutional Research Committee and with the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards. The protocol was performed using input from several peer-reviewed papers in the academic literature^2,13,14,15. Imaging was performed on the authors themselves for the normal images and as part of routine educational ultrasound scans done for teaching purposes for the positive images, with preceding verbal consent obtained as per institutional standards. The patients were selected based on certain criteria. Specifically, the inclusion criterion was any patient with hypotension, and the exclusion criterion was patient refusal to undergo an ultrasound exam.

1. Safety procedures

1. Utilize nonsterile nitrile or latex gloves depending on the patient's allergies. Additional safety precautions may be required based on the clinical context. Please refer to the respective institution's infection control policies, and follow any precautions in place.

2. Probe selection

1. For infants (i.e., children younger than 1 year of age), perform the sonographic evaluation of the IVC with either a low-frequency or high-frequency (>5 MHz) ultrasound transducer, depending on the infant's body size.
   ​NOTE: IVC evaluation in infants is a specialized pediatric topic beyond the scope of this review. The remainder of this review solely focuses on imaging the IVC in individuals over 1 year of age.
2. For individuals over 1 year of age, visualize the IVC with any low-frequency (≤5 MHz) ultrasound transducer, such as a linear phased-array sector arc probe or curvilinear probe.
   NOTE: The linear phased-array sector arc probe is commonly referred to as a phased-array probe. This term is misleading, since all modern ultrasound transducers use phasing to steer the ultrasound beam^16,17. However, for the sake of brevity, throughout this review, we will use the term phased-array probe instead of linear phased-array sector arc probe.
   1. The phased-array probe is the optimal probe for both main types of external cardiac ultrasound: transthoracic echocardiography (TTE) and focused cardiac ultrasound (FoCUS)¹⁸. When performing either TTE or FoCUS to evaluate the heart, continue using the phased-array transducer for the IVC portion of each exam rather than switching to another low-frequency probe.

3. Machine preset

1. Set the machine to the cardiology convention by using the Cardiac Preset function, which sets the indicator to the left of the screen. Set the screen refresh rate to >20 Hz.
   NOTE: The IVC evaluation can be performed in the Abdominal Mode. However, for the same points mentioned in step 2.2.1, it is far more convenient to utilize the same presets for both a FoCUS exam and a POCUS IVC exam.
2. Set the mode to B-Mode (2-dimensional grayscale). Set the depth to 6-20 cm, depending on the depth of the IVC in each patient.

4. Scanning technique

1. Apply ultrasound gel to the transducer.
2. Obtain the anterior IVC short-axis (ANT IVC SAX) view.
   1. Position the patient in the supine position with both hips flexed, if tolerated by the patient.
   2. Place the ultrasound probe centered on the patient's anterior midline just caudal to the xiphoid process in the coronal plane, with the transducer indicator mark pointing toward the patient's left (Figure 1).
   3. Adjust the depth so that the IVC and aorta appear in the middle third of the screen and the spine is visible (Video 1).
   4. For setting the axis, fan the ultrasound beam cranially or caudally until both the IVC and the abdominal aorta appear in the short-axis cross-section as rounded structures (Video 1).
   5. Decrease the gain until the blood in the IVC is either completely black or just a few specks of grey are visible (Video 1).
   6. Once all the settings are done, click on Acquire.
3. Obtain the anterior IVC long-axis (ANT IVC LAX) view.
   1. Position the patient in the supine position with both hips flexed, if tolerated by the patient.
   2. Position the probe for obtaining the ANT IVC SAX view as described in step 4.2, center the view on the IVC, and rotate the ultrasound probe 90° counterclockwise, without translating the probe, such that the probe's indicator faces cranially at the end of the rotation (Figure 2).
   3. Adjust the depth so that the IVC appears in the middle third of the screen and the liver tissue is visible deeper than the IVC (Video 2).
   4. For setting the axis, fan the ultrasound beam toward the patient's left or right until the IVC appears as a rectangular, intrahepatic structure spanning from cranial to caudal on the screen. (Video 2).
   5. Decrease the gain until the blood in the IVC is either completely black or just a few specks of grey are visible (Video 2).
   6. Once all the settings are done, click on Acquire.
   7. Optional: Quantify the IVC anterior-to-posterior (AP) diameter (Figure 3).
      1. With a live image of the IVC optimized as per step 4.3.6, click on Freeze. Click on Caliper or Measure, depending on the machine's measurement button.
      2. Move the trackball to the anterior wall of the IVC approximately 1-2 cm caudal from the hepatic vein confluence. Click on Select.
      3. Move the trackball to the posterior wall of the IVC opposite the point in step 4.3.7.2, such that the line between the two points is roughly perpendicular to the long axis of the IVC. Click on Select, and then click on Acquire.
4. Obtain the right lateral IVC long-axis (RL IVC LAX) view.
   1. Position the patient in the supine position with the legs flat and the right arm moved away from the patient's side, either overhead or outstretched laterally, so as to allow access to the right flank.
   2. Place the probe transducer in the coronal plane with the indicator pointing cranially in the sixth or seventh right intercostal space just anterior to the right mid-axillary line (Figure 4).
   3. Adjust the depth so that the IVC appears in the middle third of the screen and the liver tissue is visible deeper than the IVC (Video 3).
   4. For setting the axis, fan the ultrasound beam anteriorly or posteriorly until the IVC is visualized as a rectangular, intrahepatic structure spanning from cranial to caudal on the screen (Video 3).
   5. Decrease the gain until the blood in the IVC is either completely black or just a few specks of grey are visible (Video 3). Click on Acquire.
5. Obtain the right lateral IVC short-axis (RL IVC SAX) view.
   1. Continue positioning the patient supine with the legs flat and the right arm moved away from the patient's side, either overhead or outstretched laterally, so as to allow access to the right flank.
   2. Continue positioning the probe in the position used for obtaining the RL IVC LAX view (see step 4.4), center the view on the IVC, and rotate the ultrasound probe 90° clockwise, without translating the probe, such that the probe's indicator faces anteriorly at the end of the rotation (Figure 5).
   3. Adjust the depth so that the IVC appears in the middle third of the screen and the liver tissue, aorta, and spine are all visible deeper than the IVC (Video 4).
   4. For setting the axis, fan the ultrasound beam cranially or caudally until the IVC and abdominal aorta are visible in the short-axis view as rounded structures (Video 4).
   5. Decrease the gain until the blood in the IVC is either completely black or just a few specks of grey are visible (Video 4). Click on Acquire.

Results

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 respiroph...

Discussion

Even when properly imaged, information garnered from the IVC should not be the sole data point used for guiding treatment. The exact same IVC size and respirophasic changes can be seen in both normal states and in pathologic conditions. Therefore, the clinical context is critically important for guiding how to interpret the IVC data. Further, when using ultrasound to assess a patient's intravascular volume status, the published literature is mixed as to what thresholds of IVC size and respirophasic change accurately ...

Materials

Name	Company	Catalog Number	Comments
Edge 1 ultrasound machine	SonoSite	n/a	Used to obtain all adequate and inadequate images/clips