An Approach to Point-Of-Care Ultrasound Evaluation of the Abdominal Aorta

Summary

This protocol reviews the steps to image the abdominal aorta with point-of-care ultrasound. We discuss image acquisition, troubleshooting imaging pitfalls and artifacts, and the recognition of life-threatening abdominal aortic pathology.

Abstract

Disorders of the abdominal aorta, including aneurysms and dissection, have potentially high rates of morbidity and mortality. While computed tomography (CT) is the current gold standard to image the abdominal aorta, the process of obtaining a CT may be time-consuming, requires the use of intravenous contrast dye, and involves exposure to ionizing radiation. Point-of-care Ultrasound (POCUS) can be performed at the bedside and has excellent sensitivity and specificity for the diagnosis of abdominal aortic aneurysm and excellent specificity for the diagnosis of abdominal aortic dissection. Additionally, POCUS is non-invasive, cost-effective, lacks ionizing radiation, requires no intravenous contrast dye, and can be performed without taking the patient from a critical care area. Screening for abdominal aortic aneurysm (AAA) can be done in primary care settings as well.

This article will review the approach to POCUS of the abdominal aorta to evaluate such critical pathology. In this paper, we will review the sonographic anatomy of the abdominal aorta as well as the choice of the ultrasound probe, description of POCUS image acquisition, and some pearls and pitfalls of using POCUS to aid in the diagnosis of potentially life-threatening abdominal aortic pathology.

Introduction

Point-of-care ultrasound (POCUS) has increased in use over the last several years and is being increasingly incorporated into various residency training programs^1,2. POCUS has great utility in critical care areas such as the emergency department and the intensive care unit, specifically to aid in the rapid diagnosis of life-threatening intraabdominal emergencies such as acute aortic dissection, as well as abdominal aortic aneurysms, especially those at risk for rupture and those that have ruptured into the peritoneum.

AAA rupture and acute aortic dissection are associated with high mortality. The mortality of ruptured aortic aneurysms ranges from 67% to 94%^3,4. The mortality associated with type A aortic dissection increases at a rate of 1% per hour after acute dissection and the mortality of type B aortic dissection ranges from 10% to 25% at 30 days⁵. Abdominal aortic dissection in isolation is rare and accounts for only 0.2% to 4% of all aortic dissections^6,7,8,9,10. Since most abdominal aortic dissections occur as an extension of thoracic aortic dissections, evaluation of the abdominal aorta for evidence of dissection may aid in the diagnosis of thoracic aortic dissection¹¹.

Computed tomography with angiography (CTA) is the gold standard for imaging pathology associated with the abdominal aorta; however, it has several drawbacks. It may be time-consuming, especially in an unstable patient, and requires a technician to perform and a radiologist or vascular surgeon to interpret the images. CTA uses ionizing radiation and requires the use of intravenous contrast dye for optimal detection of pathology. Furthermore, the performance of CTA requires potentially unstable patients to leave the critical care area. In contrast, POCUS is non-invasive, cost-effective, and lacks the ionizing radiation and contrast dye that CT requires. It can also be performed and interpreted by the same individual in real time and does not require the patient to leave the monitored area.

A systematic review of emergency department POCUS for diagnosing AAA by Rubano et al. revealed a sensitivity of 99% and specificity of 98%, with a positive likelihood ratio of 99 and a negative likelihood ratio of 0.01¹². This pooled analysis evaluated the test characteristics over a varied group of operators, including resident and attending physicians with a wide range of training in POCUS.

The test characteristics for the POCUS evaluation of abdominal aortic dissection are different from those of AAA and may vary depending on the origin of the dissection. Sonographic findings of an intimal flap separating the true and false lumens have a sensitivity of 67%-79% and a specificity of 99%-100% for aortic dissection^13,14. As most aortic dissections found in the abdomen are an extension of a thoracic aortic dissection, additional POCUS applications of the heart and lungs to evaluate for pericardial effusion, aortic root dilatation, and left pleural effusion may be performed, but will not be the focus of this paper¹³.

Finally, it is important to note that the United States Preventative Services task force provides a Grade B recommendation for a one-time ultrasound screening for AAA in men aged 65-75 who have ever smoked. This is particularly relevant to the primary care setting.

This review will describe a step-by-step protocol for the performance of POCUS in the bedside evaluation of the abdominal aorta, specifically to evaluate for AAA and abdominal aortic dissection. This protocol assumes a basic knowledge of diagnostic ultrasound, including physics, instrumentation, as well as medical knowledge of anatomy and pathologic states of the abdominal aorta and major branching arteries. Readers are advised to refer to other sources for prerequisite knowledge.

Protocol

All ultrasounds in this protocol were performed on human subjects and were conducted following the ethical standards of the University of Illinois Hospital and the Declaration of Helsinki and its revisions. The imaging was performed on the authors themselves and patients in the emergency department as part of routine education and clinical care with preceding verbal consent as is the standard for the institution. Images collected illustrate both normal anatomy and physiology as well as abnormal findings collected at the University of Illinois Hospital. Images used to illustrate scanning techniques were performed on members of the writing team. All ultrasound images are free of any identifying information. The subsequent protocol was designed using sources from peer-reviewed journals and book chapters^{10,15,16,17,18,19}. For this review, the protocol will focus on obtaining US images of adults.

1. Safety

NOTE: POCUS studies can be performed with non-sterile gloves, either nitrile or latex, depending on patient allergies. Additional safety measures may be taken based on clinical context and institutional policies.

1. Examine the ultrasound system for cleanliness prior to use and clean the machine and probes in the appropriate manner after use. The cleaning material and process are dictated by the ultrasound unit manufacturer and institutional standards.

2. Selection of probe

1. For most adults, the abdominal aorta is best visualized with a 2.5-3.5 MHz curvilinear probe, which has a large footprint and a wide field of view with a convex beam shape. This probe will generally provide excellent resolution and measurement capabilities.
2. Alternatively, use the phased array probe (1-5 mHz), typically used for echocardiography and often informally referred to as the cardiac probe.
   ​NOTE: The phased array probe can be useful especially when attempting to visualize the proximal abdominal aorta as it exits through the diaphragmatic hiatus. This is especially true if the space just inferior to the xyphoid process is too narrow to accommodate the wider curvilinear probe. The phased array probe has a rectangular footprint and a triangular beam shape with a narrower field of view than the curvilinear probe but should be adequate to achieve the imaging goals.

3. Machine presets

1. Use the abdominal preset on the machine regardless of the probe used.
2. Set the mode to B mode or 2-dimensional grayscale.
3. Set the depth to 20 cm.
   NOTE: This is typically adequate to visualize the vertebral body which is an important landmark for the aorta.
4. Adjust the depth once the aorta is visualized to keep the aorta in the midfield of the screen.
5. Consider using harmonic imaging to provide better visualization if imaging is challenging due to excessive bowel gas.
   NOTE: Harmonics uses the resonance characteristics of tissue and creates a higher-resolution image with fewer artifacts.
6. Choose a lower frequency range for patients with a high body mass index to improve image acquisition.

4. Scanning technique

1. Apply ultrasound gel to the transducer.
2. Position the patient supine with the abdomen exposed. Hip flexion, if tolerated by the patient, will relax the abdominal muscles and may improve image acquisition.
   NOTE: Bowel gas can impede image acquisition. To improve image acquisition in the presence of bowel gas, the operator can apply firm, continuous pressure, known as graded compression, to the scanning area for a few minutes, displacing the bowel gas. Evaluation of the aorta in the coronal plane can also avoid bowel gas encountered in the transverse plane (see step 4.3.5).
3. For a thorough evaluation of the abdominal aorta, obtain the images listed below.
   1. Obtain images of the proximal aorta in the transverse plane.
      1. Orient the transducer in the transverse plane with the indicator toward the patient's right. Ensure that the indicator position matches the indicator on the screen (Figure 1A).
      2. Place the transducer just distal to the patient's xiphoid process and apply light pressure to visualize the anterior aspect of the vertebra with its hyperechoic shadow-casting arch (Figure 1B).
         NOTE: The liver will appear in the upper left corner of the screen and acts as an acoustic window. The aorta will appear just above the vertebral body as an anechoic circle on the right side of the screen, corresponding to the patient's left. The inferior vena cava (IVC) is on the left side of the screen, corresponding to the patient's right. The IVC has a thinner wall than the aorta and is often collapsible even with light pressure.
      3. Slide the transducer caudally until the celiac trunk is visualized. The celiac trunk is short and quickly bifurcates into the hepatic artery and splenic artery. When the two arteries are visualized together, this is called the seagull sign (Figure 2).
      4. Capture these images for later review by clicking the button on the system that records clips.
      5. Slide the transducer caudally to encounter the superior mesenteric artery (SMA), which comes off the anterior aorta and very quickly courses inferiorly, typically following a parallel path with the aorta. The splenic vein courses anterior to the SMA and the left renal vein courses between the SMA and the aorta (Figure 3).
      6. Capture these images for later review by clicking the button on the system that records clips.
      7. Measure the AP diameter of the suprarenal aorta by optimizing a live image of the aorta in this location and then pressing the system's freeze button.
      8. Press caliper or measure and move the track ball or touchpad of the system to the outer edge of the anterior wall, the adventitia, and click select.
      9. Move the trackball or touchpad again to the outer edge of the posterior wall and click Select. Wait for the system to generate a measurement (Figure 4).
      10. Save this image as a still image containing the measurement by clicking the button on the system that saves still images .
          NOTE: The upper limit of normal of the AP diameter of the aorta is 3.0 cm. Any measurement >3 cm is considered aneurysmal^15,16,20,21.
   2. Image the distal aorta in the transverse plane.
      NOTE: The distal aorta comprises two-thirds of the abdominal aorta and begins just distal of the renal arteries. The majority of AAAs occur in this distal segment.
      1. As with the proximal aorta, continue scanning in a transverse plane visualizing the entirety of the aorta through the bifurcation.
      2. Capture these images for later review by clicking the button on the system that records clips.
      3. When a live image of the distal aorta is optimized, measure the AP diameter of the infrarenal aorta.
      4. Press caliper or measure and move the trackball or touchpad of the system to the outer edge of the anterior wall, the adventitia, and click select.
      5. Move the trackball or touchpad again to the outer edge of the posterior wall and click Select. Wait for the system to generate a measurement.
      6. Save this image as a still image containing the measurement by clicking the button on the system that saves still images.
         NOTE: It is prudent to obtain at least two measurements of the distal aorta given its greater length and increased likelihood of having aneurysmal dilations.
      7. Adjust the depth as the abdominal aorta courses caudally through the abdomen, since it becomes more superficial and tapers slightly.
   3. Obtain a video clip of the aortic bifurcation into the left and right iliac arteries (Video 1-see Supplemental File 1: Supplemental Figure S1).
      1. Continue scanning caudally, adjusting the depth as necessary to maintain the aorta and vertebral body in the middle of the screen.
      2. Scan through the aortic bifurcation into the left and right iliac arteries.
      3. Capture images while scanning through the bifurcation.
   4. Obtain images and video clips of the aorta in the longitudinal plane.
      1. Place the probe in the proximal abdomen, starting again in the subxiphoid area.
      2. It is often easier to begin in the transverse plane with the indicator toward the patient's right. Once the transverse view of the aorta is optimized, rotate the probe clockwise, following the aorta as the image becomes longitudinal on the screen and the indicator is pointing toward the patient's head (Figure 5A).
      3. Acquire images while scanning caudally examining for aneurysmal dilatations.
      4. Capture these images for later review by clicking the button on the system that records clips.
         NOTE: The celiac trunk and SMA are easily visible projecting from the anterior aorta in long axis view (Figure 5B). It is advisable NOT to measure the diameter of the aorta in the longitudinal axis. If the US beam intersects the aorta tangentially, as opposed to in its midline, the measurement will be falsely smaller than if it were through the maximal AP diameter (Figure 6).
   5. Optional: Obtain a longitudinal view of the aorta in the coronal plane. This view is useful if obtaining views in the transverse or longitudinal planes is difficult.
      1. Begin with the transducer in the coronal plane at the midaxillary line on the patient's right with the indicator pointing cranially (Figure 7A).
      2. If the patient is able, position them in the left lateral decubitus position for better image acquisition.
      3. Scan the aorta in a coronal plane. The aorta will be visualized deep to the IVC if both vessels are imaged (Figure 7B).
      4. Capture these images for later review by clicking the button on the system that records clips.

Results

Adequate exam
One of the biggest challenges in obtaining accurate results from an abdominal aortic ultrasound is the lack of consensus about measurement. As noted in protocol step 4.3.1.10, any diameter of the abdominal aorta greater than 3 cm is considered aneurysmal^15,16,22,23. There is, however, great variation in the methods used to measure the aorta's diameter, a...

Discussion

Timely diagnosis of AAA and aortic dissection is key in the treatment of these high-morbidity conditions. POCUS used in the diagnosis of AAA leads to improved outcomes and significantly decreases the time to diagnosis and operative intervention when compared with traditional imaging²⁷. POCUS has high sensitivity and specificity for AAA and high specificity for aortic dissection^12,13,19,

Materials

Name	Company	Catalog Number	Comments
M9 Ultrasound Machine	Mindray	n/a	Used to obtain all adequate and inadequate images/clips